Introduction

Terahertz (THz) waves refer to electromagnetic radiation whose frequencies fall within the range of 0.1–10 THz1. The THz region has gained significant interest in recent years as sixth-generation (6 G) wireless communications and highly connected networks, such as the Internet of Things1, push carrier frequencies towards the millimetre wave (mmWave) and THz bands. Owing to its ultra-high bandwidth, data rates in excess of terabits per second are achievable using THz waves2. To drive this push into THz frequency wireless communications and bring the technology ‘out of the lab’, smart THz devices are required, which are compact, programmable and consist of an ultrafast reconfigurability1,2,3,4. However, these types of devices are rare in current electronics and photonics technologies, creating a so-called ‘THz Gap’5. Particularly important are ultrafast and electrically tuneable/programmable materials that are required to develop fundamental building blocks of THz wireless networks, including modulators, switches and polarisation converters3.

Graphene, a 2D allotrope of Carbon first isolated in 20046, is a promising material for THz devices. Graphene features a unique band structure7,8 leading to desirable electrical properties9 for THz technologies. Graphene consists of ultrafast carrier dynamics with mobilities that can exceed 100,000 cm2V−1 s−1 at room temperature10. This permits graphene to respond to electromagnetic fields at THz frequencies. Further, the conductivity of graphene can be tuned by external electrostatic doping11, that is, a bias voltage. Therefore, graphene is a desirable candidate material to develop ultrafast electrically tuneable and reconfigurable THz devices12,13, operating beyond currently available technologies.

Large area (cm scale) graphene films are required for practical THz devices. This requires films developed by Chemical Vapour Deposition (CVD) and not exfoliated flake materials (micron in size)14. CVD graphene films have been incorporated in tuneable THz modulators12,15,16,17,18,19,20,21 as well as self-biased stacks22, absorbers23, and, more recently, polarisation controllable devices24,25,26,27, to name a few. While these proof of concepts demonstrate tuneable THz devices, CVD films are polycrystalline in nature and contain wrinkles, defects and charge dopants, with typical carrier mobilities of the order of 1000 cm2V-1 s-1 28, two orders of magnitude below freestanding exfoliated graphene29. This limits the practical use of CVD graphene films in real-world applications.

The utility of CVD graphene in THz components is improved when coupled with metamaterial structures23. Metamaterials imitate the periodicity of a crystal lattice30 through subwavelength dielectric or metallic subunits. These structures provide control of the polarisation, amplitude, and phase of electromagnetic radiation31. Tuneable THz metamaterial devices are essential for high-value, emerging applications including target tracking, THz communications, THz sensing and THz imaging31. By coupling graphene with a metasurface, electromagnetic wave properties can be simultaneously controlled by the metasurface and tuned by the graphene32,33,34,35,36,37,38,39. Despite some promising results to date, graphene-based metamaterial devices are also compromised due to the poor electrical quality of the CVD-derived graphene film. As such, adequately tuning these devices typically requires high bias voltages or ion gels19,20,40,41,42,43,44,45, impractical for real-world applications.

Hexagonal-Boron Nitride (h-BN) is a large bandgap (5.9 eV) material29 with lattice parameters that closely resemble graphene. The structure of h-BN is such that it has no dangling bonds or charge traps, making it an excellent substrate for 2D material optoelectronics46. 2D h-BN is commonly used with exfoliated graphene flakes as a dielectric layer or encapsulation material29,46. When encapsulated in h-BN, exfoliated graphene can exhibit carrier mobilities of 140,000 cm2V-1 s-1, similar to results obtained from free-standing graphene29.

