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Abstract
Broadband and switchable versatile polarization metamaterial is crucial in the applications of imaging, sensing and communication, especially in the terahertz frequency. Here, we investigated versatile polarization manipulation in a hybrid terahertz metamaterial with bilayer rectangular rods and a complementary vanadium dioxide (VO2) layer. The VO2 phase transition enables a flexible switching from dual-band asymmetric transmission to dual-band reflective half-wave plate. The full width half maximum (FWHM) bandwidths of dual-band asymmetric transmission are 0.77 and 0.21 THz, respectively. The polarization conversion ratio (PCR) of the reflective metamaterial is over 0.9 in the frequency ranges of 1.01-1.17 THz and 1.47-1.95 THz. Angular dependences of multiple polarization properties are studied. The proposed switchable polarization metamaterial is important to the development of multifunctional polarization devices and multichannel polarization detection.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz (THz) technology has received considerable attention due to its potential in the fields of spectrum measurement, imaging and communication [1–5]. Effective THz functional devices are highly desired as crucial components of terahertz systems. However, conventional materials weakly interact with terahertz waves, which seriously hinders the rapid development of THz devices. Recently, metamaterials provide a new avenue to enhance the interaction between matter and terahertz waves. Metamaterials are composed of artificial elements on the subwavelength scale, exhibiting fantastic electromagnetic properties that are inaccessible in naturally occurring materials [6]. Metamaterials can powerfully manipulate amplitude, phase, polarization and propagation direction of the electromagnetic wave [7–16], providing an opportunity for developing THz functional devices. Especially, the realization of metamaterial-based polarization devices has attracted increasing attention due to their high efficiency, compactness and multifunctional integration. Polarization manipulation may be implemented by means of chirality, anisotropy, plasmonics and Pancharatnam-Berry (PB) phase [17–21]. Various polarization properties and applications, such as polarization conversion [22–25], optical activity [26,27], circular dichroism [17,28,29], holograms [30–32], asymmetric transmission (AT) [19,28,33], and wavefront shaping [34,35], wireless communication [36,37], play an indispensable role in practical microwave, terahertz and optical systems. In addition, dynamically tunable polarization behaviors have been accomplished by hybridizing metamaterials with active materials, including graphene, silicon, Dirac semimetal (DSM) and VO2 [38–43], however many efforts have always been focused on on/off switching of single polarization response. Facing the requirements and challenges of integrated photonics, developing versatile switchable metamaterials devices has become an emerging research topic by use of the phase transition of VO2 [44–50]. Zhang et al. proposed a switchable hybrid metamaterial between AT effect and reflective quarter-wave plate (QWP) incorporating DSM and VO2 [44]. Ding et al. demonstrated bifunctional broadband metasurfaces that can be switched from an absorber to a reflecting half-wave plate (HWP) based on VO2 phase transition, therein the HWP has cross-reflectance higher than 60% and PCR over 95% in the frequency range of 0.63 to 1.12 THz [46]. Luo et al. presented a metal-VO2 switchable wave plate metamaterial, which can be switched from HWP to QWP with a relative bandwidth of 71.8% [47]. Li et al. designed a metasurface with embedded VO2, showing switchable bifunctional properties between band-stop filter and the AT effect under different external temperature [50]. Shu et al. proposed an electrically driven polarization switching of the reflected VO2 metasurface between the waveplate and the mirror [51]. However, most of these metamaterials focus on reflective polarization manipulation. Even though there have been a few reported transmissive and reflective versatile polarization switching, they suffer from complicated structural designs. Besides, switchable terahertz metamaterial between dual-band AT effect and HWP still remains unexplored.
In this work, we propose a bifunctional hybrid metamaterial with a polarization switching effect of dual-band AT and dual-band HWP. The metamaterial is composed of bilayer twisted rectangular aluminum rods and a complementary VO2 layer. The flexible switching of the polarization states can be manipulated by temperature-controlled conductivity of the phase change material VO2. The hybrid metamaterial can realize mutual dual-band AT of linearly polarized wave when the VO2 film is in an insulating phase. Two FWHM bandwidths are 0.77 and 0.21 THz, respectively. The hybrid metamaterial can realize x- to y-polarization conversion and act as a reflective dual-band HWP when the VO2 film is in a metallic phase. The reflection coefficient ryx is higher than 0.8 with PCR over 0.9 in the frequency ranges of 1.06-1.17 THz and 1.47-1.95 THz. In addition, the effect of angle of incidence is investigated on the switchable polarization performances. Such versatile terahertz polarization switching is definitely beneficial to the development of integration and miniaturization for on-chip optical systems.
