Characterization of linear diattenuator and retarders using a two-modulator generalized ellipsometer (2-MGE)

The two-modulator generalized ellipsometer (2-MGE) is a spectroscopic polarization-sensitive optical instrument that is sensitive to both standard ellipsometric parameters from isotropic samples as well as cross polarization terms arising from anisotropic samples. In reflection mode, the 2-MGE has been used to measure the complex dielectric functions of several uniaxial crystals, including TiO₂, ZnO, and BiI₃. The 2-MGE can also be used in the transmission mode, in which the complete Mueller matrix of a sample can be determined (using 4 zone measurements). If the sample is a linear diattenuator and retarder, then only a single zone is required to determine the sample retardation, diattenuation, the principal axis direction, and the depolarization. These measurements have been performed in two different modes: 1) Spectroscopic, where the current wavelength limits are 260 to 850 nm, and 2) Spatially resolved (current resolution ∼30-50 microns) at a single wavelength. The latter mode results in retardation, linear diattenuation, and principal axis direction "maps" of the sample. Two examples are examined in this paper. First, a simple Polaroid film polarizer is measured, where it is seen that the device behaves nearly ideally in its design wavelength range (visible), but acts more as a retarder in the infrared Second, congruently grown LiNbO₃ is examined under bias. These results show that there are significant variations in the electric field-Pockels coefficient product within the material. Spectroscopic measurements are used to determine the dispersion of the r₂₂ Pockels coefficient.