1. INTRODUCTION
Polarization is a fundamental property of electromagnetic waves, describing the orientation of the electric field vector and playing a crucial role in wave–matter interactions and information encoding [1–4]. In the terahertz (THz) regime, precise polarization control is crucial for enhancing signal transmission rate [5], improving imaging contrast [6–9], and enabling polarization-sensitive detection [10]. Spatially tailoring the polarization distribution allows the generation of vector beams [11,12], offering expanded capabilities in various applications. In particular, certain vector beams, such as vortex beams carrying orbital angular momentum [13,14] (OAM), allow for high-dimensional modulation and OAM multiplexing [15–17], further increasing the capacity of THz communication [18,19]. Furthermore, vector beams carrying structured polarization states enable advanced spectroscopic techniques for the identification of chiral molecules [20,21] and high-dimensional optical encryption [22,23].
To achieve such flexible polarization control in the THz regime, metasurfaces [24–26] have emerged as a powerful platform with significant advances in both static [27–29] and dynamic [30–32] designs. Polarization control [33,34] is typically achieved through asymmetric structures, and by strategically arranging unit cells with varied polarization conversion capabilities, vector beams with tailored polarization distributions can be generated. While these solutions are reliable, they are limited in adaptability and cannot meet the demands of dynamic applications. Active metasurfaces that incorporate tunable materials or devices provide greater flexibility, yet pixel-level reconfigurable polarization control remains a challenge. Programmable metasurfaces offering spatially dynamic polarization modulation are essential for next-generation THz technologies, enabling functions such as adaptive imaging [35], secure communication [36], and high-precision sensing.
Liquid crystal (LC) has been widely employed in active THz metasurfaces due to its birefringence [37–41] and electro-optic effects [42], enabling dynamic manipulation of THz waves. By integrating LC with asymmetric resonant structures, active metasurfaces can achieve tunable polarization conversion and vector beam switching, providing high flexibility in polarization control. Furthermore, pixel-level addressability enables programmable control of spatial THz waves. Recent advancements have demonstrated their effectiveness in spatially controlling amplitude [43–47] and phase [48–51], unlocking promising applications in beamforming [52–55] and compressed sensing [56]. However, despite these developments, programmable metasurfaces capable of spatially reconfigurable polarization conversion remain largely unexplored, limiting their potential in advanced polarization-sensitive THz applications.
In this work, we designed and implemented a programmable THz metasurface capable of generating spatially varying polarization states. Each unit cell functioned as a reconfigurable polarization converter, with external input voltages applied to tune the polarization state at the pixel level. To validate the proposed concept, we fabricated an programmable device and experimentally verified its polarization conversion capabilities. Additionally, by incorporating a predesigned nonuniform arrangement of unit cells, we verified pixel-level phase presetting without affecting active polarization control. This work provides a foundation for advanced applications in polarization imaging, communication, and stealth technology.
Fig. 1. Principle of programmable LC metasurface for THz polarization conversion. The programmable metasurface is composed of an array, with each pixel individually controlled by an FPGA. Each pixel functions as a reconfigurable polarization converter. When an x-polarized plane wave is normally incident on the metasurface, the reflected wave becomes y-polarized when the voltage bias is removed (OFF). When the bias is turned on (ON), the reflected wave retains its original x-polarization.
2. RESULTS
The programmable LC metasurface consists of an array (see Fig. 1 ), with each pixel independently controlled via a digital-to-analog converter (DAC). A field-programmable gate array (FPGA) regulates the voltage bias of each pixel, enabling the flexible-generation of THz beams with customized polarization patterns. In its preset state, the metasurface displays an N-shaped polarization pattern. Upon receiving specific instructions, the FPGA dynamically adjusts the voltages across the pixels, causing the LC in the addressed regions to rotate in response to the applied electric field. As a result, when an x-polarized wave is incident on the metasurface, the reflected polarization pattern follows the N-shaped configuration: x-polarization is retained within the “N” region, while the surrounding area undergoes a conversion to y-polarization.
When the reflected wave passes through an x-polarizer, the electric field remains uniform within the N-shaped region, while no THz wave is detected in the surrounding area. Conversely, when transmitted through a y-polarizer, the outer region exhibits a uniform field distribution, whereas the N-shaped region shows no detectable THz wave. These complementary field patterns highlight the metasurface’s capability to modulate polarization direction, enabling the precise-generation of customized polarization distributions through the array.
Fig. 2. Diagram of the unit cell and simulated THz reflection spectra. (a) Exploded view of the unit cell structure. The top and bottom layers are quartz substrates. The metallic structure is fabricated onto the quartz substrates, and the middle layer is 20 µm thick LC. (b) Simulated phase difference and amplitude ratio spectra for the and configurations at 0.413 THz. (c) Calculated elliptical polarization state, azimuth angle, and ellipticity of the reflected wave as the LC undergoes reorientation under x-polarized incidence at 0.413 THz.