In this work, we demonstrate that using CVD h-BN as a substrate/intermediate layer in a h-BN/graphene heterostructure produces improved tuning in a graphene film, well beyond that which was achievable with the same graphene film on its own. A plain film h-BN/graphene heterostructure electrically tunes a broadband THz pulse (0.2–1.4 THz) by over 30 dB with a readily accessible 24 V bias. This represents a greater than 20 dB improvement over an equivalent graphene film not in a heterostructure with h-BN, using equivalent testing conditions. The utility of this approach is verified through the development of a highly tunable THz polarisation converter. In this paradigm, the hBN/graphene heterostructure is built on top of a (reflective) gold layer. A metamaterial design of periodic double-ring slots is patterned and etched through the gold/h-BN/graphene trilayer. This device exhibits benchmark tuning performance, tuning the ellipticity and rotation angle of a polarised 0.25 THz beam by 0.73 and 41° respectively, with only a 15 V bias. This work demonstrates that CVD h-BN layers can provide an effective means of improving graphene tuneability and performance in THz devices. The experimental results presented are pertinent to wide-ranging THz devices incorporating graphene, beyond the demonstrated polarisation conversion, promising new and varied THz opto-electronic devices with new and/or enhanced functionalities.

Results

Wide tuning in large area CVD h-BN/graphene heterostructures

The tuning range of a standalone CVD graphene film and graphene/h-BN heterostructure was tested using THz time domain measurements in transmission geometry, shown in Fig. 1a, b. Float zone Silicon (resistivity > 10,000 Ωcm) was used as the substrate material with two electrical connections to provide a bias voltage. The two designs, graphene-only and graphene/h-BN heterostructure were built from CVD films via a wet transfer process (see Methods) and are depicted in Fig. 1c. Initially, the THz signal transmitted through each structure with no applied voltage was recorded and is presented as the red curves in Fig. 1a, b. Hereafter, a voltage was applied to increase the graphene conductivity and hence, THz absorption. The maximum applied voltage (thus maximum THz absorption) was found to be 24 V before reproducibility of the effect could not be sustained. This is due to the fact that very-high voltages risk damaging the graphene film and result in device degradation or breakdown. The attenuated THz signal at 24 V is given as the blue curves in Fig. 1a, b. When 24 V is applied to the h-BN/graphene heterostructure (Fig. 1b) a significantly larger tuned signal is observed, with the attenuated signal approaching the noise floor of the THz time domain system. Here, the transmitted THz beam is tuned by greater than 30 dB across the 0.2–1.4 THz spectrum, with a maximum of 45 dB at 1.35 THz. When compared with the graphene-only case in Fig. 1d, the inclusion of the h-BN layer has dramatically improved the available tuning of a like-for-like graphene film (10 dB) by over 20 dB across the available THz spectrum for the same bias voltage and measurement conditions. Interestingly, the reduction in transmitted power is solely due to large absorption within the graphene film. Simultaneous reflection measurements show that the reflected THz power is consistent with applied bias. This is presented in Supplementary Information; Fig. 1.

Fig. 1: Comparison of the tuning range in a standalone graphene film and in a h-BN/graphene heterostructure from 0–24 V.
figure 1

a 0 V (Red) to 24 V (blue) tuning of graphene-only film. Inset: Corresponding power spectrum referenced against the bare Si substrate. b 0 V (Red) to 24 V (blue) tuning of h-BN/graphene heterostructure. Inset: Corresponding power spectrum referenced against the bare Si substrate. c Schematic of the investigated device structures. The chip size is 2 x 2 cm with a 1.8 x 1.5 cm graphene film. The contact pads are 2 × 1.8 mm. d Difference in transmitted power comparing 0–24 V as a ratio for the graphene (green) and h-BN/graphene (green) films. Note the structure in the inset blue curve andgreen curve for h-BN/graphene is due to the noise floor of the measurement system being reached.

