Introduction

Terahertz (THz) waves, which constitute an important segment of the electromagnetic spectrum, occupy the region between far-infrared and microwave frequencies. The THz frequency range is 0.1–10 THz and corresponds to wavelengths of 3 mm–30 µm1. THz waves have a broader bandwidth than microwave. Moreover, multiple atmospheric transmission windows are present in this range, which makes the THz frequency a valuable choice for near-field wireless communication. The exceptional data transmission and seamless integration with sensing capabilities also make THz waves an important component in next-generation wireless communication technologies2. Additionally, the characteristic absorption peaks of various key substances, including explosives, drugs, and biomolecules, are in the THz frequency range, which endow THz technology potential applications in military, security check, biomedical imaging and other domains3,4,5,6. Currently, most THz applications rely on the free-space path, but the transmission of THz waves through free space is affected by the interference of water vapor and beam diffusion5. There is no available method to visualize the THz light, which increases the difficulty of THz light path construction and adjustment. Waveguides are efficient for power transmission between THz devices and offer anti-interference, low loss capabilities and a great flexibility in optical path adjustment, so waveguides are valuable for THz applications7,8,9,10.

Optical fibers have long been recognized for the efficient interconnection of optical components with flexibility, minimal loss, efficiently transmit, and integration of modulator. The integration of optical modulators in fibers significantly enhances the anti-interference and stability of optical systems. Similarly, interconnecting the THz components with flexible, low-loss THz fibers is necessary for THz applications, and the fiber-integrated THz modulation devices can ensure seamless operation. Due to the inevitable material absorption on THz waves11 fiber designs for THz waves are primarily based on hollow-core fibers, including photonic crystal fibers (PCFs), dielectric waveguides, suspended-core fibers, and anti-resonant fibers (ARFs)12,13. Among these types, ARFs can realize THz wave transmission with a low loss, a broad bandwidth, and vapor isolation due to a compact structure, so ARFs have been widely considered to be an optimal waveguide solution for THz applications14. The incident light in ARFs has various interacting phenomena such as mode coupling, anti-resonance and leaky mode resonance, and various functional ARFs have been proposed for THz wave modulation. Mode coupling in ARFs can be achieved by controlling the diameter ratio between core and cladding tubes, which has led to the development of large-mode-area single-mode ARFs for THz transmission15,16,17. ARFs with an obvious polarization filter and birefringence have great advantages, and they have been developed based on selective mode coupling and tuned anti-resonant effects18,19,20,21.

However, these light modulation techniques in ARFs require high precision in structural dimensions, and small deviations in the fiber geometry can lead to undesirable mode coupling or increased transmission losses. The structural precision is crucial for the device performance, which also challenges the fabrication of the ARF-based THz devices. Metamaterials, as a series of artificially constructed micro-structured arrays, can achieve superlative electromagnetic responses beyond the underlying material, and the metamaterial has become a novel process to enhance the light modulation of the fibers. In fibers, metamaterials have been constructed in the cladding, core and tip to realize various functions for incident lights. The cladding of fibers can be constructed using a multilayer metamaterial22,23 a metal-line array metamaterial24,25 or a nonlinear crystal-glass metamaterial26 to enhance the fiber properties of sensors, light transmission and all-optical frequency conversion. Various metamaterial structures have also been applied to the fiber core to realize the functions of fiber sensors27 polarization filter28 wavelength filter29 mode convertors30 and transmission in subwavelength geometry31. As a two-dimensional form of metamaterial, the metasurface is easy to integrate on the fiber tip to modulate the output light from the fiber32 and has realized functions such as beam collimating33,34 phase modulation35second harmonic generation36 enhanced fiber sensors37,38,39 and photodetector arrays40. For THz waves, the interaction with natural materials is too weak to directly modulate the light, and metamaterials become an important method for THz wave modulation. Additionally, the wavelength of THz wave is generally hundreds of microns, which simplifies and promotes the applications of metamaterial structures for THz waves. Metamaterials have been applied into THz fibers and realized various functions such as plasma frequency modulation41 tunable band-stop fiber filter42 and mode confinements43.

