Terahertz MEMS actuators and applications

Abstract

In recent years, micro-electromechanical systems (MEMS) actuators have emerged as innovative solutions for enhancing the dynamic control of terahertz devices, leveraging their advantages of miniaturization, low power consumption, and high integration. This paper provides a comprehensive review of the fundamental technological advancements in terahertz MEMS actuators, with a particular emphasis on the analysis of the performance characteristics of various driving mechanisms and the integration strategies. Furthermore, it systematically presents the diverse forms of terahertz MEMS actuators utilized in terahertz switches and tunable resonators, highlighting the significant advancements they have made in applications including sensing, frequency and polarization tuning, beamforming, and logical operations. By leveraging cutting-edge microfabrication techniques and functional materials, terahertz MEMS actuators are capable of achieving wideband tuning, high-sensitivity sensing, and the modulation of intricate electromagnetic responses. Additionally, the review examines prospective development trajectories, offering theoretical insights and technical strategies to support the transition of terahertz technology from laboratory settings to practical applications in domains such as 6 G communication, high-resolution imaging, and intelligent sensing.

Introduction

Terahertz (THz) waves are defined as electromagnetic waves that operate within a frequency range of 0.1–10 THz¹, situated in the electromagnetic spectrum between the microwave and infrared regions. In contrast to infrared wavelengths, terahertz waves possess the ability to traverse a majority of materials, with the exception of metals and water. Further more, terahertz waves are classified as non-ionizing radiation, and numerous molecular spectral characteristics are found within the terahertz frequency range². Consequently, terahertz waves exhibit significant potential for application across various domains, including security screening, quality assurance, molecular diagnostics, high-speed communication, and high-resolution imaging³. The advancement of high-speed communication has significantly enhanced the data transmission rates for individual users by several orders of magnitude. Concurrently, there has been an increased demand for reduced communication delays and enhanced reliability. Additionally, the convergence of terahertz and optical communication technologies presents unprecedented challenges for hardware systems and devices⁴. A particularly critical issue is the limited electromagnetic interaction between natural materials and terahertz waves, which significantly hampers the research and development of functional devices capable of effectively controlling the propagation of terahertz waves. This limitation is fundamentally responsible for the phenomenon known as the “terahertz gap”⁵.

The pronounced attenuation characteristics of terahertz waves pose significant challenges for the direct application of technologies developed for microwave and millimeter-wave frequencies in the design of low-loss terahertz switches and wideband tuned resonators, which are fundamental components of terahertz systems^6,7. The concept of micro-electromechanical systems (MEMS) actuators was first explored in the mid-1970s⁸, with commercial products emerging in the 1990s⁹. Recently, the integration of MEMS technology with terahertz devices has emerged as a promising strategy to address these challenges. MEMS technology offers advantages such as miniaturization, high integration, and low power consumption, demonstrating considerable potential for the manipulation of terahertz waves¹⁰. The incorporation of MEMS within terahertz components represents a burgeoning area of research, providing enhanced reconfigurability, superior performance, and the ability to achieve novel functionalities that are difficult to attain through conventional methods¹¹.

One of the most effective approaches to achieving reconfigurability in terahertz devices involves the regulation of their geometric structures¹², which in turn modifies the corresponding terahertz response. Due to their inherent tunability and reconfigurability, MEMS actuators have garnered significant attention in the realm of terahertz devices¹³. Typically, these actuators consist of patterns of metal or dielectric materials at sub-wavelength scales, with structural forms including cantilever beams, solid-supported beams, and separation ring resonators (SRRS), among others. These microstructures are engineered to execute microscale movements under specific driving modes, thereby altering the transmission state of the signal or inducing electromagnetic resonance at designated frequencies¹⁴. By adjusting the geometric parameters or structural configurations of MEMS actuators, the propagation characteristics of terahertz waves can be effectively modulated, encompassing critical parameters such as amplitude, phase, polarization, and frequency¹⁵. In comparison to traditional terahertz control technologies, MEMS provides a broader tuning range and enhanced wavefront control efficiency, which is essential for the advancement of high-performance terahertz systems¹⁶.

This paper reviews the driving mechanisms and the integration strategies of various MEMS actuators utilized in the terahertz domain. It analyzes the operational principles, structural forms, advantages, and limitations of different actuators. Furthermore, this paper will investigate the pivotal role of MEMS actuators in terahertz devices and systematically present their recent advancements across various terahertz application fields, including sensing, frequency and polarization conversion, beamforming, and on-chip logic operations. Through a comprehensive analysis of these topics, this review aims to provide valuable insights for the research, development, and application of MEMS drivers in terahertz devices, thereby facilitating the progress and practical implementation of terahertz technology.

Terahertz MEMS actuator

The practical implementation of terahertz technology has historically faced limitations due to device-related challenges, including low-loss integration and high-precision response. MEMS actuators have emerged as essential functional components, facilitating advancements in terahertz switches and tunable resonators.

Current research on terahertz MEMS actuators emphasizes innovative approaches to material and device structure development. At the material and processing levels, the collaborative design of substrate materials—such as silicon on insulator (SOI), quartz, and polydimethylsiloxane (PDMS)—alongside conductive and insulating layers like gold and silicon nitride, combined with advanced microfabrication techniques such as deep reactive ion etching (DRIE) and wafer-level packaging, has significantly enhanced device performance stability and integration. In terms of functional implementation, electrostatically driven switches have achieved notable advancements in low insertion loss and high isolation performance within the terahertz frequency range, facilitated by topological optimizations including cantilever beams and comb tooth structures. Additionally, tunable resonators utilizing thermal expansion effects, piezoelectric materials, or phase change materials offer continuously adjustable solutions for wideband frequency tuning and polarization control¹⁷. Furthermore, the incorporation of microfluidic channels and flexible printing processes has broadened the application scope of MEMS devices in dynamic sensing and multi-port control systems. Notwithstanding considerable advancements, terahertz MEMS actuators continue to encounter obstacles, including substantial dimensions and inadequate environmental stability.

The fundamental aspect of the terahertz MEMS actuator is its driving mechanism, which significantly influences the performance attributes and potential application contexts of the actuator¹⁸.

Electrostatic actuation, the most prevalent technology for current THz switches and tuners, operates on Coulombic attraction between electrodes and can be further categorized by its structural topology. The cantilever beam configuration is prized for its compactness and rapid response, achieving a driving voltage of 50 V and isolation greater than 30 dB¹⁹. The fixed-fixed beam structure offers higher rigidity, demonstrating an insertion loss as low as 1.2 dB at ~30 V²⁰. In contrast, the electrostatic comb-drive, particularly when optimized with multi-segment serpentine springs, enables a very low driving voltage of 6.8 V and an exceptionally fast response time of 2.28 µs across a DC-380 GHz bandwidth²¹. However, its relatively large physical footprint often poses integration challenges.

Thermal actuation primarily relies on two physical effects: thermomechanical deformation and phase transition. Thermal expansion actuation exploits the coefficient of thermal expansion mismatch in composite materials, enabling precise, continuous, and large-scale structural deformation via voltage-controlled heating. For instance, a V-shaped thermal actuator can continuously tune the resonant frequency from 1.374 to 1.574 THz²², albeit with a characteristically slow response time. Phase-change actuation utilizes drastic property changes accompanying the thermally induced insulator-to-metal transition in materials like VO₂. This approach can achieve a wider tuning range, as demonstrated by a stereoscopic metasurface supporting multi-frequency storage from 0.35 to 0.5 THz²³, but demands extremely stringent material fabrication processes.

Magnetic actuation provides a non-contact control paradigm by employing external magnetic fields to manipulate magnetic thin films or Ferromagnetic Shape Memory Alloys (FSMAs). For example, resonant frequency can be modulated by magnetically controlling the deflection of a magnetic-film-coated cantilever beam above a resonator²⁴. Utilizing the martensitic-austenitic phase transformation in Ni-Mn-Sn FSMA films under a magnetic field enables polarization conversion across a broad 1.04–1.96 THz range²⁵, benefiting from fast response and exceptional deformation recovery. Key challenges remain in controlling phase transition temperatures, fatigue life, and fabrication complexity.

Pneumatic actuation induces large, continuous, and bidirectional deformation of flexible membranes or microstructures by applying positive/negative pressure differentials via microchannels. This mechanism excels in dynamic reconfiguration, exemplified by a vertical displacement of 60 µm at the center of a spiral structure under a very low pressure of just ±10 Pa²⁶ enabling dynamic polarization switching in the THz band. Independent tuning of dual absorption peaks has also been achieved via negative pressure control of a PDMS membrane²⁷. The necessity for external pneumatic connections, however, increases system integration complexity.

Finally, Piezoelectric actuation exploits the inverse piezoelectric effect in materials like PZT-5H to achieve very fast response and exceptionally high control precision, albeit with inherently small displacement. A representative example is a piezoelectric micro-gripper integrated into a SRR, which produces a displacement of 0.3 µm under a high driving voltage of ±200 V to modulate electromagnetic coupling²⁸. This mechanism offers low power consumption but requires high drive voltages and faces challenges related to small stroke and intricate fabrication.

This section provides a comprehensive review of fundamental driving mechanisms, critical device design, system integration strategies and the micromachining processing pertinent to THz MEMS actuators. It aims to furnish theoretical insights and technical guidance for the advancement of next-generation high-performance THz devices.

Terahertz MEMS switch

In the terahertz frequency range, the quantum tunneling effect imposes significant limitations on high-frequency applications, resulting in substantial switching losses in tunnel semiconductors and insufficient isolation. Additionally, the preparation of graphene and liquid crystals exhibits inconsistent switching performance, which is susceptible to environmental influences. Conversely, mechanically switched RF MEMS switches demonstrate commendable RF performance within the terahertz band, effectively fulfilling the low-loss requirements of contemporary terahertz tunable communication systems.

Typically, terahertz RF MEMS switches are actuated electrostatically, with predominant configurations including cantilever beams, fixed support beams, and electrostatic comb drives. The cantilever beam design is simple and compact, making it suitable for small-scale integration and applications requiring fast response. In contrast, fixed support beams, which are anchored at both ends, exhibit high rigidity but are constrained by attractive forces. Electrostatic comb drives are noted for their low power consumption and high-frequency capabilities; however, they necessitate a larger size and high-voltage actuation. The selection of various actuator types requires a careful consideration of factors such as displacement, integration environment, power consumption, and process compatibility. Furthermore, performance enhancements can be achieved through structural optimization or the implementation of hybrid drive systems.

Current research on MEMS actuators focuses on enabling switching control in terahertz waveguides and addressing the inherent impedance mismatch and loss issues within single pole multi-throw(SPMT)switches. Due to THz wavelengths’ shortness, device dimensions and aspect ratios are minimized. Multi-segment meandering beams achieve lower drive voltage and faster response while maintaining low aspect ratio. Currently, electrostatically actuated comb-drive switches represent the highest performance level within the domain of THz MEMS switches. However, their comb-drive structure results in relatively large physical dimensions, posing challenges for integration. Furthermore, MEMS waveguide reconfigurable surface (MEMS-RS) is also a significant focus.