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Importantly, being a nm-scale thick dielectric material, the h-BN layer does not produce any additional absorption when incorporated with the graphene. This was tested with a h-BN film on Si, with no detectable change found between a THz beam passing through the Si or Si/h-BN, as detailed in Supplementary Information; Fig. 2. As such, incorporating the CVD h-BN layer globally improves the electrical tuneability of CVD graphene without appreciably altering the THz signal. This tunable response is not limited just to absorption, but overall to the interaction of the graphene with a THz wave. As such, more complex graphene-based THz devices can be designed and fabricated, for instance, a tuneable THz polarisation converter described below.

Highly tuneable THz polarisation converter based on a gold/h-BN/graphene metastructure

To test if the high tuning performance of the CVD h-BN/graphene heterostructure translates to a real-world THz device, a THz polarisation converter was designed, modelled, fabricated and tested. The THz polarisation converter is based on dual arrangement of metallic slotted split ring resonators in a 2-dimensional array (see Figs. 2, 3). This arrangement converts a linearly polarised THz beam to circular polarisation. Adding graphene with the metallic layer enables the polarisation conversion to become tuneable. Specifically, the rotation angle and ellipticity of the THz beam can be controlled by the conductivity of the graphene layer, which is tuned via an external bias voltage. Further, including h-BN in a metal/h-BN/graphene trilyer, the results from Fig. 1 suggest this device structure should exhibit a larger tuning range compared to a device that does not contain the h-BN.

Fig. 2: Modelling of the polarisation converter metasurface.
figure 2

a Element model. b Surface currents of pure gold model. c Surface currents of tri-layer model with a 246 mS graphene conductivity.

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Fig. 3: Fabrication process for tested devices.
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The device chip is 2 x 2 cm. The metasurface and first electrode are 1.8 × 1.4 cm. The second electrode is 0.2 × 1.8 cm and there is a 0.2 cm gap between the metasurface and second electrode a summary of process used for producing the Au/h-BN/graphene trilayer polarisation converter. b Optical image of the trilayer patterned metasurface. Note: the gold appearance of the metasurface rings is a result of ambient yellow light in the cleanroom where this image was taken, and not due to unetched gold. c Schematic of the operation principle of the device in THz reflection geometry.

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To model the device performance, full wave simulations were carried out using the Frequency Domain Solver in CST Studio Suite 2022. The element model is shown in Fig. 2. It consists of two anisotropic slotted rings with dimensions of D = 0.23 mm, w = 0.02 mm, α1 = 50°, α2 = 30°, r1 = 0.06 mm, and r2 = 0.1 mm. The two rings are split at -45° and +135° with respect to x-axis. The element is illuminated by a y-polarised THz wave. Considering the slotted model of pure gold without a graphene/h-BN layer, due to the anisotropy, the outgoing wave will be rotated by 90° to x-polarisation. By adding the same patterned graphene and h-BN layers, the surface conductivity of the element will be changed, thereby varying surface currents and the induced electric fields. The simulated surface current distributions at 238.5 GHz for the pure-gold model and the tri-layer model with a graphene conductivity of 246 mS are given in Fig. 2b, c. It is noticed that the current density has been reduced along the edges of the dual slots as the surface conductivity decreases, thus dampening the ideal y-to-x polarisation conversion functionality. Correspondingly, the radiated components of y-polarisation and the converted x-polarisation will be modified with different amplitudes and phases, thus enabling polarisation variations among linear, elliptical, and circular states for different graphene conductivities.

The components in the x- and y- polarisation, rotation angle and ellipticity predicted from the simulation for different graphene conductivities are given in Supplementary Information; Fig. 3. The electrically tuneable polarisation converter was fabricated via the process outlined in Fig. 3a. A 285 µm thick float zone (resistivity > 10,000 Ωcm) silicon wafer was used as the device substrate. On the back surface, 220 nm of gold was sputtered to provide a ground. On the front surface, a hard mask was used to define electrical contacts and the metasurface region. The first layer of the device stack was then deposited as a 220 nm gold layer. Onto this layer, a multilayer CVD h-BN film was wet transferred with sufficient coverage to ensure graphene transferred on top could contact all functional areas of the device chip. A multilayer CVD graphene film was subsequently wet transferred on top of the h-BN with a small overlap at each end to electrical contacts. A second photolithography step defined the slotted metamaterial array. Ion beam etching with Argon gas removed material from the exposed regions of the trilayer generating the desired metasurface structure. As evident from Fig. 3b, high-quality patterning of the graphene/h-BN/gold trilayer was obtained and the metasurface pattern was sufficiently etched through each layer material in the stack.