2. Discussion and results
Rectangular rod, as a typical building element of metamaterial, has been extensively investigated to achieve anomalous refraction, AT effect of circularly polarized wave and birefringence [19,23,52,53]. Here, the VO2 film will be incorporated into the design of bilayer switchable metamaterial for realizing dynamical terahertz response and bifunctional polarization manipulation, as schematically shown in Figs. 1(a) and 1(b). In Fig. 1(a), mutual dual-band asymmetric transmission is shown in the hybrid metamaterial when the VO2 film is in the complete insulating state, while reflective half-wave plate is obtained in the hybrid metamaterial in Fig. 1(b) when the VO2 film is in the complete metallic state. The polarization functional switching of the metamaterial is dictated by the thermally-controlled phase transition of VO2. In Fig. 1(c), the unit cell of the hybrid metamaterial consists of twisted bilayer rectangular aluminum rods, a cyclic olefin copolymer (COC) dielectric layer and a complementary VO2 film. The bilayer rectangular aluminum rods are coaxial and geometrically identical, but there is a twist angle of 45° between the front and back aluminum rods. The structural dimensions of the rectangular rods are given below, w = 35µm, l = 80µm and t1 = 200nm. The bilayer rectangular rods are separated by a COC layer with a thickness of t = 20µm. The 200nm-thick VO2 film is patterned complementary to rectangular aluminum rod. The periods of the unit cell along the x and y directions are a = 100µm and b = 95µm, respectively. The polarization properties of the hybrid metamaterial are performed using 3D full-wave numerical simulations (CST Microwave Studio). The periodic boundary conditions are applied in both x and y directions. Here, aluminum can be regarded as a lossy metal with the conductivity of 3.56×107 S/m, and the refractive index of COC is 1.53 with a loss tangent 0.004 at terahertz frequencies [54]. Drude model is applied to describe the permittivity of VO2 in the THz frequencies [47],
Here σ0 = 3×105 S/m and ωp (σ0) = 1.4×1015 rad/s. The conductivity values 200 S/m and 105 S/m are assumed to denote the complete insulating and metallic phases [46]. The calculated reflection and transmission coefficients can be denoted by Rdij (rdij =| Rdij |) and Tdij (tdij =| Tdij |). The subscripts i and j stand for the polarization state of the output and incident waves, and the superscript d corresponds to forward (f, along the -z axis) or backward (b, along the + z axis) wave propagations.
Fig. 1. Schematics of bilayer switchable metamaterial based on VO2 phase transition. (a) Mutual dual-band asymmetric transmission of the hybrid metamaterial when the VO2 film is in the complete insulating state. (b) Reflective half-wave plate of the hybrid metamaterial when the VO2 film is in the complete metallic state. (c) Stereogram of the unit cell in the proposed hybrid metamaterial. The unit cell is composed of twisted bilayer aluminum rods and a complementary VO2 layer.
The VO2 film is in the complete insulating state at room temperature. The hybrid metamaterial is equivalent to an array of 45°-twisted bilayer rectangular rods. Transmission coefficients are plotted in Figs. 2(a) and 2(b) for normally incident forward (-z) and backward (+z) polarized waves. It can be seen that the responses of co- and cross-polarization coefficients meet the reciprocity theory [55]. The spectrum of tfyx manifests two distinct resonant peaks with maxima of tfyx = 0.75 and tfyx = 0.73 at 1.24 and 1.75 THz, respectively. The peak values are limited by the metamaterial’s reflection and absorption, and the performance may be improved by introducing an antireflection coating layer or adopting all-dielectric meta-atoms [18,56]. While the cross-polarization coefficient tbyx for backward wave propagation is very small, it has an averaged value of around 0.1 in the lower frequency range and even reduces to zero at about 1.41 THz. These results indicate that the metamaterial allows polarization conversion transmission of incident x polarized wave, while it prevents the cross-polarization transmission of x polarized wave for the reversed propagation. Similarly, the other transmission coefficient tfxy has a resonance maximum of 0.75 at 2.52 THz, where tbxy exhibits a weak transmission less than 0.2. The phenomenon, referring to the difference in the transmission intensity for a fixed incident polarization state between two opposite propagation directions, is known as asymmetric transmission. The strength of the AT effect can be determined by the relevant parameters:
Fig. 2. Polarization properties of bilayer hybrid metamaterial with dielectric VO2 film. The simulated results of the four transmission coefficients along the (a) forward and (b) backward wave propagations. (c) Asymmetric transmission parameters of circularly and linearly polarized waves along the forward propagation. (d) Circular dichroism.
