**3.2 Polarization devices based on the dielectric metasurface**

In order to implement the high-efficiency polarization devices at the THz frequency, Si micro-brick arrays have been investigated in this design [9]. Since there is very low absorption loss for the silicon crystal around 1THz, the refractive index *n* and the dielectric constant å are 3.41 and 11.6. The anisotropic property with symmetric axes along x and y directions makes it possible to control x and y polarized field separately without the cross-polarization coupling effect. In this structure, Si micro-brick arrays are attached to the TPX substrate for the purpose of low absorption loss, the suitable refractive index matching, stable mechanical properties and the adhesion. The Si micro-brick unit with substrate is shown in **Figure 5**, where a double-phase modulation THz metasurface is based on the Si micro-bricks, in which the side length of the Si micro-bricks, *L*x and *L*y are
