Multivalued electro optic response in near-field THz polarimetry

A robust calibration method to correct instrumentation artifacts in near-field THz polarimetry is reported. An unexpected nonlinear dependence of the NIR quarter waveplate angle with respect to each NIR half waveplate angle was observed in one of two balanced response curves using a time-domain spectroscopy configuration. Signal degradation stems from this multivalued electro-optic response and is associated with an unwanted frequency dependence. Our calibration method removes these artifacts from ZnTe absorbance spectra by normalizing all E-field measurements at every THz polarization angle across the bandwidth of interest (~0.5 – 100 cm^-1) and sampling the ZnTe region from which sample measurements are subsequently made. Remaining THz polarization dependencies are additionally removed by normalizing absorbance spectra with respect to the E-field maximum of the reference THz angle spectrum. Reproducible absorbance results were obtained on several triose phosphate isomerase (TPI or TIM) protein crystals with several apertures to sample different regions of the ZnTe crystal. Serendipitously, this method also corrects for previously reported deleterious effects due to defect nonuniformities in ZnTe electro-optic crystals commonly used in near-field polarimetry.