To gain insight into the underlying physical origin of the AT effect, the instantaneous surface current distributions at three resonances are presented in Fig. 3. For a normally incident x-polarized wave as shown in Figs. 3(a) and 3(b), the bonding and antibonding modes are individually excited at 1.24 and 1.73 THz, where the strongest surface currents are always excited along the long sides of the resonators. When the incident wave is y-polarized, an approximately antisymmetric surface current mode is exited at 2.52 THz as shown in Fig. 3(c). The coupling strength between the front and back layers can be tailored by changing the thickness of the spacer layer.
Fig. 3. Surface current distributions at three resonant asymmetric transmission peaks when the VO2 film in the bilayer metamaterial is in the complete insulating phase. (a) 1.24 THz, (b) 1.73 THz and (c) 2.52 THz.
When the VO2 film working at above critical temperature is metallic state, the hybrid metamaterial becomes a typical sandwiched structure that contains single-layer 45°-oriented rectangular rod and a continuous metallic plate separated by COC dielectric spacer. The reflection coefficient ryx is higher than 0.8 with the maximum of 0.88, while rxx is less than 0.38 within the frequency range of 1.06-2.07 THz as shown in Fig. 4(a). Further, polarization conversion rate (PCR) can be used to evaluate the polarization conversion ability of the hybrid metamaterial,
The PCR maintains over 0.9 in the frequency ranges of 1.01-1.17 THz and 1.47-1.95 THz, and even reaches to 1 at 1.07 and 1.71 THz as shown in Fig. 4(b), indicating that the incident x-polarization is largely converted to y-polarization across a wide frequency range. The working principle of the cross-polarization conversion is next introduced, and the incident x-polarized wave is decomposed into two orthogonal components along the u- and v-axes. The incident and reflected electric fields are given as:
Here, the cross-polarization conversion coefficients are almost zero (ruv = rvu ≈ 0) due to symmetry of the structure along the u- and v-axes, and the reflected electric field is mainly dictated by the co-polarization. The magnitudes of two co-polarization coefficients are almost equal (ruu = rvv) and the phase difference (Δφ = φuu - φvv) is approximately 180° within the frequency range of 1.06-1.17 THz and 1.47-1.95 THz in Figs. 4(c) and 4(d). As a result, there are two key conditions for achieving half-wave plate.
Fig. 4. (a) The spectra of co- and cross-polarization reflection coefficients for normally incident x-polarized wave excitation along the forward propagation (-z). (b) Polarization conversion rate. (c) The reflection coefficients and (d) phases of the hybrid metamaterial when the VO2 film is in the complete metallic state for normal u and v polarization incidences, as depicted by the inset.
In order to further identify the physical mechanism of the observed reflective polarization conversion, current distributions of the 45°-oriented rectangular rod with respect to the x axis are shown in Fig. 5. The incident electric field Ex is decomposed into Eu and Ev, as illustrated in Fig. 5(c). At 1.07 THz, the rectangular rod can be regarded as an electric dipole resonator that couples with the Eu-component in Fig. 5(a), accompanied by 0° phase shift. While the Ev-component is reflected off by the hybridization of aluminum rod and metallic VO2 film, π phase shift is induced by the continuous metallic plate. An electric dipole is excited by the Ev-component at 1.71 THz as shown in Fig. 5(b), and the structure doesn’t resonate with Eu-component. Therefore, the phase difference between Eu- and Ev-component is π in the two frequencies, which generates cross-polarization conversion for the incident x-polarized wave. The underlying mechanism in relation to phase difference is consistent with the numerical simulation as shown in Fig. 4(d).
Fig. 5. Surface current distributions of the 45°-oriented rectangular rod with respect to the x axis at the two resonances when VO2 film is complete metallic phase. (a) 1.07 THz and (b) 1.71 THz. (c) Schematic diagram of the decomposed electric field vector.
