Dual rotating-compensator multichannel ellipsometer: Instrument development for high-speed Mueller matrix spectroscopy of surfaces and thin films

Abstract

A multichannel ellipsometer in the dual rotating-compensator configuration has been developed for applications in real time Mueller matrix spectroscopy of anisotropic surfaces and thin films. The sequence of optical elements for this instrument configuration can be denoted PC1r(5ω)SC2r(3ω)A, where P, S, and A represent the polarizer, sample, and analyzer. In this sequence, C1r and C2r represent the first and second compensators, rotating with angular frequencies of 5ω and 3ω, respectively, where ω=π/TC is the base angular frequency (corresponding to 2 Hz) and TC=0.25 s is the fundamental optical period. The instrument can provide 170 point spectra over the wavelength range from 235 nm (5.3 eV) to 735 nm (1.7 eV) in all 16 elements of the unnormalized Mueller matrix with minimum acquisition and repetition times of TC=0.25 s. In this study, instrumentation calibration procedures are demonstrated in the transmission geometry without a sample, including (i) alignment of the two MgF2 zero-order biplate compensators, (ii) determination of the retardance and phase spectra for the compensators, (iii) determination of the offset angles for the optical elements, and (iv) characterization of the spectral response function of the ellipsometer. Calibration procedure (iv) allows the (1,1) element of the transmission Mueller matrix to be determined; thus the unnormalized Mueller matrix can be obtained. The dual rotating-compensator multichannel ellipsometer is assessed with respect to its performance in transmission without a sample, and is then applied in the transmission geometry to study anisotropy, depolarization, and light scattering effects for a MgF2 helicoidally sculptured thin film on a glass substrate. Numerous future applications of this instrument are anticipated for real time analysis of complex surfaces and thin films in the reflection geometry as well.