Multi-timescale infrared quantum cascade laser ellipsometry

We recently introduced a versatile infrared laser ellipsometer for sub-decisecond spectroscopy [Opt. Lett. 44, 4387 (2019)] and 0.03mm(2) spot-sized hyperspectral imaging [Opt. Lett. 44, 4893 (2019)]. Here we report on the next device generation for thin-film sensitive simultaneous single-shot amplitude and phase measurements. The multi-timescale ellipsometer achieves 10 mu s time resolution and long-term stability over hours at high spectral resolution (0.2 cm(-1)). We investigate the temporal stages (from minutes to milliseconds) of fatty acid thin-film formation upon solvent evaporation from acetone-diluted microliter droplets. Optical thickness variations, structure modifications, and molecular interactions are probed during the liquid-to-solid phase transition. Multi-timescale ellipsometry could greatly impact fields like in situ biosensing, microfluidics, and polymer analytics, but also operando applications in membrane research, catalysis, and studies of interface processes and surface reactions. (C) 2022 Optica Publishing Group