The integration of various MEMS actuator configurations into terahertz waveguides is crucial for achieving reduced insertion loss and improved isolation. Du et al.¹⁹ developed a MEMS switch featuring a double cantilever configuration and a T-shaped low-impedance transmission line circuit based on standard 0.13 μm Silicon-Germanium (SiGe) BiCMOS processes. The implementation of wafer-level packaging was accomplished through a tection-type anchoring design and packaging structure, which effectively mitigated the effects of residual stress and radiation loss. The switch’s beam thickness is 2 μm, enabling a low driving voltage of 50 V. Within the frequency range of 180–250 GHz, the device exhibits an insertion loss (IL) of 1.2–2.7 dB, a return loss (RL) exceeding 12 dB, and an isolation(ISO) greater than 30 dB. Utilizing the same fabrication process, Wipf et al.²⁹ constructed a single-pole single-throw (SPST) switch structure, which incorporates a capacitive switch comprised of low-impedance microstrip lines and movable metallic films. This switch effectively mitigates the impact of process variations, such as surface roughness and fluctuations in metal layer thickness, on the resonant frequency. With a driving voltage of 75 V applied to the high-voltage electrode, a capacitance ratio of 8.78 was achieved. This switch demonstrated a minimum IL of 0.44 dB, an optimal ISO of 24.67 dB, and a RL exceeding 9.6 dB within the frequency range of 220–325 GHz. Feng et al.³⁰ subsequently fabricated a contact MEMS switch characterized by ultra-low loss in the frequency range of 500–750 GHz. This switch features a cantilever beam with a height of 2.5 μm and a width of 7 μm. By progressively reducing the isolation capacitance at the tip of the cantilever and employing a 60 V electrostatic comb drive, the device achieves a balance between electric field strength and the risk of dielectric breakdown, thereby ensuring stable operation within the 500–750 GHz frequency band. The switch exhibits an IL of 0.7–2.7 dB and a RL greater than 12 dB in this frequency range, with an ISO exceeding 17 dB. Furthermore, they³¹. fabricated silicon-based and quartz-based terahertz MEMS switches, achieving performance metrics with IL below 3 dB and ISO greater than 12 dB at 750 GHz. Zhang et al.²⁰ designed a J-band coplanar waveguide (CPW) integrated fixed support beam switch, which features a comb-shaped grounding structure and a Si₃N₄ isolation layer. In the “UP” state, the switch applies ~30 V through electrostatic actuation to maintain the cantilever beam in direct contact with the signal line, resulting in an IL of only 1.2 dB at 220 GHz and less than 4 dB across the 220–270 GHz frequency range. In the “DOWN” state, the capacitor contact formed via the Si₃N₄ layer shorts the signal to ground through a large capacitor. Within the frequency range of 220–320 GHz, the switch achieves an ISO greater than 16 dB, thereby ensuring a favorable balance between low loss and high isolation in both operational states. Then, they³² proposed an arc-shaped beam waveguide switch, the WR-2.8 waveguide package was utilized, and a 1500 Å Si₃N₄ dielectric isolation layer was deposited via Plasma-Enhanced Chemical Vapor Deposition (PECVD). The parasitic losses associated with the roughness of the sacrificial layer were mitigated through the chemical mechanical polishing (CMP) process. The switch operates with a driving voltage of 30 V and achieves the IR of less than 2 dB, the RL greater than 12 dB, and the ISO exceeding 16 dB within the frequency range of 220–280 GHz (Fig. 1a, b). To facilitate the implementation of multi-port terahertz MEMS switches, they also investigated strategies to address the mismatch and transmission losses inherent in high-frequency single-pole multi-throw MEMS switches. A specialized double-contact hybrid solid-supported beam switch³³ was designed, which is a compact symmetrical single-pole four-throw (SP4T) switch, exhibiting a maximum IR of 1.8 dB and a RL at the input port exceeding 15 dB. The entire switch is intended to be fabricated on a 50 μm fused silica substrate utilizing surface microfabrication techniques.

Fig. 1

THz MEMS Switch. a Arc-shaped beam waveguide switch b Insertion loss/isolation and return loss of arc-shaped beam switch in DOWN/UP state³². c The 3D schematic of the multi-throw RF-MEMS switch²¹. d Schematic block diagram of the crossover switch, illustrating the thru (green) and isolated (red) ports’ signal path in the crossover and straight states. e Perspective view of the crossover switch implementation, including the hybrid couplers and SPST switches connected to four E -plane transitions to standard WR-3.4 waveguide³⁸. f SEM images of a finished encapsulated RF-MEMS switch⁴³. Reprinted with permission from refs. ^21,32,38,43

Due to the relatively short wavelength of terahertz electromagnetic waves, the dimensions of switching devices and the aspect ratio of the beam are typically minimized. To effectively lower the driving voltage and reduce the response time while maintaining a low aspect ratio, a multi-segment tortuous beam configuration has been implemented in the design of the switch. Yu et al.²¹ introduced a low-voltage-driven, wideband multi-throw switch design, resulting in the fabrication of a three-pole three-throw (3P3T) six-port MEMS switch structure. The design incorporates an electrode layout with progressively narrowing tip widths to diminish coupling capacitance, while a multi-stage folding cantilever design further reduces the required driving voltage. This switch demonstrates an IL of less than 0.66 dB across the DC-380 GHz frequency range, an ISO level exceeding 32 dB, and a RL of less than −15 dB. The switch operates at a driving voltage of 6.8 V and exhibits an exceptionally rapid response time of 2.28 µs (Fig. 1c).

In a separate study, Li et al.³⁴ developed a low-voltage terahertz MEMS switch featuring a double-ended fixed serpentine spring beam. Utilizing a quartz substrate, this switch design decreases spring stiffness through the incorporation of serpentine bending springs and achieves a reduction in coupling capacitance to one-third of the theoretical maximum by employing four metal contact points. This switch achieves an IR of less than 1.3 dB within the terahertz frequency range of 300–350 GHz, with an ISO level greater than 15.5 dB, a driving voltage of 24 V, and a maximum von Mises stress of 89 MPa. Additionally, they¹⁵ fabricated an eight-contact, four-metal beam double-ended fixed serpentine contact MEMS switch structure. This topology, featuring double-ended eight-contact points, mitigates the risk of single-point failure due to residual release during the manufacturing process. The IL for this switch is recorded at less than 0.83 dB within the 100–200 GHz range, with a measured ISO level exceeding 19.88 dB and a driving voltage of 23.17 V. The maximum von Mises stress observed at the junction between the outer spring and the plate is 62.8 MPa, which remains below the yield strength of the gold material (120 MPa), thereby ensuring elastic recovery. The RL is greater than 19.90 dB in the open state and less than 0.58 dB in the off state.

The electrostatic comb-tooth switch currently represents the highest performance among terahertz MEMS switches. Nonetheless, its comb-tooth configuration results in a relatively large physical size, which poses challenges for integration. Rahiminejad et al.³⁵ developed a contact single-pole double-throw (SPDT) waveguide switch that features a U-shaped waveguide and a rotating metal disk. This design incorporates an electromagnetic bandgap (EBG) metal column array along with a waveguide isolation structure. By integrating a four-arm cross-shaped flexible suspension, which has a thickness of 50 μm and a spring constant of 60 N/m, with three sets of double-sided comb tooth actuators, the switch achieves a rotational drive of ±4.5° at a voltage of 70 V. The performance metrics of this switch include an IL of less than 2.5 dB, an ISO greater than 30 dB, and a RL exceeding 20 dB within the frequency range of 500–750 GHz. Notably, the switch exhibits no mechanical damage after undergoing 430,000 switching cycle tests.

Utilizing the double-mask SOI MEMS microfabrication technique, Oberhammer et al.³⁶ introduced the MEMS-RS technology and successfully developed a SPST waveguide switch. This switch features a switchable metallic surface structure that comprises a vertical array supported by a horizontal bar. The MEMS actuator, driven by a voltage of 40 V and employing a comb-tooth mechanism, facilitates the dynamic transition between short-circuit and non-short-circuit states within the TE10 waveguide mode. The design integrates a standard WR-1.5 waveguide with a gold film surface, resulting in a compact chip size of 3.07 mm × 10.58 mm. High-precision assembly with the waveguide flange is achieved through an omega-shaped alignment structure. The switch demonstrates an IL of less than 3 dB, ISO exceeding 15 dB in the blocked state, and a RL greater than 9.6 dB across the frequency range of 500–750 GHz. They³⁷ investigated the loss characteristics of the MEMS-RS switches, revealing that the MEMS reconfigurable surface contributes only 0.5–1 dB to the insertion loss, with the majority of losses attributed to the inadequate thickness of the waveguide side wall metal and surface roughness. In reliability testing, the comb drive actuator, operating at a voltage of 28 V, successfully completed 100 million cycles in an uncontrolled laboratory environment while maintaining stable performance, with no failures recorded during a continuous 10-day closed test. This design represents the first MEMS waveguide switch capable of operating in frequency bands exceeding 750 GHz, thereby advancing integrated applications in areas such as terahertz communication and spectrum analysis.

Additionally, the researchers³⁸ fabricated a four-port MEMS waveguide cross-switch operating in the 220–260 GHz range, employing collaborative control technology that integrates hybrid couplers and dual SPST switches. This configuration includes two multi-step hybrid couplers and four sets of capacitor-inductor composite MEMS-RS blocking units. Signal path switching within the waveguide TE₁₀ mode is achieved through a 40V-driven double comb-tooth actuator. The design incorporates the WR-3.4 waveguide standard interface, which is vertically assembled with a four-layer SOI chip. To ensure assembly accuracy within ±5 μm, twelve groups of bent waveguides and an omega-shaped flange alignment structure are integrated. The switch exhibits an IL ranging from 0.9 to 1.4 dB, ISO greater than 29.3 dB, and RL exceeding 14 dB in the cross state within the 220–260 GHz frequency band. In the through state, it achieves an IL of 0.8–1.3 dB, ISO greater than 29 dB, and RL exceeding 13.6 dB. Furthermore, the ISO between cross-ports is greater than 13.7 dB, while the ISO between straight-through ports exceeds 34 dB (Fig. 1d, e).

Based on a similar process, Theodore Reck et al.³⁹ developed a SPST waveguide reflection switch, which features a reconfigurable waveguide module comprising a 20 μm-wide movable metallic separator and a stepped impedance filter. The control of electromagnetic wave transmission within the frequency range of 500–750 GHz is facilitated by a double-folded comb-tooth drive actuator, which operates at a driving voltage of 60 V. This switch demonstrates an IL of ≤3 dB, an ISO level exceeding 20 dB, and a RL greater than 20 dB within the specified frequency range. Notably, it exhibits resilience against operational failure due to partition adhesion after enduring 5.07 million cycles, thereby outperforming conventional metal contact switches. Additionally, they⁴⁰ fabricated a silicon-based SPDT waveguide switch. In this design, the silicon-based actuator is entirely retracted from the waveguide path in the open state via an electrostatic comb driving mechanism, enabling the switching of terahertz wave paths. The switch achieves an ISO level greater than 23 dB, an IL of ≤1 dB, and a RL exceeding 20 dB within the 420–500 GHz frequency range. This device employs a Y-shaped waveguide branch structure and operates through the coordinated functionality of two SPST switches. The design incorporates a comb drive structure featuring pre-bent beams and trapezoidal teeth, which not only ensures stability for a significant displacement of 150 µm but also enhances high-frequency performance. Furthermore, a high yield rate exceeding 85% is attained through the DRIE process. In a related study, Armin Karimi et al.⁴¹ constructed MEMS-based SPST waveguide switches to facilitate path switching for sub-millimeter waves in the 220–290 GHz range by manipulating the relative displacement of two surface sets. In the ON state, these surfaces maintain a 20 µm gap, resulting in an IL of 0.7–1.2 dB and a RL greater than 17 dB. Conversely, in the OFF state, the surfaces are positioned to achieve a contact gap of less than 300 nm, yielding a high ISO level of 28.5–32.5 dB. The switch is designed with an axially aligned WR-3.4 standard waveguide port, with overall dimensions measuring only 3 mm × 3.5 mm × 1.2 mm.

