A THZ Tunable Metamaterial Reective Polarization Converter Based Vanadium Oxide Film

In this paper a simple and tunable reflective polarization converter has been investigated numerically based on metamaterial which composes of a two-corner-cut square patch resonator with a slit embedded into Vanadium dioxide film (VO2) and reflective ground layer. All the results obtained by the CST Microwave Studio show that the polarization conversion ratio (PCR) above 90% is achieved from 2.22-5.42THz at the temperature about 25°C under the linearly and circularly polarized wave incidence normally. In addition, the influences on electromagnetic polarization properties have been demonstrated with the insulator-to-metal phase transition of the Vanadium dioxide film (VO2) film by the method of varying the temperature. At the same time, to be demonstrated, the physical mechanism of changeable polarization conversion has been discussed by the distributions of current densities. According to the results, the designed metamaterial could be applied in the area of temperature-controlled sensing, THz wireless communication, tunable polarized devices.


Introduction
Polarization is an enormous important property of the electric-magnetic wave for providing real application in information processing, imaging and sensing [1,2]. As a fundamental phenomenon, polarization converter, that is traditionally accomplished by natural birefringent material or dichroic crystals, has recently acquired much attentions from researchers in different areas. The results can occur attributed to the advent of Metamaterials (MMs). The artificial electromagnetic materials (MMs) not only overcome shortcomings of the effect weakly and the volume bulky incurred by traditional materials but also open a promising platform to control the polarization states of light [3,4]. With regard to MMs converter, many interesting phenomena have been reported in THz spectrum, such as dual-band [5], tri-band [6], multi-band [7] and broadband [8,9] polarization converter.
However, there are only a few attempts to realize dynamic modulation.
The MMs integrated with dynamic inclusions can accomplish tunable polarization conversion modulation. At the present, the thermal-controlled tunable MMs have been seldom investigated [17,18]. In respect of temperature-controlled phase change MMs, VO2, as a novel phase-controlled material, exhibits an insulator-to-metal phase transition [19,20] with conductivity of VO2 changes depending on the various temperatures. Hence, in certain perspective, it is possible and worthwhile that tunable polarization conversion can be achieved by the phase transition of VO2.
In this work, a simple and tunable reflective polarization converter based on metamaterial is proposed in THz spectrum, which composes of a two-corner-cut square patch resonator with a slit embedded into Vanadium dioxide film (VO2) and reflective ground layer. Numerical calculation shows that reflective curve for both linear polarization and circular polarization incidence can hold the same trend with PCR above 90% from 2.22 to 5.42THz. In addition, to obtain tunable polarization conversion performance, the polarization responses with the respect to the various conductivities of VO2 have been investigated in detail. At last, to be interested, the surface current densities depending on different conductivity from 200, 10 4 to 10 5 S/m have also been researched.
Therefore, such dynamic changes of phase transition metamaterial offer a new route for acquiring multifunctional devices, such as switches, polarization modulators. The proposed MMs ( Fig. 1(a)) consist of periodical identical unit cells ( Fig. 1(b)) which composed of hybrid layer and metallic ground layer separated by polyimide dielectric layer with a relative dielectric constant of 3.5 [21] and a thickness (t1) of 10um ( Fig. 1(d)), as schematically shown in Fig. 1(c). It is obviously that the hybrid layer (first layer) comprises a metallic two-corner-cut square patch resonator with a slit and VO2 film. The top and bottom metallic layers are made by the same gold dealt with the Drude model

Descriptions of the Designed MMs and Simulation Model
and a thickness (t2) of 0.2um. Among the hybrid layer, the properties of VO2 film can be expressed by the Bruggeman effective model [23]: Where d  and m  denote dielectric constants of the insulating and metallic phase, respectively.
Moreover, V indicates the volume fraction of the metallic regions. According the above parameters given, the dielectric constant of VO2 can be obtained arbitrary. In our simulation, the relative permittivity of VO2 can be set as 9 [23], in which the conductivity is 200 S/m at the temperature about 25 °C [23]. All the designs, of which the relevant other parameter given below (Table 1), can be carried out by CST Microwave Studio accomplished by frequency domain solver. Periodic boundary condition and port boundary condition are applied to X/Y direction and Z direction, respectively.
The alphabet i and r denote incidence and reflection, respectively. For the incident circular polarization, the subscripts about y and x can be replaced L and R, respectively.

Results and discussions
To characterize the performance of the designed MMs, the reflective amplitude (|rij|) and PCRi for linear and circular polarization incident wave have been investigated, respectively. The subscript i and j represent either x, y or R, L. As exhibited in Fig. 2(a) Where pr=|rxy|/|ryy| and φr=arg(rxy)−arg(ryy). The θ denotes the angle between reflective and incident polarization plane, while the η characterizes the polarization state of the reflective wave. When the η=0° and θ≠90°, the reflective wave keep linear polarization state with an angle of θ referred to the incident wave, but not cross-polarization state. What is more, the pure cross-polarization wave can be obtained when η=0° and θ=90°. As is shown in Fig. 3, the ellipticity η of the incident y-polarized wave stands less than 18° from 2.22 to 5.42THz at whole frequency band and keeps nearly 0° at the around resonant frequency of 2.39, 3.78 and 5.27THz. The deep meaning implies that a near linear polarization wave is obtained around the three resonant frequency. Moreover, Furtherly the cross-polarization conversion can be maintained when the polarization azimuth angle θ keeps ±90° at the same three resonate frequency points in Fig. 3. cross-polarization conversion. The Ey component of induced E has no impact on polarization for its same direction with incident Ei [29]. However, the current densities reflect gradually weakly between top and bottom layer when σVO2 increases from 10 4 to 10 5 S/m as is shown in Fig. 5

Conclusions
A simple and tunable reflective polarization converter is proposed based on metamaterial in THz spectrum, which composes of a two-corner-cut square patch resonator with a slit embedded into Vanadium dioxide film (VO2) and reflective ground layer. The results indicate that high PCR can be obtained for the incident linear and circular polarization wave. In addition, the dynamic thermal tunable polarization converter can also be accomplished by adjusting the conductivities of VO2. To further research, the mechanism of tunable polarization conversion is also analysed by investigating the current densities at the top and bottom layer under the condition of changeable conductivities.
According to the results, the designed metamaterial could be applied in the area of temperature-controlled sensing, THz wireless communication, tunable polarized devices.