Objective Polarization is an important characteristic of terahertz (THz) waves and is widely applied in THz communications, imaging, sensing, detection, and other fields. Traditional THz polarization converters have large volumes, narrow bandwidths, and poor integration capabilities because they are based on the birefringence of materials. Metasurface polarization converters provide a potentially useful alternative. A metasurface is an ultrathin material comprising a planar array of subwavelength unit cells that can control THz waves with high performance. However, conventional metasurface polarization converters have a single function and a fixed bandwidth; thus, they cannot provide dynamic modulation. In this paper, we propose a THz-bandwidth, tunable polarization converter based on a vanadium dioxide (VO2) hybrid metasurface. VO2 is a typical phase-transition material that can change from an insulating state to a metallic state under an external optical, electrical, or thermal stimulus when its temperature exceeds 68 ℃. During the phase transition, its electrical conductivity increases by about four orders of magnitude. Our proposed device can modulate the polarization of a THz wave by dynamically controlling the phase transition of VO2.
Methods The proposed polarization converter comprises three layers: a metal substrate, an intermediate polyimide (PI) dielectric layer, and a VO2 composite metasurface. The resonant structure of the converter comprises a metal ring, with two metal rods crossing a diameter, and a piece of VO2 embedded in the middle of the two metal rods. We used the full-wave electromagnetic simulation software CST Microwave Studio 2019 to simulate the performance of this converter. In the simulation, we used periodic boundary conditions in the x- and y-directions, and open boundary in the z-direction. The THz wave is incident normal to the surface from the -z-direction, and the polarization angle is set at a counterclockwise deflection of θ=45° relative to the +x-axis. We obtained reflection amplitude spectra for co-polarization and cross-polarization from the simulation both before and after the VO2 phase transition and calculated the polarization conversion ratio (PCR) and the relative bandwidth of the device using the simulation results. We simulated the current distributions in both the resonant structure and the metal substrate to clarify the physical mechanism responsible for THz-wave polarization conversion and the effect of the VO2 phase transition on the polarization conversion bandwidth. We also analyzed the effects of the thickness of the PI layer and the polarization angle of the incident THz wave on the polarization conversion.
Results and Discussions When VO2 is in the insulating state, the reflection of the cross-polarization component exceeds 80% over the frequency range of 1.58--2.08 THz and PCR exceeds 95%. However, when the VO2 becomes metallic state under electrical triggering, the frequency band with a cross-polarization conversion rate above 80% is reduced to 2.04--2.08 THz, although the PCR in this frequency band remains above 95% (Fig. 2). The relative bandwidth with such a large PCR is thus reduced from 27% to 1.9%. We analyzed the current distribution at the three resonant frequency points f1=1.61, f2=1.88, and f3=2.07 THz. When VO2 is in the insulating state, resonant frequencies occur at 1.61 and 1.88 THz because of polarization conversion caused by magnetic resonance of the incident wave, while a third resonant frequency at 2.07 THz is due to polarization conversion caused by electric dipole resonance of the incident wave. The superposition of these three resonant frequencies forms a broad band. When VO2 is in the metallic state, however, the magnetic resonances at 1.61 and 1.88 THz are eliminated, but the electric dipole resonance at 2.07 THz is not affected. Consequently, the band becomes a narrow-band resonance at this single frequency. We also analyzed the effects of the thickness of the PI dielectric layer and the incident polarization angle on PCR. When the polarization angle of the incident THz wave ranged from 40° to 55°, PCR was >80% (Fig. 5).
Conclusions We have designed a bandwidth-tunable THz polarization converter based on the phase-transition material VO2. When VO2 is triggered to undergo phase transition, the device transforms from a multifrequency, resonance-superposition, broadband device to one with a single resonant frequency. Consequently, the bandwidth for which the cross-polarization reflectivity exceeds 0.8 changes from 1.58--2.08 THz to 2.04--2.08 THz; the relative bandwidth for which PCR is >95% thus decreases from 27% to 1.9%. By analyzing the resonant modes and the accompanying surface-current distributions, we can explain both the mechanisms responsible for the resonances in the device and the reason for the bandwidth change caused by the phase transition. We also analyzed the effects of the thickness of the dielectric medium and the polarization angle of the incident THz wave on the PCR before and after the phase transition of VO2. The proposed method has potential and important applications in THz communication, sensing, detection, and imaging systems.