With the exception of electrostatic drive mechanisms, research on MEMS switches utilizing alternative actuation methods is relatively limited, primarily due to the intricate nature of their manufacturing processes. The majority of current studies employ the strategy of incorporating pre-fabricated driver modules to facilitate the actuation function. Nevertheless, the integration of these supplementary driver modules often results in a considerable enlargement of the device’s overall dimensions. Berkel et al.⁴² successfully developed a high-isolation piezoelectric motor-driven SPDT waveguide switch that operates within the frequency range of 250–310 GHz. This switch utilizes a piezoelectric motor to manipulate a metal U-shaped waveguide arm, thereby directing the signal path between two output waveguide ports. The EBG structure effectively mitigates the leakage of the parallel plate mode, resulting in an IL of less than 0.6 dB, a RL exceeding −20 dB, and an ISO level below −75 dB within the specified frequency range (250–310 GHz).

The implementation of high-quality packaging technology for terahertz MEMS switches is crucial. Packaging that minimizes RF interference can significantly enhance the practicality of these switches and broaden their potential applications. Goritz et al.⁴³ developed a capacitive shunt switch utilizing direct etching stop (VES) and metal substrate transverse etching stop (mLES) techniques, in conjunction with SiO₂ wet etching (Fig. 1g). This switch demonstrates an IR of less than 2 dB, an ISO greater than 20 dB, and a RL exceeding 12 dB within the frequency band of 110–170 GHz. An 8-inch wafer-level package was achieved through the use of a 3 μm thick Ti/TiN/AlCu/Ti/TiN stacked metal grid. A reliable and cost-effective MEMS switch packaging process has been proposed, and upon completion of the packaging, the switch’s functionalities are fully operational. Furthermore, the consistency pass rate of the S parameters across the frequency range of 110 to 170 GHz exceeds 75%.

Terahertz MEMS tunable resonator

MEMS tunable resonators have garnered significant attention⁴⁴ from both academic and industrial sectors due to their compatibility with existing electronic systems, rapid response times, high control precision, and elevated quality factors. These characteristics enable them to exhibit exceptional sensitivity to minute variations in resonant frequency, even at ambient temperatures⁴⁵. The experimental development of terahertz MEMS resonators is poised to be a critical area of research, particularly in relation to 6 G communication and integrated sensing applications⁴⁶. Terahertz MEMS resonators are frequently utilized in conjunction with metamaterials, which serve as their unit cells. Metamaterials possess unique properties that are not found in natural materials⁴⁷. By modifying the structural design of periodic subwavelength units, it is possible to achieve controlled anisotropy, thereby influencing the frequency response of the medium as well as the polarization, amplitude, and phase of incident electromagnetic waves.

This section reviews recent research progress in terahertz MEMS tunable resonators, focusing specifically on their primary driving mechanisms. Electrostatic drive employs electrostatic force to control the position of microstructures. Thermal drive encompasses two distinct approaches: thermal expansion drive and phase change drive. Thermal expansion drive leverages the initial out-of-plane bending induced by differences in the thermal expansion coefficients of composite beam materials. Precise, continuous, and large-scale structural deformation is achieved through voltage-regulated thermal expansion, offering stable and adaptable tuning capabilities, albeit with a relatively longer response time. Phase change drive, conversely, achieves tuning by exploiting the significant alterations in material physical properties that occur during thermally induced phase transformations. While this method can attain a wider tuning range, it imposes stringent demands on material preparation processes. Magnetic drive facilitates frequency modulation via external magnetic fields. Notably, FSMAs present an innovative solution for tunable metamaterials, distinguished by their exceptional deformation recovery ability, non-contact control characteristics, and rapid response. Additionally, the dynamic regulation of terahertz waves by modulating the microstructure of magnetic fluids using magnetic fields represents a significant application direction for functional materials. Other notable driving mechanisms include pneumatic drive and piezoelectric drive.

SRRs represent the predominant fundamental architecture utilized in electrostatically driven terahertz MEMS tunable resonators. Han et al.⁴⁸ developed an SRR configuration that incorporates tunable capacitors on a low-loss quartz substrate. This design features a metal cantilever beam, measuring 8 μm in width, suspended with a disc positioned at the center of a 100 μm × 100 μm square SRR unit. The variation in capacitance between the cantilever and the underlying electrode is modulated by an electrostatic drive of ±33.5 V, facilitating dynamic adjustments to the LC resonant frequency of the SRR. This innovation results in a high-performance switchable filter and modulator operating within the terahertz frequency range, allowing for a resonant frequency shift from 695 GHz to 480 GHz, and achieving a notable transmittance switching effect of 16.5 dB at 480 GHz. The advantageous low-loss properties of the quartz substrate ensure that the device retains high transmittance even outside the resonant frequency range (Fig. 2a, b).

Fig. 2

THz MEMS tunable resonator. a A close-up view of the unit cell in the SEM image of MEMS-SRR. b SEM image of MEMS-SRR array⁴⁸. c The cross-sectional view of ~120 nm vanadium dioxide (VO₂)on a 500 nm SiO₂/ sapphire substrate and the resistance-temperature variation of the VO₂ film. The dotted line indicates the interface between VO₂ and SiO₂. d Optical microscope images of a helical cantilever beam bending upward at 30 °C and a helical cantilever beam being completely flat at 90 °C⁵⁴. e Schematic diagram of pneumatically driven metamaterials⁵⁶. f Schematic diagram and working principle of the adjustable terahertz metamaterial absorber with built-in pneumatic drive mechanism⁵⁷. Reprinted with permission from refs. ^48,54,56,57

In a separate study, Toshiyoshi et al.⁴⁹ designed a capacitor-embedded SRR, interconnected by two metal brackets, each 8 μm wide. This configuration not only supplies the necessary driving voltage as an electrical connection but also preserves the electromagnetic coupling of terahertz waves. Consequently, this design allows the SRR to function at frequencies of 284 GHz in the OFF state and 266 GHz in the ON state. Additionally, Pitchappa et al.⁵⁰ introduced an electrostatically switchable MEMS terahertz resonator characterized by polarization-insensitive properties. This design features an octagonal ring structure with microcantilever beams positioned at each corner, which provides the requisite rotational symmetry and ensures consistent response characteristics to terahertz waves across varying polarizations. Experimental findings indicate that the modulation range of this device in both inductor-capacitor and dipole resonant modes reaches 0.16 THz and 0.37 THz, respectively, with a maximum transmittance variation of 70%.

Composite beams constructed from various materials exhibit discrepancies in their thermal expansion coefficients, resulting in an initial stress mismatch within the beam. This initial condition causes the beam to bend outward from its plane, which can subsequently be altered through electrostatic actuation, facilitating the tuning of the resonator. Ma et al.⁵¹ introduced a double-layer cantilever beam resonator comprising a 20 nm thick Al₂O₃ film and a 500 nm thick Al film. By applying a direct current (DC) bias voltage, the cantilever beam transitions from a warped configuration to a substrate state, enabling multi-band resonant frequency operation within the terahertz spectrum. Feng et al.⁵² developed adjustable helical metamaterials that modify the spacing between the inner and outer helical rings via electrostatic actuation. Utilizing the Hauf-Grigull gas refractive index-pressure relationship, they achieved a sensitivity of 0.34 THz/RIU for the detection of environmental explosives.

Electrothermal actuation facilitates accurate, continuous, and extensive structural deformation via voltage-regulated thermal expansion, thereby offering a stable and adaptable tuning mechanism for MEMS actuators. While it is compatible with micromachining techniques, it exhibits a moderate response time. Li et al.²² proposed a broad and continuously tunable V-shaped terahertz metamaterial structure, which employs a thermal driver to adjust the resonant frequency of a double-cut split ring resonator, designed and simulated using the MetalMUMPs process. The V-shaped thermal driver allows for continuous tuning of the resonant frequency across a wide range from 1.374 to 1.574 THz, with the transmission curve exhibiting a pronounced sharp decline at each resonant frequency, indicative of robust resonant performance. Xu et al.⁵³ introduced an electrothermal actuator (ETA) platform alongside cross-shaped metamaterials (CSM) to design metamaterials with optical logic functionalities. The reconfigurability and stretchability of CSM are achieved by applying varying DC bias voltages to the ETA.

The tunable resonators can be accomplished by leveraging the phase transformation of the material during heating, which induces significant alterations in the material’s physical properties. This approach can yield a relatively extensive tuning range, albeit with stringent requirements for the material fabrication process. The phase-change material VO₂⁵⁴ is utilized in terahertz metamaterials, which feature a three-layer cantilever design (Au-Cr-VO₂). Upon undergoing a phase change, VO₂ experiences volume shrinkage that alters the internal stress and modifies the curvature of the cantilever. (Fig. 2c, d) The VO₂ stereoscopic metasurface developed by the Prakash²³ achieves three-dimensional to two-dimensional structural reconstruction through stress mismatch, supporting multi-frequency storage in the range of 0.35–0.5 THz.

Magnetically-driven MEMS actuators can utilize a cantilever beam structure coated with magnetic thin films, allowing for the modulation of resonant frequency through an external magnetic field. Burak Ozbey et al.²⁴ successfully fabricated a series of cantilever beam structures integrated with magnetic films, positioning them above an electrically small resonant ring (eSRR). The degree of bending in these cantilever beams was manipulated via an external magnetic field to adjust the parallel plate capacitance.

The expansion of applications for magnetic drive functional materials represents a significant trend in the development of MEMS actuators. FSMA offer an innovative approach for tunable metamaterials, owing to their remarkable deformation recovery, non-contact control capabilities, and rapid response times. Wang et al.²⁵ developed tunable terahertz polarization conversion metamaterials driven by magnetic fields, utilizing Ni-Mn-Sn FSMA films. This design facilitates any polarization transformation across a broad frequency range from 1.04 to 1.96 THz, and enables the conversion of linearly polarized light to circularly polarized light within the frequency ranges of 0.87–0.92 THz and 2.07–2.12 THz. The Ni-Mn-Sn alloy undergoes a martensitic-austenitic transformation in response to a magnetic field, resulting in significant alterations in shape and electromagnetic properties. Nonetheless, the implementation of shape memory alloy-based magnetic drive structures presents challenges, including the control of phase transformation temperatures, fatigue life, and the complexity of preparation processes. Future research endeavors may concentrate on the development of novel shape memory alloy materials, the optimization of phase transformation characteristics, and the exploration of broader application contexts.