A. Design Principle and Unit Cell Structure
Figure 2 (a) shows an exploded view of the unit cell structure, which is periodically arranged to form a programmable metasurface with a period of µ. The structure includes two quartz substrates, each with 200 nm thick gold layers deposited on the upper and lower surfaces to form the metallic patterns and ground plane, respectively. An LC layer (LC_DN_03) is sandwiched between these metallic layers, creating a metal-insulator-metal (MIM) resonant structure. The geometric parameters of the upper metallic pattern predominantly determine the polarization conversion capability. The pattern is symmetric along the directions and incorporates split-ring resonators, which are generated by etching the metallic layer. The inner ring radius is 107 µm, while the outer ring radius measures 165 µm, and the pattern includes a fan-shaped segment with a subtended angle of .
The design principle is based on the polarization decomposition theory [57,58]. The incident -polarized wave is decomposed into electric field components of and . The anisotropic nature of the unit cell pattern along the directions leads to differing resonant frequencies for the 45° and polarized waves, resulting in a phase shift between them. When the amplitudes are equal at the operating frequency and the phase difference reaches 180°, polarization switching occurs, converting the reflected wave to its orthogonal polarization.
When a voltage bias is applied, the of the LC layer changes due to the electro-optic effect, thereby shifting the resonant frequency. When the change of () approaches 1.2, the amplitudes of the two orthogonal polarization components equalize, and the phase difference approaches 0, preserving the original polarization state at the output. By leveraging the tunable characteristics of the LC, an electrically reconfigurable polarization converter is realized, effectively functioning as a half-wave plate.
In the simulations, the LC permittivity varied from 2.45 to 3.55 in 0.1 steps, with the OFF state at 2.45 and the ON state at 3.55. Figures 2 (b) and 2 (c) show that for incident waves polarized at , the phase difference decreases from 180° to 0°, and the amplitude ratio increases from 0.98 in the OFF state to 1.26 in the ON state. The results demonstrate that the metasurface achieves the designed polarization control effectively. Further analysis reveals significant changes in the output wave direction and ellipticity for an x-polarized incident wave. The ellipticity shifts from in the OFF state to in the ON state, indicating linear polarization throughout the switching process. Additionally, the azimuthal angle changes from 90.4° to 6.5°, demonstrating that orthogonal polarization switching is realized. Overall, these results suggest that as the dielectric constant increases, the output wave transitions from y-polarization to elliptical polarization in intermediate states, ultimately achieving x-polarization.
Based on the simulation results, the Jones matrix for this metasurface can be derived as follows [59]:
where represents the amplitude ratio between and , and is the phase difference between them. When and , the Jones matrix describes a half-wave plate. On the other hand, when and , the matrix represents a planar reflector, which preserves the polarization state of the incident co-polarized wave.
B. Electrically Reconfigurable Polarization Converter
We fabricated a reconfigurable metasurface-based polarization converter using microfabrication techniques based on the previously designed unit cell. This device effectively enables polarization conversion between linear polarization (x- and y-polarized) and circular polarization (left- and right-handed). Figures 3 (a) and 3 (b) illustrate the reflection coefficients for co-polarized and cross-polarized waves under x-polarized incidence, exhibiting pronounced modulation as the biased voltage varies. At simulated center frequency center, the co-polarized reflection coefficient increased from 0.02 to 0.80, while the cross-polarized reflection coefficient decreased from 0.88 to 0.11. Similar trends were observed for circularly polarized incidence, verifying the device’s capability to transition between the half-wave plate and planar reflector functionalities.
Fig. 3. Simulated and measured co-polarized and cross-polarized reflection spectra. Simulated amplitude spectra for co-polarized (a) and cross-polarized (b) reflections, with corresponding polarization conversion ratio (PCR) (c), under ON and OFF conditions. Measured reflection coefficients for co-polarized (d) and cross-polarized (e) components and the corresponding PCR (f) as a function of driving voltage from 0 to 18 V.