The polarisation converter was measured using a THz time domain spectroscopy (TDS) system in reflection geometry as detailed in the Methods section. Figure 4 shows the tuning performance of the double ring slot polarisation converter fabricated on a gold/h-BN/graphene trilayer stack at a bias voltage of 0–15 V. The device is illuminated by a y-polarised incident wave. With a beam diameter of approximately 4 mm, around 200 elements of the metasurface are illuminated. Figure 4a shows the reflected x- and y-polarisation amplitudes, demonstrating polarisation conversion at a resonant frequency of around 0.245 THz with peak x-polarisation components. The amplitudes of the reflected two polarisations vary distinctly at different frequencies. However, in Fig. 4b the phase difference between x- and y-polarisations is observed to increase consistently with applied voltages in a wide bandwidth. This confirms tuneability of the reflected polarisation state with voltage applied to the graphene. This is reflected in variation to both the polarisation angle and ellipticity of the reflected beam as given in Fig. 4c, d. For fixed ellipticity (-0.42) at 0.234 THz, the angle is tuneable by 41° (See Fig. 4e). While at 0.242 THz, the ellipticity has been maximally tuned by 0.73 across the 0-15 V biasing range (See Fig. 4f), providing a transition from near linear polarisation to near circular polarisation. These experimental results agree well with simulations provided in the Supplementary Information; Fig. 3. For comparison, a similar device was fabricated, which omitted the h-BN layer. As expected, this device displayed much poorer tuning performance. Without the h-BN layer, the maximum tuning of the angle and ellipticity is 32o and 0.44, respectively, for the same measurement conditions. These results are detailed in the Supplementary Information; Fig. 4. and are comparable with the literature for polarisation-controllable devices which do not contain a h-BN layer.

Fig. 4: Summary of the voltage-tuneable performance of the trilayer polarisation converter with 0-15 V bias.
figure 4

a Reflected components of the polarisation Ex and Ey. b phase c polarisation angle d ellipticity e polar plot at 0.234 THz (fixed ellipticity) f polar plot at 0.242 THz. Note: In (f) a fixed angle is not depicted for clarity.

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Figure 5 directly compares the tuneable response of the polarisation converter with an equivalent device omitting the h-BN layer. With h-BN, the polarisation angle tuning at 15 V is typically above 30° across a relatively broad 0.23–0.26 THz frequency range (not including at resonance). Whereas, in the device without a h-BN layer, this is consistently below 10° across the same range. Similarly, the tuned ellipticity is above 0.7 across a 0.242–0.251 THz range, compared to maximum of 0.4 across a much smaller range. Thus, h-BN dramatically improves the achievable tuning range of graphene in the metasurface structure.

Fig. 5: Performance comparison of h-BN/graphene and graphene-only devices.
figure 5

Total available tuning performance of the a angle and b ellipticity for the measured polarisation converters. Blue line- h-BN included trilayer stack. Red dashed line – h-BN excluded stack.

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Discussion

The use of h-BN, a 2D dielectric material, as an intermediary layer in a stacked heterostructure with a graphene device provides a significant improvement to the electrical tunability of the graphene. The reason for this is two-fold. Firstly, the similarity in crystal structure of h-BN and graphene results in a more optimised graphene transfer. The CVD graphene films investigated were observed to adhere much better to the h-BN film compared to metal or a bare silicon substrate, due to the strong van der Waals interaction. Secondly, h-BN has lattice parameters that are similar to graphene and an atomically smooth surface free of dangling bonds and charge traps29,46. In comparison to devices based on exfoliated flakes, this translates to reduced roughness, lower intrinsic doping and chemical reactivity29, leading to better performing electrical devices. This performance gain appears to scale to large area CVD thin films of graphene on h-BN, as shown in both unpatterned graphene (20 dB tuning gain) and the patterned graphene polarisation converter (benchmark angle and ellipticity tuning) in this study.