Pneumatically driven MEMS actuators facilitate non-contact and continuously adjustable dynamic responses through pneumatic control, achieved by creating a pressure differential across a microstructure that operates without a substrate⁵⁵. The implementation of pneumatic actuation has been shown to enable significant, continuous, and bidirectional deformations of the microstructure. Su et al.²⁷ introduced a concentric metallic structure comprising an inner layer of ELC inductors and an outer layer of SRR rings. This configuration, in conjunction with an elastic PDMS suspension film and a metallic ground plane, establishes a compressible air cavity between the layers. By applying negative pressure, the PDMS membrane is induced to deform downward, concurrently adjusting the gap of the metallic structure and the thickness of the air cavity, thereby allowing for independent tuning of dual absorption peaks at 1.24 THz and 1.49 THz. The absorption rate of the lower frequency peak increased from 9.1% to 97.5%, corresponding to a blue shift of 116 GHz, while the higher frequency peak decreased from 96.8% to 12.5%, with a blue shift of 100 GHz.

Kan et al.²⁶ proposed a vertically deformable chiral metamaterial, characterized by a MEMS actuator structure based on a planar Archimedes helix. The vertical deformation of the helix is regulated by pneumatic pressure, enabling the formation of three-dimensional helical shapes with left-handed (LH) and right-handed (RH) configurations. The pneumatic actuation allows for a central actuation distance of 60 μm, facilitating greater deformation compared to electrostatic actuation. This device utilizes SOI wafers as the substrate, with a planar Archimedes helical structure fabricated through photolithography, RIE, and backside wet etching, featuring a line width of 6 μm. A 45 nm gold layer is deposited on the helical surface to enhance the terahertz response. The design incorporates an upper and lower split chamber structure, with each chamber connected to a nitrogen (N₂) pressure source via independent gas lines. The deformation of the screw is driven by modulating the pressure differential between the chambers. A positive pressure of +10 Pa is applied to the upper chamber while maintaining normal pressure in the lower chamber, resulting in an upward displacement of the screw’s center to form a left-handed screw. Conversely, applying a negative pressure of −10 Pa to the lower chamber pulls the center of the spiral downward, creating a right-handed spiral. At a pressure of ±10 Pa, the vertical displacement of the spiral center reaches 60 μm, demonstrating a linear relationship with the applied pressure. This innovative design represents the first polarization modulator exhibiting both strong optical activity and chiral dynamic switching within the terahertz frequency range, making it suitable for applications such as real-time imaging and molecular circular dichroism spectroscopy, thereby addressing the existing gap in practical dynamic polarization devices in this domain.

Actuators characterized by a singular structural design frequently encounter limitations in terms of tuning range and functional diversity. In order to maximize the benefits of MEMS actuators in the terahertz frequency range and to broaden their applicability, the integration of microchannel and microfluidic technologies within MEMS microsystems has emerged as a significant area of research.

Microchannels fabricated from flexible materials such as PDMS and SU-8 are increasingly being utilized as airflow channels in pneumatic MEMS actuators, marking a notable trend in research. Zou et al.⁵⁶ demonstrated the ability to switch between three polarization states at frequencies ranging from 1.3 to 1.5 THz by dynamically altering the geometric configuration of a metallic helical structure via air pressure modulation. This device comprises an elastic PDMS film, a symmetrically positioned metal helical structure on the upper side, and a sealed cavity containing microchannels on the lower side. By applying positive and negative air pressure through these microchannels, the deformation of the PDMS film is induced, allowing the planar helical structure to be dynamically transformed into either a right-rotating or left-rotating three-dimensional helical configuration. Notably, in the absence of pressure, the planar helix functions as a quarter-wave plate, facilitating the conversion between linear and circular polarization. When subjected to a pressure of +100 kPa, a right-rotating helix is formed, resulting in a reversal of the circular polarization rotation direction, while a pressure of -50 kPa yields a left-rotating helix that maintains the original rotation direction (Fig. 2e).

Chuhuan et al.⁵⁷ developed a MEMS resonator that integrates a gas cavity, a microchannel, and a 4 μm thick elastic PDMS film. This device utilizes positive and negative air pressure applied through an external microchannel to induce upward bulging or downward sinking of the PDMS film, thereby altering the gap width of the top double-slit SRR. This mechanism enables bidirectional tuning of the absorption peak within the frequency range of 4.58–5.02 THz (Fig. 2f). Furthermore, Wen et al.⁵⁸ designed tunable metamaterials utilizing electrostatic comb-shaped actuators, employing a double concentric semi-circular ring resonator structure. By varying the gap between 2–20 μm and the longitudinal displacement from 0 to 36 μm, a dual-band tuning range of 82 GHz in both transverse electric (TE) and transverse magnetic (TM) modes was achieved. Following the integration of PDMS microchannel packaging, a linear refractive index sensitivity of 0.186 THz/RIU was attained, with a maximum quality factor (Q) reaching 248.

The advancement of Through-Silicon Via (TSV) technology presents a novel approach for the creation of microchannels. In this configuration⁵⁹, a gold SRR is positioned on a silicon substrate featuring a silicon through hole. The airflow through the TSV generates pressure, resulting in the upward bending and deformation of the SRR’s cantilever. The bending angle of the cantilever ranges from 10° to 45°. In TM mode, the resonant frequency corresponding to the bending angle varies from 0.721 to 0.796 THz, with a tuning range of 0.075 THz and a linearity of 0.997. In TE mode, the resonant frequency associated with the bending angle spans from 0.805 to 0.945 THz, exhibiting a tuning range of 0.140 THz and a linearity of 0.983. Mechanical model analysis indicates that a micronewton-level pressure of 3.1 to 4.4 pN can induce a significant frequency shift, with the corresponding flow velocity sensitivity reaching up to 24.739 THz/(m/s²). This design represents a novel integration of MEMS mechanical control with metamaterial dual-mode resonance, marking the first instance of such a combination.

MEMS actuators are capable of facilitating dynamic control within microfluidic systems. In a notable study, Meng et al.⁶⁰ developed an embedded terahertz microfluidic chip utilizing cycloolefin copolymer (COC) and constructed a “sandwich” type microfluidic channel structure through laser engraving and micro-milling techniques. This chip exhibits a transmittance exceeding 90% within the 0.1–0.6 THz frequency range. It incorporates a valveless micro-pump powered by a 25 Hz vibration motor, enabling precise fluid control at a rate of 2.2 μL/min. The device supports automatic injection and dynamic detection of ammonium salt solutions, facilitating real-time monitoring of ethanol concentrations ranging from 0-20%.

The utilization of piezoelectric materials presents a novel continuous tuning mechanism. The limitation of this design is that it is capable of producing only relatively minor mechanical displacements when subjected to high voltage. Lalas et al.²⁸ employed a piezoelectric micro-gripper as the fundamental component of a SRR. The micro-gripper’s two arms produce displacement in response to applied voltage, thereby altering the resonator’s gap size. The architecture of this MEMS piezoelectric micro-gripper consists of a gold conductive layer, a PZT-5H piezoelectric material serving as the driving layer, and a Cr/Pt adhesion layer. The gap is measured at 0.6 μm, with a unit period of 18 μm. Upon the application of a positive voltage, the PZT layer expands in the direction of polarization, resulting in the outward bending of the cantilever beam and an increase in the SRR gap. Conversely, when a negative voltage is applied, the PZT layer contracts, causing the cantilever beam to bend inward and decrease the gap. When subjected to a voltage of ±200 V, the displacement of the micro-gripper’s two arms reaches 0.3 μm. The electromagnetic coupling strength of the SRR is modulated by varying the micro-gripper’s gap, thereby dynamically adjusting the LC parameters associated with electromagnetic resonance. Notably, the power consumption of piezoelectric micro-grippers is relatively low, as they do not require current for biasing purposes.

Micromachining for terahertz MEMS actuators

The development of terahertz switches and tunable resonators with high robustness necessitates stringent criteria for microfabrication techniques. Recent advancements in technologies, including DRIE and multi-layer chip bonding, have significantly facilitated the fabrication of intricate three-dimensional microstructures. These technology has been validated as effective in the development of various high-performance terahertz devices⁶¹, thereby establishing a technical basis for the integration of terahertz MEMS actuators⁶².

The fabrication of terahertz MEMS actuators involves a sequential process that includes substrate preparation, deposition of metal layers, photolithographic patterning, deposition and etching of sacrificial layers, and the subsequent release of these sacrificial layers. In terms of substrate selection for terahertz MEMS actuators, silicon substrates are particularly advantageous due to their high resistivity, which results in a consistent transmittance across the terahertz frequency spectrum, thereby facilitating a broad terahertz response⁶³. The SOI process has been applied in various domains. With advancements in heterogeneous integration technology, the SiGe BiCMOS process¹⁹ has been incorporated into the fabrication of terahertz actuators. Quartz glass is widely acknowledged as the most appropriate substrate material for terahertz radio frequency (RF) devices, owing to its loss tangent of less than one-thousandth within the terahertz frequency range. Certain MEMS necessitate the use of flexible substrates, prompting the consideration of PDMS as a potential alternative substrate. PDMS²⁷ can be processed into deformable films via casting techniques, and cavities can be etched into the PDMS substrate to create aerodynamic structures.

Gold (Au) is the material of choice for terahertz MEMS actuators, attributed to its superior electrical conductivity, resistance to corrosion, and favorable mechanical properties, including toughness and ductility. Prior to the metal sputtering process, an adhesive layer is typically composed of chromium (Cr) or titanium tungsten (TiW). Additionally, the insulating layer applied to the substrate and electrode is frequently comprised of thermolyzed silica³¹ or silicon nitride(Si₃N₄)²⁰. In their research, Han et al.⁶⁴ implemented a three-step lithography technique to fabricate electrostatic MEMS. To streamline the fabrication process, they eliminated the Cr layer that typically exists between the Au and silicon dioxide (SiO₂) layers in the design, opting instead for a 10 nm thick Cr layer solely as an adhesion layer during the actual device manufacturing. This modification not only simplifies the manufacturing process but also preserves satisfactory electrical performance. Furthermore, in pursuit of cost-effective alternatives, aluminum(Al) was employed for the integration of microfluidic channels⁶⁵, while functional materials such as lead zirconate titanate (PZT-5H) were deposited for use as micro-grippers²⁸. Regarding the beam materials for terahertz MEMS actuators, the study considered not only metals like Au and Al but also composite beams that incorporate both metals and oxides. Tao et al.⁶⁶ exploited the disparity in thermal expansion coefficients between silicon nitride and gold, applying thermal variations to induce bending in the cantilever beam. Additionally, Xu et al.⁶⁷ developed a wound resonant electrothermal actuator utilizing photoresist (PR), Au, and Si₃N₄.

PR, SiO₂, and Al have been utilized as sacrificial layer materials in various manufacturing processes. Several release techniques, including reactive ion etching (RIE)³¹, potassium hydroxide (KOH)⁶⁶ wet etching, and isotropic dry etching⁶⁸, have been employed to facilitate this process. To enhance the quality of the release, numerous innovative strategies have been integrated into the manufacturing workflow. For instance, to mitigate the drilling and erosion of the device’s sidewalls during the hydrofluoric acid (HF) release process, Zhao et al.⁶⁹ implemented a silicon protective layer on the device surface through low-pressure chemical vapor deposition, effectively addressing the detrimental impact of drilling and erosion on the device’s RF performance. Similarly, Feng et al.³¹ utilized an aluminum sacrificial layer to minimize the thermal deformation of Au and applied a chromium barrier layer over the Au surface to prevent adverse chemical interactions between the aluminum sacrificial layer and gold. Goritz et al.⁴³ deposited 100 nm of silicon-rich nitride (SiR_N) onto the electrodes to achieve vertical release termination and maintain the structural integrity of the electrodes. They also employed 1.5 μm pore metal meshes and low-temperature, high deposition rate (HDR) SiO2 deposition techniques for the packaging of MEMS devices, thereby preventing contamination from particles and moisture.