The measured reflection spectra of the device are presented in Figs. 3 (c) and 3 (d). In the OFF state, where no voltage is applied, the co-polarized reflection coefficient remains below 0.13 within the 0.41–0.42 THz range, while the cross-polarized reflection coefficient exceeds 0.8. This behavior indicates that the incident x-polarized wave is effectively converted into a y-polarized wave within this frequency range. Upon increasing the applied voltage, both co-polarized and cross-polarized resonance frequencies exhibit a redshift. At the experimental center frequency center, the co-polarized reflection coefficient rises from 0.05 to 0.80, while the cross-polarized reflection coefficient decreases from 0.80 to 0.22, demonstrating voltage-controlled polarization modulation. These opposing trends in reflection coefficients highlight its capability for adaptive polarization conversion. The experimental results exhibit strong agreement with simulation predictions. The difference in center mainly arises from discrepancies between the liquid crystal parameters used in the simulation and those of the actual material, as well as fabrication tolerances.
To quantify the polarization transformation, we define the polarization conversion ratio (PCR) as [60–62]
Figure 3 (c) presents the simulated PCR, which remains above 0.96 between 0.41 and 0.42 THz in the OFF state and falls below 0.05 upon activation (ON state). In experimental measurements at 0.418 THz, the PCR decreases from 0.99 to 0.01 as the applied voltage increases, validating the capability of the metasurface for highly efficient polarization state modulation. These results demonstrate the tunability of the LC-based metasurface, providing precise and dynamic control over the polarization characteristics of the reflected THz wave.
C. Electrically Programmable Polarization Converter
To achieve dynamic and spatial modulation of THz polarization, we designed and fabricated a programmable metasurface with an array. As shown in Figs. 4 (a) and 4 (b), the device consists of a bottom gold layer segmented into square patches, each measuring . Each patch is individually connected to a peripheral bonding electrode interfaced with a DAC, enabling precise voltage control at each addressable pixel. We measured both the co-polarized and cross-polarized reflection spectra of the device to evaluate the polarization modulation performance (see Section 2 of Supplement 1 for details). Figure 4 (c) illustrates the PCR response spectra between the ON and OFF states. In the OFF state, the PCR remains consistently above 0.92 within the frequency range of 0.414 to 0.420 THz. Upon switching to the on state, the PCR drops and remains below 0.12 throughout the same frequency range. These results indicate that pixel-level segmentation does not degrade the modulation efficiency of the device.
Fig. 4. Characterization of programmable metasurface for dynamic THz polarization patterning. (a) Optical image of the fabricated device. (b) Microscopic image of the patterned metallic structures on the upper layer. (c) Measured PCR spectra. (d) Measured normalized co-polarized and cross-polarized electric field intensity distributions at 0.414 THz for various programmed patterns.
The capability of programmable THz polarization patterning was further assessed under x-polarized incidence using five distinct control configurations. As shown in Fig. 4 (d), voltage patterns corresponding to “N,” “J,” “U,” “smile,” and “cry” were sequentially applied. In the half-wave plate mode (OFF state), the reflected cross-polarized component is enhanced while the co-polarized signal is suppressed. In contrast, in the mirror mode (ON state), the co-polarized reflection dominates. The spatial distribution of reflected THz intensity reveals high-fidelity reconstruction of the programmed patterns in the co-polarized channel, with complementary features in the cross-polarized channel. These experimental results confirm that the metasurface enables dynamic, two-dimensional control of THz polarization with high spatial resolution.
D. Integrated Control of Phase and Polarization in Programmable Metasurface
While the programmable metasurface enables efficient spatial control of polarization, we extend its functionality by incorporating phase engineering, allowing comprehensive manipulation of THz wavefronts. This is achieved by designing unit cells with varied in-plane orientations, enabling simultaneous modulation of both polarization and phase. In the OFF state, where no bias is applied, each unit cell shown in Fig. 5 (a) behaves like a half-wave plate with its optical axis oriented at 45°. This configuration enables modulation of the phase difference between orthogonal polarization components relative to the slow axis. As shown in Fig. 5 (b), rotating the unit cell by an angle maintains a nearly constant cross-polarized electric field amplitude of while inducing a linear phase shift of . Figure 5 (c) further confirms that under circularly polarized incidence, clockwise rotation from the 45° reference results in efficient polarization conversion into the cross-polarized component. When the orientation is rotated by 90°, the polarization conversion efficiency remains high, but the induced phase shift reaches 180°.
Fig. 5. Programmable polarization converter with tailored phase distribution. (a) Schematic of the top layer of a unit cell, with the in-plane rotation axis used for phase control. (b) Simulated phase and electric field amplitude of the transmitted cross-polarized wave under left-handed circularly polarized incidence, plotted as functions of the rotation angle. (c) Simulated PCR versus the orientation angle at the operating frequency under circularly polarized incidence in both ON and OFF states. (d) Spatial arrangement of unit cells with two distinct orientation angles configured in a checkerboard pattern. (e) Measured distributions of cross-polarized electric field intensity and phase, along with co-polarized amplitude, for all the pixels in the OFF state. (f) Measured normalized electric field intensities for co-polarized and cross-polarized components in four different configurations.