To develop a metasurfaced ARF for efficient polarization-dependent THz wave modulation, a metasurface was integrated on the surface of a fiber core to realize a metasurfaced-core ARF, addressing the limitations of conventional ARFs in structural precision and modulation capability. At present, the investigations of metamaterial-combined fiber for THz waves are mainly in the stages of principle exploration and structural design, and in this paper, the metasurfaced-core ARF was realized using the fabrication method of “film to fiber.” The cladding tubes of the ARF were constructed using a metasurface-coated low-density polyethylene (LDPE) film that was subsequently encapsulated in a polytetrafluoroethylene (PTFE) shrinkable tube, forming the metasurface-core ARF. Simulation investigations indicate that the proposed ARF with a 4-mm core-diameter has a simulated loss of 0.18 dB for the transmission polarized light and an absorption polarized loss exceeding 100 dB/m. Based on the proposed “film to fiber” method, a metasurfaced-core ARF sample with a core diameter of 4 mm and a length of 16 cm was produced. The measurement results showed an obvious anti-resonant effect and a strong polarization-dependent filtering capability with an approximately 7-dB difference in power spectral density. The ‘film-to-fiber’ fabrication method offers a simplified manufacturing with high dimensional accuracy, while the metasurface enhances polarization selectivity, achieving a superior performance than the traditional ARFs without metamaterials.

Fiber design

The primary method to decrease the transmission loss of ARFs is to maximize the light confinement in the core while minimizing the light interaction with the surrounding fiber material. One promising approach is to apply water-drop-shaped cladding tubes, which has been shown to significantly reduce light-cladding interactions and transmission loss44,45. This paper proposed an ARF design that incorporated water-drop-shaped cladding tubes to enable efficient THz wave confinement in the fiber. As depicted in Fig. 1(a), six water-drop-shaped tubes were symmetrically arranged around a central core, and the core diameter is Dcore. Each cladding tube consisted of an arc and two straight sections, and both straight sections were tangent to the arc. The arc, which is characterized by diameter d, spans an angle of (180 + 2θ)°. Length L of the two straight segments can be determined as follows:

$$L=d/2(tan\theta)$$
(1)

In this fiber design, both cladding tubes and inner side of coating are constructed from the same film and result in a consistent wall thickness of T. In hollow-core fibers composed of low-loss materials, the primary source of transmission loss is attributed to confinement loss α, which can be obtained as follows:

$$\alpha=\left(\frac{\text{20}}{\text{ln}10}\right)\left(\frac{2{\uppi\:}f}{\text{c}}\right)\text{Im}({n}_{\text{eff}}),$$
(2)

where f is the frequency of incident light, c is the speed of light, and Im(neff) is the imaginary part of the effective refractive index.

LDPE with a refractive index of approximately 1.51 + 0.0004i at 1 THz was selected as the cladding material and PTFE with a refractive index of approximately 1.42 + 0.0012i at 1 THz46 was selected for the coating layer as presented in Fig. 1(a). The refractive index values of these two materials exhibit no remarkable variations within the 0.5–3 THz band and are nearly constant. The transmission characteristics of the ARF were numerically investigated using the Comsol Multiphysics software with a manual-controlled mesh. The basic maximum mesh size was set to λ/6, while the cladding tube walls were refined to a maximum size of 10 μm to ensure accuracy. Perfectly matched layers (PMLs) with a thickness of 500 μm were applied as boundary conditions to absorb outgoing waves, thereby simulating an infinite free space.