The advantages of an improved graphene transfer when using a h-BN intermediary layer are exemplified when building more complex THz devices with stacked material layers and metasurface patterning. The better adhesion of the graphene minimised wrinkling, water retention and air gaps beneath the film, which contribute to a degradation of the electrical performance through, for example, charge scattering. This compounds into a more robust, reproducible fabrication process, where the photolithographic patterning and etching more accurately transfer the device design to the h-BN/graphene heterostructure.

Consequently, the THz polarisation converter based on the h-BN/graphene heterostructure presents benchmark tuning performance at a practically low applied voltage. Ellipticity tuning in this device (0.73) is above earlier reports (see Table 1) with a maximum tuning of 0.44 (similar range to our device without the h-BN layer). The CVD h-BN layer was intuitively applied to optimise the available tuning range of the graphene in our staked multilayer polarisation device. It could also be used as a gating dielectric or substrate material. Therefore, wide-ranging THz devices based on CVD graphene can immediately see an electrical tuning performance enhancement using a CVD h-BN film as a buffer layer.

Table 1 Comparison of Graphene Tuneable THz Polarisation Converters
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In this study, a h-BN/graphene heterostructure is investigated and shown to exhibit large broadband electrical tuning of THz waves. Greater than 30 dB modulation of a 0.2–1.5 THz beam is presented, which is 20 dB improvement more than an equivalent standalone graphene film without the h-BN layer. This is achieved without compromise; the h-BN does not absorb or interact with the THz radiation in any measurable way. The exceptional tuning of this CVD h-BN/graphene heterostructure can be translated into more complicated high-performance THz devices. A THz polarisation converter, operating at 0.24 THz, based on this heterostructure was developed, modelled, fabricated, and experimentally tested. The presented device, consisting of a photolithographically patterned double-ring slotted array metasurface in a gold/h-BN/graphene trilayer, exhibits a benchmark ellipticity and rotation angle tuning of 0.73 and 41°, respectively. This large tuning range is achieved at a practically low applied bias voltage of 15 V. The h-BN in the heterostructure stack significantly improves graphene tuning performance when compared to a similar device without incorporating the h-BN layer. This trilayer approach has proven to be a simple and effective method to significantly improve the electrical tuning properties of CVD graphene devices at THz frequencies. As such, it is well-suited to be implemented in a vast array of electrically tuneable graphene-based THz devices.

Methods

Polarisation converter fabrication

The Polarisation Converter was built on a 2 x 2 cm float zone (undoped) 285 nm Si substrate with a quoted resistivity of 10,000–100,000 Ωcm. A 220 nm gold layer was deposited onto back side of the Si using magnetron sputtering to provide an RF ground. A second gold layer of 220 nm subsequently deposited onto the front face of the substrate using a hard mask. This defined both the gold region (1.4 x 1.8 cm) of the metasurface and electrical contacts (0.2 x 1.8 cm). The metasurface region and first electrical contact (source) were separated from the second contact (drain) by a 2 mm gap defined by the hard mask. Commercially sourced multilayer (50 nm thick) chemical vapour deposition (CVD) h-BN (1.6 x 1.8 cm) was transferred on top of the gold region and between the electrical contacts with a small overlap using a wet transfer technique. A multilayer (50 nm thick) CVD graphene film47 (1.8 × 1.8 cm) was subsequently wet-transferred on top of the h-BN layer, such that the graphene overlapped the ends of the h-BN, contacting the gold pads.