The polarization ellipses of the output wave are presented in Fig. 6 with different VO2 conductivity (provided that the hybrid metamaterial works in different temperatures). Figures 6(a) and 6(b) show the polarization states of transmitted light at 1.24 and 2.52 THz for normally incident x- and y-polarized waves, respectively. At room temperature (σVO2 = 200 S/m), the rotated polarization azimuth and ellipticity angles of the transmitted y-polarized wave are -76° and 10°, while those of the transmitted x-polarized wave are -88° and 5° [57], indicating that the polarization state of the transmitted wave is changed to its cross-polarization. As the conductivity increases, the intensity of transmitted wave gradually diminishes. When the VO2 film is complete metallic phase, the intensity is almost zero. These results imply that the dual-band AT effect in the hybrid metamaterial is switched “off” during the phase transition. Figures 6(c) and 6(d) illustrate the polarization properties of the reflected wave at 1.07 and 1.71 THz for normal x-polarized incidence. The reflected wave gradually changes from elliptical- to y-polarization accompanied by enhanced intensity with increasing VO2 conductivity. The polarization azimuth and ellipticity angles of the reflected wave are 87° and -0.6° at 1.07 THz, and those of the reflected wave are -85° and 0.7° at 1.71 THz when the conductivity is 200000 S/m. When the insulator-to-metal phase transition of the VO2 film occurs, the hybrid metamaterial manifests itself as a dual-band half-wave plate.
Fig. 6. Polarization ellipses of the output wave with different VO2 conductivity. Polarization ellipses of the transmitted wave (a) at 1.24 THz for normal x-polarized and (b) at 2.52 THz for normal y-polarized illumination. Polarization ellipses of the reflected wave at (c) 1.07 and (d) 1.71 THz for normal x-polarized illumination.
It is of great significance to investigate how angle of incidence affects the polarization performance of the transmissive metamaterial. The polarization properties of the transmitted wave vary with the angle of incidence in Fig. 7 when the VO2 film is in an insulating phase. The metamaterial allows mutual dual-band AT effect of linearly polarized wave at small incident angle as shown in Fig. 7(a). The bandwidth of the low-frequency AT effect becomes narrower when the incident angle gradually increases, resulting from a large red-shift of the resonance at 1.73 THz. In addition, the high-frequency AT effect nearly vanishes when θ = 50°. The maxima of the AT effect at high and low frequencies are 0.60 at θ = 50° and -0.62 at θ = 30°. The AT effect of circular polarization shows a distinct shift to lower frequency with increasing incident angles and it has a maximum of 0.45 at 2.0 THz around θ = 58°, as shown in Fig. 7(b). Figure 7(c) demonstrates an angular-robust CD pass band at around 1.20 THz with a constant value of 0.53 and the maximum of the CD spectrum is -0.72 at 1.82 THz when θ = 45°.
Fig. 7. Polarization performances of the transmissive hybrid metamaterial for different incident angles when the VO2 film is complete insulating phase. (a) $\Delta _{\textrm{lin}}^x$. (b) $\Delta _{\textrm{cir}}^ + .$ (c) CD.
The relationship between the properties of the half-wave plate and angle of incidence is shown in Fig. 8. The dual-band half-wave plate performance is in “on” state within the incident angle range of 0°-40°. With the increase of incident angle, the properties of half-wave plate at 1.71 THz gradually fade away. Instead, another pass band of the half-wave plate occurs at higher frequencies, and ryx is higher than 0.8 with the PCR of 0.95 in the frequency range of 2.00-2.15 THz.
Fig. 8. Polarization properties of half-wave plate as a function of incident angle. (a) ryx. (b) PCR.
3. Conclusion
In conclusion, we have demonstrated a bifunctional hybrid metamaterial that consists of bilayer twisted rectangular aluminum rods and a complementary VO2 layer. The hybrid metamaterial can be flexibly switched from transmissive to reflective polarization properties during the phase transition of the VO2. The metamaterial can implement mutual dual-band AT effect with the AT parameters of 55% when the VO2 film is in the complete insulating state. The metamaterial can realize dual-band half-wave plate with PCR of over 0.9 in the ranges of 1.01-1.17 THz and 1.47-1.95 THz when the VO2 film is in the complete metallic state. The proposed switchable THz metamaterials are beneficial in designing polarization devices and pave an avenue to the development of miniaturized or integrated THz systems.
Funding
National Natural Science Foundation of China (62175049); Natural Science Foundation of Heilongjiang Province (ZD2020F002); Fundamental Research Funds for the Central Universities (3072021CF2508, 3072021CFT2501); 111 Project(B13015) to the Harbin Engineering University.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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