In the simulation, when the core diameter Dcore was set to 4 mm, d was set to 2.4 mm, θ was set to 30°, and Tex was set to 500 μm, the loss of 1-THz wave in the ARF had a periodical variation with changes in T. As described in Fig. 1(b), when T was 30–300 μm, two distinct low-loss transmission bands emerged at approximately T ≈ 70 μm and T ≈ 200 μm. When T was maintained at 70 μm, Fig. 1(c) illustrates the transmission spectrum of the ARF. In the frequency range of 0.5–3 THz, two significant transmission bands were observed at approximately 1.05 THz and 2.55 THz. This periodic transmission behavior with respect to T and f demonstrates that the fiber has a strong anti-resonant effect for incident light. Furthermore, the impact of the external coating layer on the anti-resonant effect of the ARF was investigated. In an ARF with T of 70 μm, the loss of 1-THz wave in fiber varying with Tex is depicted in Fig. 1(d). There is a periodic variation in fiber loss with increasing Tex: three peaks appeared in the range of 200–600 μm with a period of approximately 150 μm. Thus, both internal cladding and external coating layers contribute to the anti-resonant effect for incident light. This additional loss from resonant effects of the external coating layer is not substantial but maintaining an anti-resonant thickness for extremely low-loss transmission remains crucial.

Fig. 1
Fig. 1
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(a) Structural diagram of the thin-film-based ARF. (b) Variation of loss with the cladding tube wall thickness. (c) Loss variation with the incident light frequency in an ARF with a 70-µm cladding. (d) Loss variation at 1 THz for an ARF with a 70-µm cladding as a function of the external cladding thickness.

Results

Simulated investigation

A micro-structured metal layer was deposited on the surface of the fiber core, and the transmission characteristics of the metal coated ARF was numerically investigated. In ARF, two symmetrical cladding tubes were coated with a copper layer on the core surface, which resulted in a metal-coating fiber as those in Fig. 2(a). The copper layer with a thickness of 100 nm occupies angle θm on the cladding tube. When Dcore was maintained at 4 mm, d at 2.2 mm, θ at 25°, Tex at 500 μm, and T at 70 μm, Fig. 2(b) illustrates the transmission losses of metal-coated ARF with θm for the x-polarized light and y-polarized 1-THz waves. The fiber demonstrated a consistent increase in loss for x-polarized light (from 0.016 dB/m to 0.8 dB/m) when θm changed from 2° to 45°. Further, in this range, there was a quasi-periodic increase in loss for y-polarized light with two peak values: one at 15.9 dB/m @11° and another at 26.8 dB/m @37°. The extinction ratio (EXTx/y) can be calculated as follows for x- and y-polarized lights:

$$\:{EXT}_{\text{x}/\text{y}}=10\text{l}\text{o}\text{g}({I}_{\text{x}}/{I}_{\text{y}})$$
(3)

where Ix and Iy are the amplitude of x-and y-polarized lights that pass through the ARF as follows:

$$\:I={1-10}^{-{\upalpha\:}/10}$$
(4)

The variation of EXTx/y with θm for a 1-m ARF is depicted in Fig. 2(c); a peak value of approximately 20 dB appeared at θm of 10°. The optimal extinction ratio for different polarized lights yields α of 0.039 dB/m for x-polarized light and approximately 10.7 dB/m for y-polarized light. At the highest loss in the fiber (θm = 36°), the y-polarized light exhibits α of approximately 26.8 dB/m, whereas the x-polarized light shows α of 0.31 dB/m, which resulted in an EXTx/y of 11.6 dB for a 1-m ARF. The simulation results indicate that the ARF with θm of 36° produced a superior transmission spectrum, as illustrated in Fig. 2(d). Thus, the copper-coated ARF has significant differences in loss between x- and y-polarized lights across a wide spectral band and an extinct loss mainly higher than 10 dB/m. The bulk coating with a metal layer is a promising solution for developing THz fiber devices, but the fiber designs must be enhanced to ensure optimal performance.

Fig. 2
Fig. 2
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(a) Structural design of the metal-coated ARF. (b) Influence of θm on the transmission losses of different polarized THz waves. (c) Corresponding extinction ratio. (d) Transmission spectra of the metal-coated ARF with θm at 36° for different polarized lights.

