Abstract
Here we demonstrate simultaneous measurements of the complex transmittance and birefringence using left- and right-handed circularly polarized terahertz (THz) pulses. We change the polarization of the THz pulses periodically by modulating the polarization of the pump pulses directed onto a ZnTe (111) crystal, and we convert linear to circular polarization using a broadband THz quarter-wave retarder. By integrating the alternating-emission system with the polarization-sensitive terahertz time-domain spectrometer, we are able to obtain the electric-field vector of the transmitted terahertz pulses for both the left- and right-handed circular polarizations. Utilizing this technique, we are able to measure simultaneously the frequency-dependent complex refractive indices (real and imaginary parts) and the orientations of the slow and fast axes of birefringent materials, a quartz disc and a barium borate crystal, in a single temporal sweep.
Highlights
Measuring the polarization dependence of the optical constants is very important for THzsensing technologies, because THz waves carry in-depth information about material properties such as anisotropy, chirality, and magnetism
Applications include ellipsometry to characterize the optical properties of bulk and thin-film materials [1], circular dichroism spectroscopy to distinguish left- and right-handed molecules [2], and polarization-resolved spectroscopy to image the elastic strain induced in a material [3]
The recent observation of the Bloch–Siegert shift in Landau polaritons provides a striking example of polarization-resolved THz spectroscopy [6], in which the dependence of the transmittance on the polarization state of the incident THz pulses was measured using polarization-sensitive (PS) THz time-domain spectrometry (TDS)
Summary
Measuring the polarization dependence of the optical constants is very important for THzsensing technologies, because THz waves carry in-depth information about material properties such as anisotropy, chirality, and magnetism. By integrating the alternating-emission system with the PS-THz-TDS system, we were able to measure two orthogonal electric fields for each incident circularly polarized pulse. Where |ET| and θT are, respectively, the absolute value of the amplitude and the orientation of the THz electric field, as illustrated in Fig. 4(a); εp and θp are the ellipticity angle and the azimuthal angle of the probe pulse, as defined in Fig. 4(b); and Ac is a proportionality constant between the electric-field strength and the ellipticity angle, which is determined by the thickness and nonlinear susceptibility of the electro-optical (EO) crystal (ZnTe). By measuring the amplitude and phase of the oscillation of the optical intensity with rotating-analyzer ellipsometry, we can obtain the amplitude and the orientation of the THz electric field. We corrected for these errors by utilizing the dependence of the measured polarization angle upon the actual polarization angle (see Appendix 2)
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