To improve the yield and reliability of terahertz MEMS actuators, CMP³² can be utilized to enhance the flatness of the beams, with perforations introduced into the beams to facilitate the release of the sacrificial layer and alleviate residual stress. In light of the significant time investment associated with the photolithography process, flexible inks have been adopted as a substitute for traditional photolithography steps, enabling the development of large-area flexible MEMS beam arrays and promoting cost-effective mass production. Toshiyoshi et al.⁴⁹ investigated the roll-to-roll printing technique for the fabrication of large-area flexible SRR arrays. In this approach, the photolithography step is supplanted by a lifting process utilizing flexible printing ink as a mold, with film deposition achieved through engraving printing. This methodology presents a viable pathway for the large-scale and economical production of terahertz MEMS devices.

Applications of terahertz MEMS actuators

The functional realization and performance breakthroughs of terahertz MEMS actuators are largely attributable to their deep integration with various cutting-edge technologies. These integration strategies transcend simple component assembly, embodying a paradigm shift in design from static configurations to dynamic reconfigurability. The evolution of their application capabilities follows a compelling logical progression: it commences with highly sensitive sensing of environmental parameters, which forms the foundational transduction mechanism. Building upon this, active control enables dynamic tuning of frequency and polarization. Extending these tuning principles into the spatial domain then gives rise to advanced beamforming technologies. Ultimately, the convergence of these dynamic control capabilities with digital logic processing opens new avenues for on-chip optical logic operations and secure communications. The following sections will elaborate on these application domains, highlighting representative demonstrations and underlying mechanisms.

Specifically, the core integration pathways can be categorized into three types. First, MEMS-Metasurface Synergistic Integration utilizes the precise displacement control of MEMS as a “tuning knob” to dynamically program electromagnetic wavefronts by either directly reconfiguring the geometric layout of metasurface unit cells or modulating the subwavelength air gap between them and the substrate. This mechanism circumvents the inherent limitations of relying solely on material properties, providing the physical basis for large-angle beam steering and multifunctional logical operations^70,71. Second, MEMS-Functional Material Fusion Integration incorporates functional materials responsive to external fields—such as phase-change materials like VO₂, graphene, or heterogenous bimaterial beams—as the core actuating or sensing elements within the MEMS structure. This approach leverages intrinsic physical effects like material phase transition, differences in thermal expansion coefficients, or electro-mechanical deformation to efficiently convert environmental stimuli into mechanical deformation, thereby endowing the devices with novel capabilities beyond conventional MEMS, such as non-volatile memory, high-sensitivity sensing, or low-power actuation^54,72. Third, MEMS-Transline Integration embeds MEMS actuators as core control units directly inside terahertz transmission lines or radiating structures. This enables system-level dynamic management of signal phase and beam direction by in situ manipulation of the propagation constant or current distribution. Such deep integration is pivotal for constructing low-loss, reconfigurable terahertz phased array front-ends, addressing the insertion loss and power consumption bottlenecks faced by traditional semiconductor technologies at terahertz frequencies^73,74. In summary, these integration strategies collectively propel the evolution of terahertz MEMS from simple actuating components towards intelligent micro-systems capable of perception, computation, and communication. (Fig. 3).

Fig. 3

The integration strategy of THz MEMS actuators with meta-device, functional materials and transline

MEMS actuators serve as essential components in terahertz communication and sensing systems, including switches and tunable resonators. They have demonstrated substantial advancements across various domains, such as sensing, frequency and polarization conversion, beamforming, and logical operations. The integration of metamaterials with MEMS resonators facilitates a departure from the constraints of static devices, enabling multi-band modulation and dynamic beam deflection through mechanical displacement. Notably, terahertz phase shifters that utilize MEMS actuators can achieve extensive phase shifts by dynamically modifying the waveguide structure or dielectric, while simultaneously minimizing insertion loss. This capability is vital for the beamforming of terahertz phased array antennas. Furthermore, MEMS-driven reconfigurable metasurfaces augment the functionality of terahertz systems, enabling logic gate operations such as XOR and NAND, thereby establishing a foundation for advanced on-chip information processing. In the realm of sensing applications, MEMS detectors that leverage thermal expansion effects and electromagnetic coupling mechanisms exhibit temperature sensitivity at the sub-millikelvin level and possess real-time imaging capabilities.

Nonetheless, the advancement of terahertz MEMS devices encounters several challenges, including the limitations associated with complex beamforming capabilities and the necessity for enhanced system integration. This section provides a comprehensive review of the significant technological advancements and typical applications of MEMS actuators within the terahertz domain, with an emphasis on analyzing innovative designs in tuning mechanisms. Additionally, it investigates the potential of heterogeneous material integration technologies, with the objective of offering theoretical support and technical references for the practical implementation of terahertz communication and intelligent perception systems.

Terahertz actuators in sensing applications

The efficacy of MEMS actuators in THz sensing fundamentally relies on their capacity to transduce changes in environmental factors into measurable variations in physical or electrical properties. This transduction mechanism enables the detection of diverse terahertz radiation forms and the conversion of environmental parameter shifts into quantifiable electrical signals⁷⁵. Essentially, MEMS actuators function as dedicated transducers in such applications, facilitating the conversion of information from the environmental domain to the electrical domain. This cross-domain signal transduction constitutes the core functionality of sensors and actuators. MEMS devices optimized for efficiently converting environmental stimuli into mechanical responses play a particularly crucial role in THz sensing. Consequently, within this context, the distinction between the “actuation” and “sensing” functionalities often becomes blurred. Serving as the core enabling component for THz sensors, MEMS actuators provide the foundation for their practical implementation.

Terahertz waves exhibit sensitivity to the molecular vibrations and rotational energy states of various substances, which confers distinct advantages in the domain of sensing applications. Nevertheless, the literature concerning MEMS resonators for terahertz detection is markedly less extensive compared to that for infrared detection. The challenges associated with terahertz detection surpass those encountered in infrared detection, necessitating enhanced sensitivity, improved signal-to-noise ratios, the utilization of specialized materials, and effective thermal noise management. The advancement of terahertz sensing technology is constrained by the absence of dependable detectors capable of stable operation at ambient temperature.

Lai et al.⁵⁹ developed a tunable terahertz metamaterial pressure sensor (Fig. 4a, b), which features a structure that integrates a SRR with a silicon substrate and TSV. The sensor operates by bending the cantilever beam of the SRR when airflow traverses the TSV, resulting in a bending angle ranging from 10° to 45°. This bending induces a variation in the resonant frequency contingent upon the polarization mode: in TM mode, the frequency experiences a blue shift from 0.721 to 0.796 THz (with a tuning range of 0.075 THz), while in TE mode, it shifts from 0.805 to 0.945 THz (with a tuning range of 0.140 THz). The sensor exhibits high sensitivity, achieving pressure detection capabilities of 0.0591 THz/pN in TM mode and 0.109 THz/pN in TE mode, alongside flow velocity detection ranging from 13.727 to 24.739 THz/(m/s)². Additionally, it is characterized by low energy consumption and holds potential applications in biological and environmental monitoring contexts. Zhong et al.⁷⁶ designed an internal triaxial structure and an external eSRR (Fig. 4c, d). The resonant frequency can be adjusted over a range of 0.32 THz by modifying the height between the inner and outer structures via an ETA. This metamaterial demonstrates polarization dependence and exhibits EIT properties. Following optimization, it achieves notable sensing performance, with a sensitivity of 0.379 THz/RIU, and an average quality factor and figure of merit of 66.01 and 63.83, respectively.

Fig. 4

Terahertz sensing applications. a 3D MTM for pressure sensing applications b The relationship diagram between the cantilever bending Angle and the induced bending force⁵⁹. c Detailed schematic diagrams and ETA simulation diagrams of TTM and unit cells based on MEMS. d The electromagnetic responses of TTM exposed to the surrounding environment with different refractive indices under different modes⁷⁶. e 3D view of the detector with dual-material microcantilevers and metasurface absorbers. f Side view of the double-material micro cantilever. Simulated thermomechanical response degrees of dual-material microcantilevers with different thickness ratios. g The measurement response of the detector⁶⁸. Reprinted with permission from refs. ^59,68,76

Among the critical attributes of the sensors, the energy harvesting capability of the MEMS actuator is paramount for achieving high sensing sensitivity. Furthermore, MEMS actuators can also facilitate the detection of terahertz radiation. Significant progress has been made in MEMS energy harvesting technology in recent years⁷⁷. This technology leverages micro-mechanical structures⁷⁸ not only to harvest ambient electromagnetic energy but also to achieve efficient energy conversion through the integration of specific functional materials⁷⁹. Within the terahertz band, MEMS actuators can be engineered to selectively absorb electromagnetic waves within target frequency ranges, thereby enabling sensing. Liu et al.⁸⁰ developed an ultra-thin tunable terahertz absorber with a thickness measuring merely 1/50 of the operational wavelength. This innovative device integrates metamaterial units that facilitate local modes alongside electrostatically actuated suspended planar films, thereby enabling effective modulation through micron-scale mechanical displacements. In its suspended configuration, the device exhibits an initial absorption rate ranging from 60 to 80%, with a switching speed of 27 μs. Zhu et al.⁶⁸ developed a terahertz detector based on a metasurface and dual-material microcantilever beams (composed of Au/Si₃N₄), (Fig. 4e–g). This detector was fabricated on a 300 μm silicon substrate. It achieves an absorption rate exceeding 90% within the 3.24–3.98 THz frequency band through the metasurface absorber, generating a mechanical response by leveraging the thermal expansion differential of the dual-material beam in conjunction with a silicon column thermal isolation structure. The device attains a photoelectric mechanical responsivity of 24.8 μm/μW and a low-noise equivalent power (NEP) of 3.82 × 10⁻¹¹ W/√Hz. The experimental performance was validated using a terahertz quantum cascade laser system, wherein the symmetrical cantilever beam structure and optical aperture design underscore its potential for real-time terahertz focal plane imaging at ambient temperature. This indicates that MEMS resonators not only achieve high-performance absorption in the terahertz band, but also demonstrate potential for applicability extension into the infrared spectrum.

Froberger et al.⁸¹ have developed a micro-mechanical terahertz detector utilizing SOI technology, which incorporates an aluminum half-wave dipole antenna in conjunction with a U-shaped cantilever beam. The detector is designed to absorb terahertz radiation at a frequency of 2.5 THz, which is subsequently converted into mechanical deformation via the thermal expansion effect. This deformation is then optically detected using a 1.5 µm laser diode. The device exhibits a remarkable sensitivity of 1.5 × 10⁸ pm/W, a NEP of 20 nW/√Hz, a bandwidth of 150 kHz, and a rapid response time of 2.5 µs, all while operating at room temperature and atmospheric pressure. In a related study, Bilgin et al.⁸² proposed a dual-material cantilever beam composed of Parylene C and titanium, along with a metamaterial absorption layer, enabling non-contact detection within the 1–5 THz frequency range through thermal mechanical response and optical readout. The pixel dimensions of this detector are 200 μm × 200 μm, with a noise equivalent temperature difference (NETD) of less than 500 mK and a refresh rate of 30 Hz.