These simulation results confirm that, in the OFF state, rotating the orientation axis retains the half-wave plate functionality while enabling fine phase tuning through the spatial arrangement of differently oriented unit cells. In contrast, the ON state exhibits a distinct behavior: for or 90°, polarization conversion is suppressed, and the metasurface behaves as a planar reflector. This occurs because switching between ON and OFF states in these orientations does not induce a 180° phase difference, thereby inhibiting cross-polarized reflection.
Leveraging this mechanism, we implemented a metasurface with a checkerboard pattern of 0°- and 90°-oriented unit cells, as shown in Fig. 5 (d). In the configuration where all four quadrants are unbiased (OFF state), the measured electric field distributions are shown in Fig. 5 (e). The cross-polarized component reaches a normalized amplitude of 0.85, while the co-polarized signal is significantly suppressed to 0.19. All regions exhibit uniform amplitude response, and the retrieved phase map shows a clear checkerboard-like distribution with adjacent domains differing by , consistent with the expected phase shift from a 90° rotation of the unit cell. To assess the robustness of the polarization tuning under spatially nonuniform configurations, we applied four distinct voltage patterns, including both striped and checkerboard biasing, to independently address different regions. As shown in Fig. 5 (f), the measured intensity maps reveal complementary co- and cross-polarized distributions that respond directly to the applied voltage patterns. These results confirm that the metasurface functions as a reconfigurable platform for joint polarization and phase control, enabling dynamic transitions between the half-wave plate and reflector-like states with pixel-level programmability.
3. DISCUSSION
We demonstrate a versatile LC programmable metasurface capable of dynamic spatial control of THz polarization, offering broad applicability across THz technologies. By independently tuning the voltage at each pixel, the metasurface enables polarization-sensitive compressed sensing imaging, facilitating the efficient extraction of polarization characteristics from imaged objects in the THz regime. Additionally, spatial modulation of polarization states allows the generation of orthogonal polarization vector fields, enhancing channel capacity in communication systems through polarization-division multiplexing and reducing interference.
The metasurface achieves a programmable, nonuniform polarization distribution by establishing a precise correlation between each unit cell and its respective phase. Predefined phase gradients ensure stable coexistence of 0° and 180° phase domains, enabling dynamic polarization encoding for applications in secure communications and adaptive wavefront control. In optical communication, it enhances data transmission by enabling polarization-division multiplexing, ensuring stable beam shaping for efficient signal alignment and reduced interference.
In summary, our programmable THz metasurface integrates a predesigned phase profile with polarization-state modulation to realize structured and dynamically tunable polarization distributions. Both experimental and simulated results confirm its ability to generate spatially varying polarization states while maintaining precise wavefront control. This platform offers promising opportunities for optical encryption, stealth technology, and other applications requiring adaptive manipulation of THz waves.
APPENDIX A: METHODS
1. Device Fabrication Process
A 200 nm thick gold layer was deposited onto a quartz substrate via magnetron sputtering. Ultraviolet (UV) photolithography was employed to define the metallic patterns, followed by wet etching to remove unprotected regions, forming the metallic structures. A spacer was placed between the upper and lower quartz substrates to create a 20 µm gap, and the assembly was sealed by bonding the two substrates with UV adhesive. The LC was heated to 106°C and introduced into the cell through thermal infiltration. The lower quartz substrate was then secured to the printed circuit board (PCB) using UV adhesive. Finally, an anisotropic conductive film tape was applied to bond the flexible printed circuit to the device electrodes, with electrical conduction established through thermal compression bonding.
2. Characterization of Spatial THz Polarization Distribution
The polarization conversion characteristics of the metasurface were evaluated using THz time-domain spectroscopy (TDS) operating in reflection mode. The emitted THz beam was first passed through a linear polarizer aligned along the x-direction to ensure a well-defined input polarization. The resulting x-polarized beam was then focused onto the sample surface. The reflected beam passed through an analyzing polarizer and was subsequently detected by a receiving antenna. The sample was mounted on a computer-controlled two-dimensional translation stage, enabling spatially resolved measurements via stepwise scanning. At each position, the detector recorded the electric field amplitude.
Two polarization configurations were employed to map the spatial polarization distribution. In the first configuration, both the transmitting and receiving polarizers were aligned along the x-direction, measuring the x-polarized component of the reflected wave. In the second configuration, the receiving polarizer was rotated to the y-direction while maintaining the transmitting polarizer along the x-direction, allowing measurement of the y-polarized component. A comparison of the two-dimensional intensity maps acquired under the x–x and x–y configurations verified the ability of the metasurface to achieve spatially varying polarization conversion.
