All-dielectric InSb metasurface for broadband and high-efficient thermal tunable terahertz reflective linear-polarization conversion

The two-dimensional (2D) equivalent form of metamaterial (MM) known as metasurface (MS) is artificial planar layered material/structure, which has aroused great attention for its unique and steerable electromagnetic (EM) characteristics unavailable in natural materials [1], [2]. MS has some advantages of low profile, deeply sub-wavelength thickness and easy fabrication [3], which can be used to construct various photoelectric devices [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], such as beam deflectors, vortex beam generators, angular momentum generators, focusing lenses, absorbers, and polarization rotators/convertors. Specifically, one of the valuable MS devices is the terahertz (THz) polarization converter due to its diverse application in imaging, communication and other optoelectronic areas with the rapid development of THz technology [15], [16]. In recent years, various chiral and anisotropic structure MSs-based polarization converters have been designed and investigated intensively in THz regime [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], which can convert the linear-polarization (LP) wave to cross-polarization or circular-polarization wave or each other. Generally, chiral structure MSs are used to design transmissive type polarization convertors [18], [19], [20], [21], while anisotropic structure ones are convenient for designing reflecting-type polarization convertors [22], [23], [24], [25], [26], [27], [28]. Although these polarization convertors have high efficiency and broad bandwidth, most of them cannot be tuned once the design and fabrication are finished, which are serious obstacle hindering widespread applications.

More recently, some feasible approaches have been proposed and investigated to achieve tunable polarization conversion for THz waves by integrating the active materials (such as graphene, VO2, photoconductive Si) into the MS structures [28], [29], [30], [31], [32], [33], [34], [35], [36]. For example, Yu, et al. proposed a broadband tunable reflecting-type polarization converter based on I-shaped metallic resonators and graphene sheets, which could switch dynamically its functionalities among different polarization state in THz regime [29]. Zhao, et al. present a VO2-assisted temperature-controlled multifunctional MS structure for polarization conversion and asymmetric transmission  [33], which can achieve the switch between reflection and transmission mode by changing external temperature. Zhao, et al. demonstrated numerically a MS structure integrated with the photoconductive Si [34], which can achieve a switchable broadband reflective LP conversion for THz waves. These polarization convertors have some advances, such as flexibility and tunability, however, which are usually demanding in terms of complicated structures and high-cost. In addition, these works are mainly based on metallic patterned resonator structures, and they will suffer high Ohmic losses when extending to the higher frequencies. Therefore, it is highly desirable to design a simple MS structure that allows tunable high-efficient polarization conversion over a broadband range for THz wave. In particular, the reflecting-type LP conversion needs to be further explored and investigated.

In this work, we will demonstrate numerically an all-dielectric InSb MS, which can realize the broadband and high-efficient temperature tunable reflective LP conversion in THz regime. The unit-cell of the proposed all-dielectric InSb MS is made of the InSb anisotropic micro-cuboid structure backed with an InSb substrate. Firstly, the reflective LP conversion property of the proposed InSb structure will be demonstrated at a given temperature of 295 K. Secondly, the physical origin of the all-dielectric InSb MS will be illustrated by analyzing the polarization decomposition and the distributions of electric field on the unit-cell. Finally, the influence of the structure parameters and surrounding temperature on the LP conversion property of the MS also will be studied systematically.

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