When θm is 10°, the electric field distribution (E field) of x-polarized light in the ARF is illustrated in Figs. 3(a), showing that the x-polarized light is perfectly guided in the fiber core. The detailed x-, y-, and z-components of the x-polarized light in fiber are presented in Fig. 3(b), indicating that the x-polarized light will not couple into the metal layer. The E field of y-polarized light as illustrated in Fig. 3(c) showing that, in contrast, due to the presence of the metal coating, y-polarized light strongly couples into the cladding and effectively delivers the energy to the coating. To elucidate the mechanism of metal coating enhancing light coupling, various components of E fields are presented in this paper. Figures 3(d) depict the x-, y-, and z-components of the E fields for y-polarized light in the ARF at θm of 10°; Figs. 3(e) present those at θm of 36°. The E fields reveal that the incident light has excited two distinct high-intensity localized electric field distributions at the edges of the metal layer, and the same time an oscillating electric field has formed within the metal layer. In the cladding tube, the electric field distribution shows a periodic outward transmission, and as the position moves away from the metal layer, it exhibits a divergent distribution. It can be concluded that the guiding modes (means the propagated light in fiber) firstly interact with the micro-structured metal layer which exciting the surface plasmon in cladding. When the metal layer has a suitable width, the surface plasmon will oscillate in the metal layer and the surface plasmon resonance (SPR) makes a strong absorption on the guiding modes. Based on the E fields, a resonant wavelength was determined to be approximately 210 μm. According to Fresnel formula, the 1-THz wave that propagates through an LDPE medium exhibits a wavelength of approximately 200 μm, which closely matches the resonant wavelength observed at the metal surface. Therefore, the metal layer with a length corresponding to the wavelength in the transmission medium effectively induces a leaky mode in the cladding. Moreover, the dual boundaries of the metallic layer play a pivotal role in eliciting resonant waves in the cladding layer.

Fig. 3
Fig. 3
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(a) Normalized electric field distributions of x-polarized light in the metal-coated ARF when θm is 10°. (b) Detail x-, y-, and z-components of the x-polarized light in fiber when θm is 10°. (c) Normalized electric field distributions of y-polarized light in the metal-coated ARF when θm is 10°. (d) Detail x-, y-, and z-components of the y-polarized light in fiber when θm is 10° and (e) those when θm is 36°. The arrows indicate the direction of the electric field.

To further investigate the plasmonic absorption dynamics in copper-coated ARFs, we developed an advanced fiber structure incorporating dual suspended copper films. As illustrated in Fig. 4(a), this fiber features a 90° rotational alignment of copper films relative to the fiber axis, creating spatially separated plasmonic interaction zones. The suspended configuration enables direct phase-matched coupling between guided modes and surface plasmons, significantly enhancing polarization selectivity. The observed distinct polarization-dependent transmission characteristics were found in this suspended-metal ARF. When Dcore was maintained at 4 mm, θ at 25 °, Tex at 500 μm, T at 70 μm, and d is varied from 1.6 to 2 mm, the transmission loss values of different polarized lights varying with θm are presented in Fig. 4(b, c). For x-polarized light (orthogonal to film orientation), the maximum transmission loss increased exponentially from 1.1 dB/m to 3.6 dB/m as cladding spacing d from 1.6 mm to 2 mm. Conversely, y-polarized light maintained stable low-loss propagation (< 0.025 dB/m) across the tested parameter range just like the original fiber. When θm is fixed at 36 °, d at 1.6 mm, the normalized E fields of x- and y-polarized lights in fiber are presented in Fig. 4(d, e), showing that the x-polarized light coupling into metal layer meanwhile part radiation coupled with cladding tubes. Accordingly, it can be concluded that the observed polarization filtering characteristic precisely results from the combined action of plasmonic absorption and plasmon coupling into guided mode.

Fig. 4
Fig. 4
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(a) Structural design of the ARF with suspended metal films. (b) Effect of suspended metal films on the transmission loss for x-polarized THz wave; (c) and y-polarized THz wave. (d) When θm is at 36°, d at 1.6 mm, normalized electric field distributions of x-polarized light in fiber; (e) y-polarized light in fiber.

