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In this paper, a broadband and highly efficient reflective polarization converter based on Z-shaped metasurface is demonstrated at sub-terahertz (THz) frequencies. The polarizer can convert a linearly polarized (LP) wave to its cross-polarized wave at three resonant frequencies, which can also convert a LP wave to a circularly polarized(CP) wave at other two frequencies. The polarization conversion efficiency for normal incidence larger than 80% is obtained from 0.116THz to 0.26THz. The surface current distributions are demonstrated to investigate the physical mechanism. Furthermore, the performance under oblique incidence shows the broad bandwidth can be sustained as incident angle increases to o 45 . The polarizer could be useful for enhancing the efficiency of polarization conversion in THz regime. Introduction Polarization converter is an important device of EM waves [1] , with the deepening of the research, the polarizer at THz frequencies has drawn tremendous attention due to its large range of applications in the area of imaging and communications [2] [3]. Conventional polarization converters use birefringence to realize phase retardation, and they often have large thickness and complicated structure[4] [5]. Recently, metamaterials(MMs) have captured extremely attention due to their unique optical properties[6], such as negative refractive index, optical activity and circular dichroism. Metasurfaces[7] are two-dimensional planar MMs, taking the advantage of their ultra-thin features, devices with miniaturization, high integration and good performance can be designed. Up to date, linear or circular polarization converters based on MMs at microwave frequencies have been demonstrated, such as E-shaped[8], L-shaped[9] and F-shaped [10]polarizes, which have multiband properties. However, the polarization conversion only take place in several resonant frequencies with these devices. In another approach, a polarizer with broad bandwidth based on cut-wire was demonstrated[11]. Unfortunately, the development at THz frequencies is restricted due to the lack of suitable materials and huge fabrication challenges. Previously, Grady et al. have designed an ultrathin broadband linear polarization converter at THz frequencies[12], after that the split-ring resonators (SRRs) and disk resonators (together henceforth referred to as DSRRs) structure were proposed which can realize broadband polarization conversion in THz regime [13]. Another reflective linear polarization converter in the near-infrared region was proposed based on ellipse-shaped MMs[14]. These methods make it possible to design a polarization converter at THz frequencies. However, designing a polarizer with higher efficiency and broader operating bandwidth remains an issue. In this paper, a broadband and highly efficient polarizer based on Z-shaped metasurface is proposed at sub-THz frequencies. The structure consists of three layers, which can realize a LP wave converts to its cross-polarization wave at the frequencies of 0.123, 0.18 and 0.248 THz, simultaneously, the LP wave converts to a CP wave at the frequencies of 0.107 and 0.273THz. The 6th International Conference on Mechatronics, Materials, Biotechnology and Environment (ICMMBE 2016) © 2016. The authors Published by Atlantis Press 427 polarization conversion efficiency for normal incidence larger than 80% can be obtained from 0.116THz to 0.26THz. The proposed structure has simple geometry, broad working bandwidth and high efficiency compared with previous designs. Design and Simulation The stereogram of the polarizer and the diagram of unit cell are demonstrated on Fig.1, respectively. The structure is consisted of three layers, the top layer is the Z-shaped metasurface, the two side arms are perpendicular to the central cut-wire, respectively. The middle layer is dielectric substrate and the bottom layer is full metallic sheet. The metal material is aluminum (with a conductivity of 3.8×S/m), and the dielectric substrate is quartz (with a dielectric constant 4.41 and a loss tangent 0.0004), the thickness for this layer is t. The angle of deflection α is 30 , which can ensure the coupling to be strongest. Optimized geometric parameters are as follows: p=550 m m , w=410 m m , t=180 m m , m=190 m m , d=30 m m , g=90 m m . Fig. 1. (a) stereogram of the proposed polarizer (b) the diagram of unit cell To better understand the polarization conversion behind the designed structure, we define r / yy yr yi E E = and r / xy xr yi E E = to represent the reflection ratio y-to-y and y-tox polarization conversions, respectively. The subscripts i and r represent incident and reflected EM waves. The subscripts x and y indicate the polarization patterns of EM waves. Define the polarization conversion ratio ( PCR ) as 2 2 2 | | /(| | | | ) xy xy yy PCR r r r = + for y-polarization incidence waves. Phase differences between the y and x components of the reflected EM wave are defined as xy φ ∆ , xy φ ∆ can take arbitrary within [ 180 , 180 ] o o − + depending on the frequency, meaning that all polarization states (circular, linear, elliptic) are possible for the reflected beam. 0 xy φ ∆ = (or 180 o ) indicates a linearly polarized state. The LP wave converts to a CP wave at the situation of | | | | xy yy r r = and 90 xy φ ∆ = , when 90 o xy φ ∆ = + ,which indicates a left-handed circularly polarized (LCP) wave, and 90 xy φ ∆ = − ,which indicates a right-handed circularly polarized (RCP)wave. Others are elliptically polarized waves. Results and discussion Finite integral method was used to analysis the polarization conversion of the proposed device. Fig. 2(a) shows the simulated results of reflectance when the incident THz beam is linearly polarized. From the Fig. 2(a), one can see that the structure can realize y-to-x polarization conversion over a broad bandwidth, this is mainly because the two side arms have strengthened the internal coupling of each unit which can results in multiple resonances. The bandwidth of the reflectance 2 | | xy r is over 80% from 0.116 to 0.26 THz, and 2 | | xy r is 0.898, 0.945 and 0.899 at the

Broadband and Highly Efficient sub-THz Reflective Polarization Converter based on Z-shaped Metasurface