Evidence of frequency-dependent THz beam polarization in time-domain spectroscopy

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Abstract
THz time-domain spectroscopy is a powerful and fast technique for measuring the complex dielectric constant of materials over a wide range of frequencies. This technique can be applied to the characterization of anisotropic materials only if the polarization of the THz beam impinging on the sample is perfectly known. In this paper, we show that the polarization of the THz beam radiated by a standard THz antenna, composed of a photoconductive switch and an extended hemispherical lens, is frequency dependent. We experimentally observe that the polarization of the THz field is linear but its direction varies over more than 10 degrees in the 100 GHz - 1 THz range. This variation can be attributed to a slight misalignment of the antenna. Using a thick slide of a highly anisotropic material acting as a polarization temporal separator, the incident THz pulse is divided into two pulses with orthogonal linear polarizations. By temporal windowing, we keep only one of the two linear polarizations, and thus free ourselves from the response of emitting antenna in terms of polarization. Then, a precise characterization of any anisotropic sample along one of its optical axis becomes possible.

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