A metasurface, which is a two-dimensional form of a metamaterial, can achieve distinctive light modulation through the collective behavior of their constituent elements. In this paper, a metasurfaced-core ARF is presented to improve the light modulation. As shown in Fig. 5(a), a matrix of microstrip lines was distributed on the cladding tube to create a metasurfaced-core ARF, which shared the structural parameters of the ARF in Fig. 2(a). Two symmetrical cladding tubes (top and bottom) of ARF were coated with a matrix of copper wires, and these copper wires were wrapped around the surface of the fiber cladding at angle θm, whereas different copper wires in the matrix were arranged at an angular separation of θg. In total, five copper wires were used to form the metasurface on each cladding tube. The transmission losses of x- and y-polarized lights in fiber varied with θm and θm as shown in Fig. 5(b). For the polarization-dependence of SPR in the metasurface structure, the results demonstrate that the metasurface in cladding induces a significant difference in loss for two polarized incident lights. When θg increased, both x- and y-polarized light losses first increased and subsequently decreased; however, a pronounced disparity remained between the loss values for the two polarizations throughout the range of θg. Additionally, θm plays a crucial role in determining the peak loss for both polarized lights. Figure 5(c) shows the extinction ratio of the 1-m fiber for x- and y-polarized lights, where the ARF with metasurfaced cladding achieved a high extinction of y-polarized light. For example, at θm = 5° and θg = 3.4°, the ARF achieved a loss of 100.8 dB/m for y-polarized light, which corresponds to a loss of approximately 0.18 dB/m for x-polarized light and an extinction ratio of 14.3 dB in a 1-m ARF. Figure 5(d) shows the transmission spectra of metasurfaced-core ARF for different polarized lights at θm = 5° and θg = 3.4°, which indicate that the fiber exhibited a broad bandwidth extinct spectrum in the range of 0.7–1.2 THz. Therefore, incorporating a metasurface into the cladding endows the ARF with excellent performance as a polarization filter compared to the simple metal layer coatings on the cladding surface.

Fig. 5
Fig. 5
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(a) Structural design of the metasurfaced-core ARF for polarization filtering. (b) Effects of θm and θg on the transmission loss for different polarized THz waves. (c) Corresponding extinction ratio. (d) Transmission spectra of the metasurfaced-core ARF for various polarized lights.

Similar to the ARF structure employing suspended metallic films, we proposed a suspended metasurface ARF configuration as illustrated in Fig. 6(a). Based on the fiber structure schematic of Fig. 5(a), the dual metasurface arrays are applied a 90° rotation relative to the fiber axis. Figure 6(b, c) presents the comparative transmission loss characteristics for x- and y-polarized lights propagating through this suspended metasurface ARF. The simulated results reveal that geometric optimization enables near-identical transmission losses for both polarization states, representing significant reduction compared to the original metasurfaced-core ARF performance. Besides, when θg is less than 5°, the y-polarized light always has a higher transmission loss than that of x-polarized light. When θm is fixed at 5°, θg at 3.6°, the E fields of two polarized lights in fiber are presented in Fig. 6(d, e), illustrating that light has evident coupling with metasurface array and the x-polarized light has a more intense resonance in metasurface array. Those evidences confirm that the polarization-selective absorption mechanism in metasurfaced-core ARFs is just the synergistic interaction between plasmonic absorption and metasurface-induced mode coupling.

Fig. 6
Fig. 6
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(a) Structural design of the ARF with suspended metasurface. (b) Effect of suspended metasurface array on the transmission loss for x-polarized THz wave; (c) and y-polarized THz wave. (d) When θm is fixed at 5°, θg at 3.6°, normalized electric field distributions of x-polarized light in fiber; (e) y-polarized light in fiber.