Terahertz actuators in frequency and polarization tuning applications

Beyond sensing applications, the terahertz-wave absorption capability of MEMS actuators can be leveraged for spatial filtering in terahertz communications. Furthermore, altering the mechanical state of a MEMS actuator enables dynamic control over the polarization response of terahertz waves, thereby facilitating polarization tuning. Frequency tuning and polarization tuning typically occur synchronously; through specialized design, their coupling can be engineered to achieve resonant excitation of a target polarization state at a specific operational frequency.

Yang et al.⁸³ presented a unit structure comprising ring-shaped and cross-shaped nanostructures. By manipulating the height between these nanostructures, the resonant frequency can be adjusted within the range of 0.530–0.760 THz, effectively covering the atmospheric window from 0.625 to 0.725 THz, which is conducive for indoor wireless optical communication applications. Zhang et al.⁸⁴ developed a dual-frequency switchable bandpass filter that facilitates orthogonal terahertz wave modulation, achieving a transmittance exceeding 95% at the two resonant frequencies of 0.481 THz and 0.931 THz. Liu et al.⁷⁰ proposed a metal-insulator-metal interlayer configuration that incorporates two movable rectangular resonators alongside a centrally located “X-shaped resonator,” utilizing materials such as Au, SiO₂, and gold. The device enables tuning of the free spectral range by 166 GHz and facilitates the switching of a fourfold resonance mode through lateral movement of the resonator gap. It demonstrates a reconfigurable frequency band coverage of 1.24–1.53 THz, achieving a 19.8% bandwidth with a maximum free spectral range of 166 GHz. This device showcases multiple resonance characteristics in the TM mode while achieving a singular resonance in the TE mode (Fig. 5a–c). Huang et al.⁸⁵ developed an electromagnetic induction transparency (EIT)-MEMS filter, which consists of a movable metal bar and a pair of fixed metal wires. By adjusting the distance between the metal bar and the wire pairs, they achieved a transmittance modulation of up to 64.5% at a frequency of 1.832 THz, with a modulation rate of 38.8%. This device operates without a substrate, effectively mitigating background intensity and interference waveforms associated with substrate use, thereby enhancing the quality factor to 28 and improving optical performance. Additionally, they⁸⁶ introduced a THz reconfigurable trapezoidal metamaterial (LS-MM) that incorporates a cantilever array. This device employs an ultra-small MEMS actuator with a beam length of 14 microns to independently reshape each LS-MM cell, thereby modulating the transmission response of THz waves. The LS-MM is constructed from gold and features two arms and a split bar. When a bipolar square wave voltage of 150 volts is applied to the electrode, the cantilever tip is drawn downward, merging the split bars into connection bars, which alters the device’s state and consequently regulates the transmission characteristics of the THz wave. Furthermore, the resonant frequency of the cantilever actuator has been recorded at 585 kHz, indicating a high-speed tuning capability. The transition from the off state to the on state resulted in a transmittance variation of 60.1% at a frequency of 0.78 THz, accompanied by a transmission phase shift delay of 1.35 radians at a frequency of 1.35 THz.

Fig. 5

Terahertz frequency and polarization tuning applications. a Schematic diagram and manufacturing process of MEMS reconfigurable TMA b Absorption spectra with different incident polarization angles c The resonance and refraction relationship diagram of TMA based on MEMS⁷⁰ d Schematic diagram of counterclockwise helical cantilever structure. e SEM image f Schematic diagram and test diagram of the THz-TDS system⁵⁴. Reprinted with permission from refs. ^54,70

P. Pitchappa et al.⁸⁷ proposed a metamaterial unit of an octagonal ring-shaped cantilever, which utilized the residual stress difference of the aluminum/alumina double-layer cantilever to make it naturally curl up after release as the initial OFF state. When an electrostatic voltage (30–40 V) is applied, the cantilever is pulled down to the base (ON state) by the electric field force, changing the electromagnetic coupling state between the octagonal ring and the cantilever. Thus, dual-frequency resonant switching is achieved at 0.44 THz and 0.575 THz, and polarization-independent dynamic modulation of the transmission spectrum is realized through voltage control (with a maximum contrast of 80%). The terahertz electro-optic switch function has been realized. Han et al.⁸⁸ proposed a reconfigurable SRR array based on electrostatic driven MEMS. They fabricated 330 μm thick MEMS reconfigurable SRR structures on fused silica substrates. Each SRR unit consists of two semi-rings and is connected by electrostatically driven cantilever plates. The cantilever plate is composed of stacked layers of gold and silica (240 nm-Au and 240 nm-SiO₂), located on one side of the ring, and forms an insulating layer (200 nm-SiO₂) between the base plate. When the applied voltage exceeds the pull-in value, the SRR is restructured from the OFF state to the ON state. Closing the air gap increases the capacitance of the SRR, thereby reducing the resonant frequency, and the modulation speed can reach 2 kHz.

Heterogeneous integration utilizing specialized materials represents an innovative approach to spectral tunning. Roy et al.⁷² introduced a terahertz MEMS metasurface switch that leverages the electro-mechanical deformation of graphene. This design features an electro-mechanical metasurface structure comprising a suspended graphene layer and a silicon dioxide (SiO₂) grating-gold substrate, which collectively function as a variable capacitor, with graphene acting as a tunable functional layer. By applying a voltage ranging from 0 to 20 V between the gold substrate and the graphene, an electrostatic force is generated that drives the free region of the graphene to deform downwards, achieving a maximum displacement of ~0.5 μm. This deformation dynamically alters the height of the air gap between the graphene and the substrate, thereby modifying the plasma resonance on the graphene surface and the matching conditions of the grating wave vector. The device successfully achieved a continuous frequency shift of the resonance absorption peak of the terahertz wave within the range of 5.0–5.5 THz, with a tuning range of 0.5 THz, and an extinction ratio of 4.67 dB at the operational frequency of 5.49 THz. The maximum bandwidth of this device is 353 kHz, constrained by the resonant frequency of the free support film. Additionally, the modulation depth of absorption is enhanced through the synergistic variation of graphene carrier density in response to voltage changes.

Huang et al.⁵⁴ developed a MEMS terahertz metamaterial driven by the phase change material VO₂. This three-dimensional metamaterial consists of units featuring a helical cantilever structure. The mechanical deformation of the helical cantilever, induced by temperature-induced phase changes in VO₂, modulates the polarization state of the incident THz wave. This mechanism allows for significant adjustments in the polarization azimuth angle (θ) and ellipticity angle (η), achieving variations of ~15° each, and demonstrates excellent modulation performance within the frequency range of 0.5–1.1 THz (Fig. 5d–f). In comparison to traditional methods, this approach offers a greater modulation depth and a broader operational wavelength range. The temperature-sensitive characteristics of this metamaterial render it particularly suitable for applications in temperature sensing and thermal imaging, thereby providing novel solutions for environmental monitoring and industrial process control.

Terahertz actuators in beam forming applications

The exponential growth of data traffic, compounded by limited bandwidth resources, has collectively propelled the rapid advancement of terahertz communication technology. Within this context, terahertz beamforming technology—which selectively transmits and receives signals in designated directions—significantly enhances signal quality and interference resistance in communication systems⁸⁹, establishing itself as a pivotal technology for next-generation networks. The integration of terahertz MEMS RF components plays a vital role in facilitating beamforming and the operation of phased array antenna systems. Within communication systems, phase shifters are essential components that significantly influence both the performance and reliability of the overall system. In comparison to conventional Gallium Arsenide, Metal-Semiconductor Field-Effect Transistor, or PIN diode switches, MEMS phase shifters exhibit reduced insertion loss, enhanced linearity, and lower power consumption at terahertz frequencies⁹⁰. Notably, at terahertz frequencies, the traditional transmission line model becomes inapplicable due to substantial losses; however, MEMS technology offers a more effective alternative⁹¹.

Research on terahertz MEMS phase shifters has increasingly concentrated on the integration of waveguide technologies. Zhao et al.⁷³ introduced a terahertz waveguide-integrated phase shifter that utilizes supermodel propagation. This innovative design facilitates a continuous phase shift within the frequency range of 220–330 GHz, achieving a maximum phase shift of 350° with an insertion loss of merely 1.87 dB. The mechanism for phase shifting involves modulating the coupling distance between two parallel silicon wafers, while a MEMS actuator is employed to control the air gap, thereby regulating the effective propagation constant.

In a separate study, Barowski et al.⁹² developed a MEMS-based dielectric channel waveguide. They outlined a fabrication technique for these waveguides using high-resistance silicon, incorporating silicon-based metamaterials as mechanical support structures to ensure robust field confinement. The functionality of the device, including phase shifting, was achieved through precise adjustments of the slot width in conjunction with MEMS actuators. The proposed slot waveguide-MEMS integrated system demonstrated effective dynamic regulation of the refractive index by varying the slot width (Δn = 0.12–0.35) and confirmed its phase modulation capabilities within the 220–330 GHz frequency range. Notably, its high field-limiting characteristics (mode field area < λ²/4) contributed to a reduction in waveguide bending loss to 0.05 dB/cm. Huang et al.⁹³ presented a fully silicon-based, self-supporting, low-loss, and wide-range tunable terahertz phase shifter operating at frequencies between 0.12 and 0.13 THz. This device is driven electrostatically and achieves continuous phase tuning exceeding 360° by modifying the waveguide structure, while maintaining an insertion loss below 1 dB. In comparison to conventional resonant or graphene-based phase shifters, this all-silicon design addresses limitations such as restricted bandwidth, elevated insertion loss, and inadequate tuning range. A significant advantage of this design is its capacity to sustain a linear phase-frequency response and ensure a flat group delay of the signal, which is essential for the efficacy of terahertz communication systems.

Dielectric phase shifters represent a significant category of terahertz MEMS phase shifters. Somjit et al.⁹⁴ introduced three innovative variants of single-stage digital passive RF MEMS phase shifters. These devices utilize MEMS actuators to manipulate a micromachined dielectric block, measuring λ/2 in length, in order to achieve relative phase shifts above a three-dimensional micromachined coplanar waveguide. Measurement outcomes indicate that at a nominal frequency of 77 GHz, the design successfully attained relative phase shifts of 45°, 30°, and 15°. The return loss between the upper and lower states exceeded 25 dB, while the insertion loss was less than 0.9 dB⁹⁵. This design demonstrates effective performance across a frequency range of 10–100 GHz, with a return loss greater than −10 dB, an insertion loss below −1.5 dB, and a relatively linear phase shift maintained throughout the entire frequency spectrum.

In a separate study, U. Shah et al. developed a three-chip integrated waveguide phase shifter utilizing MEMS-RS technology. This device employs a symmetrical comb-tooth drive actuator, operating at a driving voltage of 30 V and achieving a displacement of ±5 μm, to facilitate dynamic blocking/non-blocking switching in the TE₁₀ mode, thereby establishing a linear phase shift function with a step size of 10°. The integration of three gold-plated silicon chips occurs within a non-standard rectangular waveguide, with the total dimensions of the chips measuring 6.8 mm × 10 mm. To prevent mechanical adhesion failure, a wet release process combined with key point drying technology is employed. The resulting 3.3-bit phase shifter achieves a 20° linear phase shift within the frequency range of 500–600 GHz, with a return loss exceeding 15 dB across the entire frequency band, an insertion loss of less than 3 dB below 540 GHz, and less than 5 dB up to 600 GHz⁹⁶.

