Experimental investigation on generalized Brewster effect for s-polarized terahertz waves at planar Si-air interfaces with a thin doped layer.

The disappearance of reflection at dielectric interfaces, called the Brewster effect (BE), is one of the fundamentals of optics. Although it occurs, in the original concept, only for p-polarized waves incident at a specific angle, called the Brewster angle (BA), which is determined solely by the permittivity of the two media, the BA for p-polarization is altered, and a similar zero-reflection can also occur for s-polarization in the cases involving magnetic material, anisotropic material, two-dimensional (2D) conducting material such as graphene, and artificial structures such as metamaterial and metasurface. They are called generalized Brewster effect (GBE) and are expected to be a noble tool for various wave control including efficient absorption. In addition, we have recently found that the GBE for the s-polarization at a flat interface sandwiching a conducting layer, similar to the 2D material case, is a key for improving the sensitivity of terahertz spectroscopy to the carrier transport at semiconductor surfaces. However, so far, there has been no report of an experiment on the GBE for s-polarization at such a flat dielectric interface. In this work, we present an experiment that clearly demonstrates the GBE of this type using a thin doped layer on the surface of a Si substrate.