Fabrication method

As descripted in Fig. 1(b), the light transmission of ARFs highly depends on the wall thickness of the cladding tubes. Commercial plastic films offer a cost-effective and versatile solution with a broad selection of materials and thicknesses, which makes them particularly suitable for THz ARF. Notably, metamaterials can be initially fabricated on these thin films and subsequently integrated into the fiber structure to enhance the modulation properties of ARFs. In this work, we use the “film-to-fiber” method to fabricate the metasurfaced ARFs, as illustrated in Fig. 7. The fabrication steps are as follows:

First, a plastic film with the designed thickness and width is selected as the substrate for microstructure deposition. Using deposition techniques such as circuit board printing or lift-off photolithography, the metasurface pattern is deposited on the plastic film.

Second, the patterned plastic film is folded and compressed to form several interconnected tubes, which correspond to the cladding tubes of ARF. Then, these tubes are arranged in a circular configuration to effectively constitute the fiber cladding structure and concurrently integrate the metal microstructures onto the cladding surface.

Third, a heat-shrink tube is applied as the coating layer to enclosure the fiber cladding. With heat, the coating layer fuses with the cladding and forms a robust bond. The ARF can be continuously fabricated by repeating these steps.

Following this procedure, the ARF fabrication is complete. Experimental results demonstrate that the all-plastic ARFs produced using this method exhibit exceptional dimensional accuracy and enhanced flexibility, which makes them highly suitable for a wide range of THz applications.

Fig. 7
Fig. 7
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Fabrication process of metasurfaced-core ARFs.

Based on the fabrication process in Fig. 5, a metasurfaced-core ARF was successfully produced. Based on the fiber design in Fig. 8(a), a 70-µm-thick LDPE film was cut into an 83-mm-wide strip and periodically scored at intervals of 8 mm and 5 mm. Then, a metallic structure was deposited onto the LDPE film via thermal evaporation (HITSemi-Ebeam650F) and shadow mask. The shadow mask was fabricated from polyimide (PI) film, as depicted in Fig. 8(b), which was patterned using a laser engraver to create strips with widths of 105 μm, lengths of 4 mm and periods of 71 μm. Two distinct shadow masks were used to deposit two separate metasurface structures on the LDPE film, and the coated LDPE film is displayed in Fig. 8(c). This film was folded, compressed into the fiber cladding, and encapsulated in a PTFE shrinkable tube. After post-annealing treatment, the metasurfaced-core ARF was realized. The fabricated fiber exhibited a core diameter of 4 mm and a length of 16 cm. The cross-sectional view of fiber in Fig. 8(d) demonstrated the fiber has uniformly distributed cladding tubes and high-quality metasurface coatings. Correspondingly, Fig. 8(e) presents the top-view morphology which revealed the fiber sample smooth and flat cladding surfaces.

Fig. 8
Fig. 8
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(a) Design of the metasurfaced-core ARF for fabrication. (b) Shadow mask to manufacture the metamaterial cladding. (c) LDPE film with metasurface coating. (d) Cross-sectional image of the metasurfaced-core ARF sample. (e) Top-view image of the metasurfaced-core ARF sample.

Experimental results and validation

The ARF sample, which incorporated two metasurface-enhanced cladding tubes, was tested using the THz Time-Domain Spectroscopy (THz-TDS; TeraMetrix, T-Ray 5000). The THz-TDS system, which was equipped with dual optical delay lines, was used to measure the transmission spectra of the fabricated ARFs. Figure 9(a) shows the experimental setup for the ARF measurement, and the fiber was positioned between a polarized THz source (polarized source) and a THz detector. In the experiments, the fiber sample was installed on a V-slot fixer and a broad-band THz radiation emitted from a THz photoconductive antenna was coupled into the fiber sample through a 6-inch focusing lens. Then, the output light from the fiber was coupled into the THz detector through another 6-inch focusing lens. When rotating the fiber in axis, the incident light will switch between different polarized and Fig. 9(b) shows the transmission spectra of the fabricated ARF (x-polarized light represents the light parallel with metasurface array and y-polarized light represent the orthogonal polarized light with metasurface array). The results reveal that the fiber sample has an obvious transmission at approximately 1 THz and distinct attenuation around 0.7–1.1 THz which is consistent with the simulation results. The fiber sample showed pronounced polarization-dependent filtering characteristics with a power spectral density difference of approximately 7 dB at 1 THz. Besides, there were obvious dips at 0.75 THz and 1.1 THz, which are attributed to water absorption effects. Figure 9(c) shows the transmission loss of fiber sample for different polarized light, and the transmission loss α was obtained as follows:

$$\:\alpha\:=\frac{-10\text{lg}\left({{T}_{fiber}/T}_{reference}\right)}{\Delta L},$$
(5)

where ∆L is the length difference of fibers, Tfiber and Treference are the magnitude intensity outputs from the measured fiber and reference, respectively. Based on the power spectrum P, the magnitude intensity is obtained as follows:

$$\:T={10}^{-\left(\frac{P}{20}\right)}.$$
(6)

Thus, the transmission loss can be obtained as follows:

$$\:\alpha\:=\frac{\left({{P}_{reference}-P}_{fiber}\right)}{2\Delta L},$$
(7)

where Preference is the power spectrum intensity propagated by the reference, and Pfiber is that by the sample fiber. Taking the air as the reference, the experimental transmission loss of the 16-cm length fiber sample for different polarized lights are presented in Fig. 9(c). The fiber sample showed a transmission band around 0.7–1.4 THz for the light paralleled to metasurface, and a significant high loss around 0.75–1.1 THz for the light perpendicular to metasurface which exhibits a consistency with the simulation results. Thus, the simulated and experimental extinction ratio values of 16-cm length fiber are presented in Fig. 9(d), indicating that the metasurfaced-core ARF can efficiently filter polarization over a broad frequency range. However, owing to the certain deficiencies in the metasurface fabrication, the corresponding measurements failed to attain the 100 dB/m as depicted in the simulation results. While the high loss for y-polarized light (> 20 dB/m) restricts long-distance transmission, the 16 cm sample still demonstrates effective polarization filtering for short-range applications like on-chip THz interconnects or sensor front-ends. Future work may optimize the metasurface geometry to balance loss and bandwidth.

Fig. 9
Fig. 9
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(a) Schematic diagram of the optical path for the fiber measurement via THz-TDS, where S is the THz source, A is the aperture, and D is the THz detector. (b) Measured power spectral density for air and fiber samples with different polarized light. (c) Experimental transmission loss across different frequencies. (d) Experimental and simulated extinction ratio between two polarized lights in the 16-cm fiber.

Method

The silver layer was enshrouded on the silicon grating through the electron beam evaporation (Ebeam650F, HITSemi, China). The transmission characteristic of fiber sample was tested using the THz-TDS (T-Ray 5000, TeraMetrix, USA). The shadow mask with PI material was patterned using the laser marking machine (HA-U5L, Han’s Laser, China).

Conclusion

This study introduced an ARF with metasurfaced core for high-performance THz wave modulations. Simultaneously, the metasurfaced ARFs were fabricated using the “film-to-fiber” method. An ARF with water-drop-shaped cladding tubes was presented with high-performance THz wave transmission at approximately 1 THz. On the cladding surface of fiber, depositing a copper metasurface enhances polarization selectivity by intentionally exploiting SPR for one polarization while maintaining anti-resonant low loss for the orthogonal polarization. Using LDPE for the cladding and PTFE as the external coating, a metasurfaced-core ARF with a core diameter of 4 mm and a length of 16 cm was realized, and the fabricated fiber showed an effective polarization filtering property with loss differences greater than 7 dB for 1 THz wave. This paper demonstrates the potential of metasurface in THz fiber technology and provides a novel method for THz wave modulation, showing promising applications in polarization-sensitive systems such as THz communication multiplexing, biomedical imaging requiring polarization contrast enhancement, and spectroscopic detection for molecular vibration analysis.