The low-loss silicon MEMS phase shifter developed by Rahiminejad et al.⁷⁴ demonstrates a phase shift of 145° at a frequency of 550 GHz through the manipulation of the silicon wafer’s position, achieving an insertion loss of 1.8 dB and a return loss of 18 dB. This device represents the sole available phase shifter for the frequency range of 500–600 GHz that combines low loss, substantial phase shift, and waveguide integration. The phase shifter operates on a thin perforated silicon film, which is either actuated by a MEMS motor and positioned within the waveguide along the E plane. The phase shift is contingent upon the membrane’s position within the waveguide, as this configuration influences the dielectric load on the fundamental TE10 waveguide mode, thereby affecting the group delay. Subsequent optimizations⁹⁷ of the film’s thickness, effective dielectric constant, and plate length have resulted in an increased measured phase shift of 350°. To validate its efficacy in electronic beam control, the phase shifter was integrated into a 4 × 1 element phased array, with the array elements being excited by leakage waves. Measurement results indicated that the antenna achieved a gain of 20 dBi and was capable of ±20° sector beam scanning.

Furthermore, Oberhammer et al.⁹⁸ successfully implemented beam shape switching utilizing a MEMS-RS switch integrated into a 2 × 8 antenna array, which they had previously proposed. The front end of this antenna is capable of radiating frequency-controllable wide beams and notch beams, with the ability to shift from −20 degrees at 238 GHz to +20 degrees at 248 GHz. The proposed reconfigurable beam control antenna front-end chip is among the first integrated silicon micromechanical devices capable of controlling beam shape within the terahertz frequency range, thereby providing significant advancements for beamforming and spatial multiplexing in terahertz communication systems.

The tunability of the integrated terahertz MEMS switch presents a viable approach for spatial beam control. Nonetheless, current investigations remain in the exploratory phase and are constrained by advancements in multi-layer glass substrates and high aspect ratio glass TSV technology, and have yet to yield a tangible physical prototype. Yang et al.⁹⁹ introduced a windmill-shaped, reconfigurable patch antenna capable of circular polarization, which operates within the terahertz frequency range. The antenna’s geometry can be altered by manipulating the on and off states of four MEMS switches. By incorporating a parasitic arm and facilitating coupling with other elements, the patch antenna is able to emit a distinct circularly polarized inclined beam for each configuration. Simulation outcomes indicate that the proposed antenna pattern can switch among four azimuth angles at an elevation angle of 30°, with a reflection coefficient of S11 < −15 dB across the frequency range of 340–350 GHz. Additionally, Wang et al.¹⁰⁰ developed a beam-steerable antenna featuring eight operational states, utilizing RF-MEMS switches. This design comprises four gradient logarithmic helical antenna elements and eight MEMS cantilever beam switches. The direction of the beam can be modified by selecting various input ports and configuring the RF MEMS switches. The choice of input port dictates the azimuth angle of the beam, while the switch state influences the vertical tilt angle. This antenna operates within the frequency range of 335–348 GHz and is capable of achieving a 90° increment in the azimuth plane, with vertical tilt angles of 32° and 50° for beam switching.

Metamaterials confer devices with the capability to meticulously manipulate the phase, amplitude, and polarization of electromagnetic waves through the design of subwavelength structures. By regulating the displacement of microscale mechanical structures, mechanically tunable metasurfaces (MTMs) facilitate beam steering control via techniques such as spatial phase modulation. The synergistic interaction between these elements has markedly enhanced the deflection efficiency, angular range, and functional diversity of terahertz beams. The profound integration of MEMS actuators with metamaterials has paved a novel pathway for the modulation of terahertz waves. Furthermore, innovative designs, including reconfigurable air-spacer metamaterials (ASMs) and cantilever beam metasurfaces, have further broadened the multifunctional regulation capabilities of terahertz waves.

To address the challenges associated with the large volume and passive response of contemporary terahertz components and devices, Sun et al.⁷¹ proposed an innovative dynamic THz beam deflector. Their approach involves the integration of a micro voice coil motor (VCM) to facilitate electrically controlled continuous adjustments of the air gap, thereby enabling effective steering of the terahertz beam. By constructing a metamaterial (MTM) that maintains an air gap between two silicon chips, they achieved a maximum deflection coefficient of 0.60 and a deflection angle of ~44.5° at an operating frequency of 0.61 THz, with the air gap set at 50 μm. This technology exhibits a modulation depth of up to 62.5%, and the deflection efficiency is enhanced by a factor of 3.2 compared to methods reliant on the regulation of the material’s dielectric constant (Fig. 6a, b). Additionally, the authors introduced³ reconfigurable ASM, which facilitate dynamic deflection of THz beams by modifying the ultra-subwavelength air gap between a metal resonator array and a gold ground plane. The ASM comprises silicon actuators, gradient metal resonator arrays, and gold-plated substrates, fabricated through surface and bulk silicon microfabrication techniques. Upon the application of voltage, the silicon actuator descends due to electrostatic attraction, altering the air gap size and consequently adjusting both the deflection angle and intensity of the THz beam. This device achieves a maximum deflection coefficient of 0.37, a deflection angle of approximately ±29°, and a modulation depth of up to 86.5% at the target angle.

Fig. 6

Terahertz beam forming applications. a The explosion diagram of MTM, schematic diagram of the unit cell structure b Simulate the far-field scattering mode of metasurface arrays with different air gaps⁷¹. c Illustration of the reflection array. d The radiation pattern of 0.6 THz terahertz waves reflected by the grating. Each grating period has two, three, four and five reflection beams respectively¹⁰¹. Reprinted with permission from refs. ^71,101

Furthermore, Liu et al.¹⁰¹ developed a high-stroke electrostatic actuator-driven reflection array, characterized by a reconfigurable reflection grating design composed of multiple subwavelength reflectors. These reflectors are actuated by five high-stroke electrostatic drivers, allowing for vertical displacement adjustments that create a sawtooth-shaped blazed grating structure. The cantilever beam, with a stroke of 600 μm, is controlled by a 5-bit digital signal within the frequency range of 0.3–1 THz, resulting in an approximately sawtooth stepped grating structure. This configuration enables efficient beam steering within a range of ±56.4°, with side lobe suppression reaching −10 dB (Fig. 6c, d). These advancements hold significant potential for applications in efficient THz beam control, particularly in next-generation high-speed wireless communication and space technology.

The integration of MEMS actuators with metamaterial technology has the potential to facilitate efficient terahertz beam steering and achieve substantial modulation depths in large deflection angles, thereby paving the way for advanced micro-systems in the future. Nonetheless, current investigations continue to encounter challenges in realizing significant angular deflection of terahertz beams and in generating complex beam patterns, necessitating the exploration of novel implementation strategies. Chen et al.¹⁰² have demonstrated full 2π phase modulation utilizing a cantilever beam metasurface, employing a row-by-row voltage control approach that resulted in a 59% conversion efficiency from plane-wave to surface-wave (PW-SW) within a bandwidth of 100 GHz. Their optimization method based on coupled mode theory allows the device to facilitate multi-beam synthesis and dynamic focusing capabilities. Despite these advancements significantly enhancing terahertz beam control technology, the ongoing challenge remains in reducing the manufacturing complexity and cost associated with MEMS and metamaterial integrated devices, which is anticipated to be a focal point for future research endeavors.

Terahertz actuators in logical operation applications

MEMS reconfigurable metasurfaces have introduced a novel approach for implementing on-chip logic operations and facilitating secure communication within the terahertz frequency range. The majority of MEMS logic gates employ mechanical resonant structures that transduce periodic variations in resonance into electrical amplitudes, a phenomenon that is intrinsic to the multifaceted behavior of micro- and nanostructured devices. By integrating the MEMS actuation mechanism with metamaterials, researchers are able to exert precise control over the amplitude, phase, polarization, and resonant frequency of the electromagnetic field. This advancement enables the development of a system-on-chip (SoC) capable of executing digital logic operations, encoding photoelectric signals, and dynamically reconstructing functionalities. Such innovations not only transcend the constraints associated with conventional passive metamaterials but also establish a technological foundation for high-bitrate, multi-channel data processing, secure encrypted communication, and the advancement of next-generation intelligent terahertz systems.

Manjappa et al.¹⁰³ introduced a MEMS-based Fano metasurface (Fig. 7a, b) capable of executing digital logic gate operations, including XOR, XNOR, NOT, NAND, and OR, at terahertz frequencies. This functionality is facilitated by manipulating two SRRs through independent voltage controls (V₁ and V₂). The device exhibits a hysteresis loop multi-input/output characteristic, which enables random access for metamaterial programming. The precise regulation of Fano resonance allows for dynamic control of near-field coupling, thereby enabling the encoding, processing, and transmission of photoelectric signals. Building upon this capability, the authors developed a secure One-Time Pad encrypted channel, wherein private messages were encrypted using XOR logical operations and subsequently transmitted via a non-secure public channel. This metasurface not only performs a variety of logical operations but also holds potential for the advancement of next-generation random access digital programmable metamaterials with high bit rate multi-channel data processing capabilities.

Fig. 7

Terahertz logical operations applications. a SEM image of CSM device b Logic voltage drive response curve graph¹⁰³ c Schematic diagram of wine-shaped cantilever based on MEMS. d SEM images of WCM devices based on MEMS e Programmable devices are functionalized as logic gate signals⁶⁷ f Schematic diagram of optical logic devices g The dual-input logic gate function implemented by CSM devices⁵³. Reprinted with permission from refs. ^53,67,103

In a related study, Li et al.¹⁰⁴ achieved dynamic control over the amplitude, phase, polarization, and spin angular momentum of terahertz waves by coordinating the regulation of the polarization angle and DC bias voltage. By incrementally increasing the polarization angle of the incident terahertz wave, the resonant frequency of the SRR transitioned from 0.74 THz to 1.16 THz, with a maximum modulation depth exceeding 70%. The polarization-related transmission intensity and resonant frequency of the device can be actively adjusted when driven by varying DC bias voltages. The driving voltage and polarization state serve as 2-bit input signals, while the transmission response is categorized into “on” and “off” states, which can be utilized for the logical functional characteristics of the logic modulator. The resonant frequency for the “NAND” logic gate is recorded at 0.439 THz, while that for the “AND” gate is at 0.732 THz.

