Terahertz ellipsometry based on the long-distance diffraction-free beam

We introduce a long-distance terahertz (THz) diffraction-free beam (DFB) to the quasi-optical THz ellipsometry. Due to the distinctive characteristics of DFBs, the reflected arm of the ellipsometer does not need a focusing lens to enhance the signal detected. Since the radiation emitted from the generator is linearly polarized, there is no need to place a polarizer on the incident arm. The THz ellipsometry with the lensless reflected arm completed the thickness measurements of non-absorbent films with thicknesses in the wavelength order. The measured values are consistent with the results measured by the micrometer. In addition, we quantitatively analyzed errors that commonly occur in this ellipsometry. The DFB generating system, which consists of two lens-axicon doublets, is fairly simple to implement. We firmly believe that the proposed approach can be easily transplanted to other types of THz ellipsometry.