We present our latest advances in laser-based infrared (IR) polarimetry regarding novel analytical possibilities for hyperspectral imaging and time-resolved studies of chemical and physical sample properties. IR polarimetry is a powerful polarization-sensitive extension of classical IR spectroscopy that facilitates in-depth sample analyses, particularly of thin films below 100 nm.

Here, a chemically homogeneous and structurally heterogeneous PNIPAAm polymer thin film, as well as a partially chemically modified and structurally heterogeneous myristic-acid film were hyperspectrally imaged, visualizing chemical composition, molecular interactions, and thickness distributions. We also studied the temperature-dependent solid–to–liquid phase transition of fatty-acid thin films via time-resolved measurements with sub-second resolution. Modeling composition and shape of the ν(C=O) vibrational band reveals the critical temperature and quantifies molecular interactions.

Hyperspectral images were taken at high spectral (< 0.5 cm⁻¹) and spatial (≈ 120 µm) resolution in the 10 min range. Time resolutions of about 10 µs per wavelength and 100 ms per spectrum are possible. These unprecedented benchmarks were achieved by employing a brilliant light source in an ellipsometric optical configuration without any moving parts. To be precise, in this EFRE 1.8/13 project (in cooperation with Sentech Instruments GmbH) we have developed a single-shot IR laser polarimeter that uses a broadband-tunable quantum cascade laser (MIRcat, Daylight Solutions) in the fingerprint region (1318–1765 cm⁻¹), and a four-channel detection unit for single-shot amplitude and phase measurements of individual laser pulses. The polarimeter enables 5D analytical investigations of complex materials, such as structured surfaces, thin-film composites below about 100 nm, and samples with an intricate or complicated optical response.

Reprinted with permission from Furchner et al., Opt. Lett. 2019, 44, 4893–4896 (©2019, Optical Society of America).

Karsten Hinrichs

ISAS e.V.

Appearances