Xu et al.¹⁰⁵ proposed an actively tunable device based on an ETA integrated with magnetic metamaterials. The device comprises sequential layers of iron (Fe), Au, and Si₃N₄, arranged from top to bottom. The Au and Si3N4 layers constitute the ETA, while the Fe layer serves as the magnetic metamaterial situated on the central plate. The device operates by inducing an upward bend in the ETA through the manipulation of temperature differentials, which arise from the disparate coefficients of thermal expansion of the Au and Si₃N₄ layers. This mechanism allows the device to distinguish between “on” and “off” states and to generate time-difference optical signals that convey digital information. Furthermore, the application of a transverse magnetic field enables the metamaterial to tilt from an initial angle of 40° to 90° within a magnetic field intensity range of 0 to 300 mT, thereby facilitating adjustments to its optical properties and enabling efficient optical switching functions. Additionally, the authors⁶⁷ introduced a programmable logic modulator device based on wound cantilever metamaterials (WCM), which dynamically regulates terahertz waves through MEMS-driven cantilever deformation and integrates dual logic gate functions of “OR” and “AND” within a single device (Fig. 7c–e). This design features a PR/Au/Si₃N₃ sandwich structure cantilever, with respective thicknesses of 1000 nm, 100 nm, and 500 nm, and achieves deformation via the ETA. By varying the polarization angle and applying different DC bias voltages, the electromagnetic characteristics of the device can be finely tuned, making it suitable for variable optical attenuators and switching applications. Subsequently, they⁵³ proposed tunable terahertz CSM that achieve optical logic functions through the integration of a MEMS electrothermal actuator platform (Fig. 7f, g). This device utilizes Au as the primary component of the metamaterial and a Si₃N₄ layer for the ETA, with both materials precisely fabricated on a Si substrate. By applying a 0.20 V DC bias voltage, a tuning range of 0.54 THz was attained, and the “XNOR” logic gate function was successfully realized at a frequency of 1.20 THz. The Au layer exhibits a relatively high coefficient of thermal expansion, while the Si₃N₄ layer has a comparatively low coefficient. The resultant deformation, driven by the thermal expansion differential, causes the metamaterial plate to tilt vertically and remain suspended in free space, thereby achieving an experimentally validated “on-off” state switch with a transmission contrast exceeding 20 dB. This capability allows the ETA to bend and deform under electrical drive, thereby modifying the CSM structure to alter its electromagnetic response characteristics. This innovative design not only addresses the limitations associated with traditional flexible substrates but also provides a more stable optical output signal, presenting a novel approach for optical logic switches.

Outlooks and future directions

The field of terahertz MEMS actuator technology has witnessed significant advancements in material innovation, optimization of drive mechanisms and system integration. Nevertheless, the large-scale implementation of this technology continues to encounter several technical challenges, including the necessity for high driving voltages, inadequate environmental stability, and low levels of system integration. Future research endeavors are anticipated to concentrate on enhancing the development of terahertz MEMS devices with an emphasis on intelligence, miniaturization, and high performance, while simultaneously broadening their application potential in areas such as 6 G communication, high-resolution imaging, and real-time sensing.

Material selection and innovation are pivotal for enhancing the performance of terahertz MEMS actuators. Research efforts will prioritize the introduction of novel functional materials and advancements in heterogeneous integration technology. Phase change materials, characterized by their volume shrinkage and abrupt alterations in electromagnetic properties during phase transitions, can offer wideband tuning capabilities for metamaterials. By optimizing the control of phase transition temperatures and improving the fatigue life of these materials, it is anticipated that low-power and highly stable terahertz reconfigurable devices can be developed. Furthermore, the incorporation of materials such as magnetohydrodynamic substances and flexible PDMS is expected to reduce manufacturing costs and improve environmental adaptability. A polarization modulator utilizing magnetic fluid can dynamically manipulate the arrangement of ferromagnetic particles via an external magnetic field, facilitating large-angle adjustable polarization while also providing the advantage of non-contact control. Heterogeneous integration technology enables the development of multifunctional devices, contingent upon the materials employed. For example, heterogeneous structure transistors can achieve performance and characteristics that surpass those of traditional all-silicon CMOS technologies¹⁰⁶. The integration of MEMS actuators with photonic and electronic components facilitates the development of terahertz microsystems characterized by low loss and high levels of integration.

The constraints associated with relying on a singular driving mechanism have led researchers to investigate the potential of multi-physics field coupling and hybrid driving strategies. While electrostatic drives are characterized by their rapid response times, their application is hindered by the requirement for high drive voltages. Conversely, thermal and piezoelectric drives offer distinct advantages, such as substantial displacement and low power consumption, respectively. Future advancements are anticipated to focus on the integration of multiple driving methods, thereby enhancing the dynamic performance of devices through collaborative optimization. The synergistic application of piezoelectric drives and phase change materials may address the fatigue life challenges associated with phase change materials while simultaneously facilitating a continuously adjustable electromagnetic response. Furthermore, magnetohydrodynamic drives present an opportunity to simplify device complexity through cost-effective manufacturing processes. Pneumatic MEMS actuators also exhibit promise in dynamic beam control and polarization modulation, owing to their non-contact and continuously adjustable properties. The introduction of a multi-physics field coupling model enables precise predictions of the device’s multi-dimensional responses, thereby providing a theoretical foundation for design considerations.

To systematically delineate the distinct roles and complementarity of various tuning strategies, Table 1 provides a comparative analysis of mainstream terahertz reconfigurable schemes, encompassing both MEMS-based actuation and non-MEMS alternatives. As summarized, MEMS actuators excel in mechanical precision and low-loss performance, whereas non-MEMS approaches (those employing phase-change materials, liquid crystals, or topological structures) often surpass in switching speed, non-volatility, and flexible integration. This dichotomy is not merely competitive but profoundly complementary. The future trajectory of reconfigurable terahertz devices lies not in the supremacy of a single technology, but in the strategic fusion of their respective strengths.

Table 1 Performance Comparison of Terahertz MEMS vs. Non-MEMS Approaches

The emergence of cascaded metasurfaces has opened new avenues for achieving flexible and powerful control over THz wavefronts. Unlike their single-layer counterparts, cascaded architectures spatially stack multiple metasurface layers along the propagation path. This configuration decomposes complex wavefront transformation tasks across individual layers and incorporates interlayer displacement or rotation as additional degrees of freedom, thereby enabling dynamic and continuous wavefront shaping capabilities that are difficult to realize with single-layer devices¹⁰⁷. Beyond beam control, cascaded metasurfaces have demonstrated diverse application potential in the THz regime. For instance, integrating phase-change materials into such layered structures has enabled the realization of THz full-adder logic gates, highlighting the promise of electromagnetic-wave-based computing¹⁰⁸. Similarly, combining cascaded metasurfaces with liquid crystals has led to reconfigurable multi-focal lenses, which exploit the birefringence of liquid crystals to achieve polarization-dependent focal length tuning¹⁰⁹. Other work has integrated photonic crystals with THz metasurfaces to demonstrate 2-bit THz coding¹¹⁰. These advances underscore the versatility of the cascaded metasurface concept and its compatibility with various active materials and tuning mechanisms.Nevertheless, independently and rapidly driving the individual elements of THz MEMS-based cascaded metasurfaces remains a significant challenge. To circumvent this limitation, Cai et al.¹¹¹ proposed an innovative approach using a cascaded system comprising two all-dielectric transmissive metasurfaces. By independently rotating these two layers at different angular velocities, they achieved dynamic and continuous control over both the THz wavefront and polarization state. Although this specific implementation did not employ MEMS actuation, the mechanically rotated cascaded metasurface paradigm presents a feasible path forward. In the future, combining the precise displacement control offered by MEMS technology with the refined wavefront manipulation capabilities of metasurfaces could provide a practical solution to the enduring challenge of dynamic wavefront control in the THz domain.

As the demand for miniaturization and multifunctionality in terahertz communication and sensing systems escalates, high-density three-dimensional packaging technology is poised to become a focal point of research. Innovations in TSV technology and wafer-level packaging processes have opened new avenues for the integration of microchannels, waveguides, and MEMS actuators. TSV technology offers novel solutions for gas microchannels, while three-dimensional microfabrication techniques facilitate the batch production of intricate three-dimensional structures. This progress lays the groundwork for the development of highly integrated terahertz phased array antennas, reconfigurable metasurfaces, and other advanced devices. By merging MEMS actuators with microfluidic systems and optical sensing modules, it is possible to create multifunctional terahertz Microsystems capable of real-time molecular detection and environmental monitoring.

The advent of artificial intelligence and adaptive algorithms¹¹² is poised to facilitate the evolution of terahertz MEMS devices from a state of “passive response” to one of “active regulation.” Optimization algorithms grounded in deep learning can be employed to design tunable MEMS actuators¹¹³ with optimal geometric configurations, thereby enabling high-precision modulation within specific frequency bands. Furthermore, adaptive tuning technologies can dynamically counteract environmental disturbances, thereby enhancing device stability. In the realm of terahertz communication, intelligent beamforming technologies can adjust beam direction in real-time through algorithmic processes, thereby accommodating the communication demands of high-speed mobile environments.

THz MEMS offer a novel approach to quantum state engineering, overcoming the limitations of conventional methods constrained by strict momentum conservation. By leveraging resonant enhancement of quantum vacuum fluctuations and relaxing phase-matching requirements, this platform enables efficient generation of entangled photon pairs. It produces tunable degenerate and non-degenerate pairs across a broad spectrum, with multi-frequency operation achievable through pump control, thereby advancing the development of integrated terahertz quantum devices¹¹⁴. As prominent solid-state sources of coherent terahertz radiation, THz quantum cascade lasers (QCLs) have long been hindered by traditional manual tuning methods, which introduce significant thermal load and operational complexity. To address this challenge, Han et al.¹¹⁵ developed a fully electronically tunable THz QCL integrated with a MEMS tuner and an open-loop low-temperature piezoelectric nano-positioning stage.Within THz MEMS architectures, quantum-level signals are highly susceptible to ohmic loss during transmission. Paradkar et al.¹¹⁶ explored indium microsphere bonding to achieve superconducting interconnects. By bonding a metallized layer beneath niobium/niobium nitride protrusions with indium, the electrical resistance approaches zero at millikelvin temperatures, reducing terahertz quantum signal transmission loss to below 0.05 dB/km—significantly surpassing conventional metal interconnects. This indium-based bonding also helps mitigate quantum state decoherence caused by signal attenuation.Another challenge for MEMS in quantum applications is device miniaturization to accommodate nanoscale wavelengths. Xie et al.¹¹⁷ developed a thinning process for piezoelectric films, progressively reducing lithium niobate film thickness from 300 nm to 67 nm and successfully fabricating suspended resonators across various thickness levels.

The future advancement of terahertz MEMS devices will significantly depend on the integration of interdisciplinary technologies. The amalgamation of 3D printing and micro-machining techniques to fabricate active mechanical structures that adhere to specified criteria will reveal distinctive mechanical characteristics¹¹⁸. Additionally, the incorporation of MEMS actuators with nanomaterials such as graphene has the potential to further augment the sensitivity and response times of these devices. In the biomedical sector, MEMS-based terahertz sensors can enable label-free biomolecular recognition through the detection of molecular vibration spectra, presenting innovative approaches for cancer biomarker identification and pharmaceutical analysis.

Despite significant advancements in laboratory settings, the industrialization of terahertz MEMS devices continues to face challenges related to standardization and large-scale production. The mechanical designs of many terahertz MEMS actuators remain relatively simple, making them susceptible to stress variations, high operating temperatures, and manufacturing tolerances. Achieving successful fabrication and packaging of MEMS devices tailored for the terahertz spectrum, while addressing reliability concerns, is essential for realizing their promising applications¹¹⁹. Current focal points include: (1) the development of high-reliability packaging technologies that are compatible with CMOS processes to mitigate issues related to adhesion failure and residual stress; (2) the establishment of cost-effective manufacturing processes; and (3) the standardization of testing platforms, including the formulation of testing standards that encompass parameters such as insertion loss, isolation, and tuning range, thereby providing a unified benchmark for evaluating device performance.

Terahertz MEMS actuators are currently at a pivotal juncture, transitioning from laboratory research to industrial application. The performance limitations they face are expected to be progressively overcome, ultimately bridging the “terahertz gap” and delivering transformative solutions across various domains, including 6 G communication, high-precision biosensing, and spatial spectral analysis. Future research endeavors should aim to establish a cohesive relationship among foundational theories, manufacturing processes, and application scenarios, thereby advancing this technology from the “scientific frontier” to the “industrial core”.