Microscopic thin film optical anisotropy imaging at the solid-liquid interface.

Abstract

Optical anisotropy of thin films has been widely investigated through ellipsometry, whereby typically an optical signal is averaged over a ∼1 cm(2) elliptical area that extends with increasing angle-of-incidence (AOI). Here, we report on spectroscopic imaging ellipsometry at the solid-liquid interface applied to a supported lipid bilayer (SLB). We detail how a differential spectrally resolved ellipsometry measurement, between samples with and without optically anisotropic thin film on an absorbing substrate, can be applied to recover in and out of plane refractive indices of the thin film with known film thickness, hence determining the thin film optical anisotropy. We also present how optimal wavelength and AOI settings can be determined ensuring low parameter cross correlation between the refractive indices to be determined from a differential measurement in Δ ellipsometry angle. Furthermore, we detail a Monte Carlo type analysis that allows one to determine the minimal required optical ellipsometry resolution to recover a given thin film anisotropy. We conclude by presenting a new setup for a spectroscopic imaging ellipsometry based on fiber supercontinuum laser technology, multi-wavelength diode system, and an improved liquid cell design, delivering a 5 ×-10 × ellipsometric noise reduction over state-of-the-art. We attribute this improvement to increased ellipsometer illumination power and a reduced light path in liquid through the use of a water dipping objective.