Elsevier

Optics Communications

Volume 468, 1 August 2020, 125802
Optics Communications

Multifunction tunable broadband terahertz device for polarization rotation and linear asymmetric transmission based on Dirac semimetals

https://doi.org/10.1016/j.optcom.2020.125802Get rights and content

Abstract

A three-layer complementary strip Dirac semimetal polarizer is proposed to achieve polarization rotation, perfect polarization conversion, and asymmetric transmission. This device can realize broadband polarization operation of linear polarized waves in the terahertz frequency domain. By changing the Fermi level of the Dirac semimetal, the polarization angle of linearly polarized light in the frequency range of 1.3 to 1.63 THz can be rotated from 0°to 90°, and the device has good robustness for linearly polarized light with different incident angles. When used as an asymmetric transmission device, its polarization conversion efficiency reaches more than 98% and the asymmetric transmission parameter reaches more than 50% in the range of 1.3–1.63 THz. The asymmetric transmission parameters, polarization conversion ratio, polarization rotation angle, and ellipticity are used to describe the performance of the converter.

Introduction

Metamaterials are widely used in electromagnetic-wave regulation, because they have unique properties, such as a negative refraction index [1], perfect lens [2], invisible cloaks [3], and negative conductivity [4], different from natural materials. Polarization is one of the important characteristics describing the propagation properties of electromagnetic waves. In recent years, research on the polarization of optical activity with metamaterials has received increasing attention, such as asymmetric transmission (AT) [5], [6] and polarization convertors [7], [8] or rotators [9]. For AT, research has focused on chiral metamaterials [10], [11], [12], surface plasmon polaritons [13], and nonsymmetric gratings [14], [15]. For polarization conversion, H-shaped [16], double-head arrow [17], and split-ring [18] resonators, as well as other structures, were proposed to achieve highly efficient polarization conversion of linearly polarized waves. However, most of these devices are made of traditional noble metals, and the operating frequency is fixed after the device has been fabricated. It cannot be dynamically tuned to be the limit of the devices’ practical application. At present, devices made of tunable metamaterials [19], [20], [21], [22] have a wider range of applications in practice. Graphene is a hexagonal array of single-layer carbon atoms whose optical response closely depends on the Fermi level and can be dynamically manipulated by the gate voltage [23]. To date, tuneable AT [24], [25], [26], [27], [28], [29] and polarization converters [30], [31], [32] based on graphene metamaterial have been widely proposed. In terms of polarization rotation, Zhang et al. [33] designed a metal grating and graphene stack structure to achieve an arbitrary deflection angle of 20° 70°. Wei et al. [34] introduced an elliptical-ring single-layer graphene polarization rotator to turn the reflective light polarization direction between nearly 0° and 90°. However, graphene is a two-dimensional single-layer material with instability and a weak coupling effect with light.

Dirac semimetals (DSs), known as three-dimensional graphene analogues, have recently stimulated in-depth investigations of terahertz sensor devices because of their remarkable properties [35], such as greater stability in ambient environments [36], higher Fermi velocity [37] and carrier mobility [38]. The dynamic conductivity of DSs can also be dynamically tuneable by alkaline surface doping [39]. DSs have higher carrier mobility, reaching 9 × 106 cm2 V−1 s−1 under an appropriate condition of 5 K, whereas the mobility of graphene can reach 2 × 105 cm2 V−1 s−1 [40] under the same conditions. These excellent properties make Dirac semimetal (DS) a new potential plasmon material for tuneable metamaterial absorbers [41], [42], plasma-induced transparency [43], [44], and polarization convertors [45]. DSs have a metal response when the frequency is lower than the Fermi level, causing DSs to show a metallic property in the terahertz frequency range. Although polarization-related devices have been widely researched, they only have independent single functions, and there have been few reports on polarizers with multiple functions. A terahertz device that implements multiple functions through one device is more popular.

In this study, a three-layer complementary strip DS polarizer was investigated to achieve multiple polarization-related functions for linearly polarized waves by dynamically adjusting the Fermi level of different DSs layers. The designed polarizer makes AT, perfect polarization conversion, and polarization rotation possible. By changing the Fermi level of the DSs, one can achieve different rotation angles and switched on/off of the AT phenomenon. It is concluded that the design can be used for optical isolators, polarizers, and broadband polarization rotators for terahertz imaging and communication.

Section snippets

Model and simulation

The three-layer DS complementary strip polarizer designed is shown in Fig. 1(a). The first layer of the structure is a DS board etched by two strip-shaped grooves parallel to the x-axis direction, which can behave as a perfect reflection layer because of the metallic properties of DSs for x-polarized incident waves. Fig. 1(b) is a side view illustrating the thickness parameters of each layer, DS-layer thickness t, dielectric-layer thickness tp, and silica-layer thickness ts. As shown in Fig. 1

Tuneable broadband polarization rotation

According to the working state of Fig. 1(e), the transmission, phase difference, and rotation angle θ corresponding to three different states were obtained, as shown in Fig. 2. As shown in Fig. 2(a), state 1 works with the mode 1 ‘on’ state, the parallel polarization component dominates, the rotation angle is close to 0° at a frequency of 1.3–1.63 THz, and the phase difference of the two transmission components Tyy and Txy is nearly 0°, indicating that the transmitted wave is a linear polarized

Conclusions

A three-layer DS grating polarizer was designed that combines multiple functions, such as AT, perfect polarization conversion, and rotation. By changing the Fermi level of modes 1 and 2 of the third layer, one can achieve a deflection of the rotation angle from 0° to 90° in a frequency range of 1.3–1.63 THz. The designed polarizer is insensitive to the oblique incident angle. When the incident angle is 80°, the difference of polarization azimuth angle is approximately 20° compared with the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 61875106 and 61775123), Key Research and Development Program of Shandong Province, China (Grant Nos.2019GGX104039 and 2019GGX104053), the Shandong graduate student tutor guidance ability promotion program project, China (SDYY17030), the National key research and development program of China (2017YFA0701000), and the Scholarship Fund of SDUST.

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