The metasurface design pattern was etched into the gold/h-BN/graphene trilayer using a standard photolithography process. The stack was spin-coated with 1.8 µm S1818 photoresist, exposed to UV light with a chrome photomask of the design pattern and developed. Subsequently, exposed gold, h-BN and graphene areas were etched away using an ion mill with 2 x 10-4 mbar Ar gas pressure. The exposed trilayer regions were etched through with a 10 mA beam current, a 500 V beam voltage and a 100 V accelerator voltage. Discharge and cathode voltages of 42 and 4 V were used, respectively. A 5 A neutraliser current was employed and the sample stage was water-cooled to prevent the photoresist from overheating causing damage. Post etching, the structure was carefully cleaned in acetone solvent and dried with N2 gas. Raman spectra and SEM information of the etched and protected regions in the metasurface are presented in the Supplementary Information; Figs. 5-6. The Raman spectra were obtained from a Renishaw Qontor InVia confocal Raman microscope with an excitation wavelength of 514 nm. SEM images were taken using a Zeiss Auriga Cross Beam FIB/SEM

Electrical connections were made to the device chip with Al wires stuck to the gold pads with conductive Silver epoxy.

2D material transfer

Both the h-BN and graphene layers in the device stack were realised using a wet transfer method. Initially, h-BN and graphene films on foil (Cu and Ni, respectively), were spin-coated with a sacrificial PMMA layer. The foil was then etched away in Nitric acid leaving the film/PMMA layer. The film/PMMA was transferred (floated) on de-ionised water before subsequent transfer onto the substrate/stack. The film was allowed to air dry for a minimum of 24 h. Finally, the PMMA was removed using anisole, before drying with N2 gas.

THz measurement of the polarisation converter

THz measurements of the polarisation converter(s) were performed on a Batop/Toptica hybrid TDS system in reflection geometry. Photoconductive antennas (PCAs) were used for THz generation and photodetection. The emitter PCA is mounted onto a mechanical stage to act as the delay line. The THz beam is directed by off-axis paraboloid mirrors (OAPMs). The polarisation of the emitter PCA is elliptically polarised but heavily favours the axis parallel to the optical bench. As such, a wire grid polariser is situated between the emitter PCA and first OAPM with the grids orthogonal to the optical bench to ensure maximum transmission incident on the sample. The sample is mounted at the focal position of the two OAPMs and was aligned to the emitter and detector before placement of the polarisers. At the focal position, the beam diameter is approximately 4 mm. With a device unit cell of 0.23 x 0.23 mm, there is approximately 230 metasurface elements illuminated by the THz beam. Due to the polarisation sensitivity of the detector (favouring polarisation parallel to the optical bench also), this detector was mounted in a rotatable stage. Thus, a reference measurement could be used to align the detector maximum signal with the second polariser. Throughout experimental measurements the second polariser orientation and detector rotation were kept synchronised to ensure maximum signal and appropriate measurement of the polarisation components.

For polarisation control, two polarisers were placed in the optical path incident and reflected from the device under consideration. In this THz-TDS system, THz generation and photodetection is achieved through PCAs, which are predominately horizontally polarised in standard operation. Therefore, polariser 1 on the incident beam path was aligned to transmit horizontally polarised light, which we label Ey (y-polarisation). On the reflected beam path, polariser 2 was free to rotate to measure the polarisation components, Ex (x-polarisation) and Ey, incident on the detector. The detector PCA was mounted to a rotation stage and kept synchronised with the 2nd polariser to minimise any local polarisation sensitivity of the detector. The experimental setup is displayed in Fig. 6.

Fig. 6
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Schematic of THz-TDS setup used to characterise the electrically tuneable polarisation converters.

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THz measurements were made point-to-point with a mechanical delay line. A scan length of 70 ps was used with a 0.05 ps step size and 0.2 s integration time.