Hyperspectral infrared laser polarimetry for single-shot phase-amplitude imaging of thin films

Diffraction-limited hyperspectral mid-infrared micro-ellipsometry

The recent introduction of quantum cascade lasers (QCL) in infrared spectroscopic ellipsometry led to decisive improvements in measurement times and signal-to-noise ratios of this powerful analytical method. In this contribution, we present another significant enhancement leading to the first, to the best of our knowledge, diffraction-limited micro-ellisometry setup in the mid-infrared spectral range with a spatial resolution better than 13.3 µm. The fast spectral tunability of the QCL combined with phase-modulated polarization enabled simultaneous acquisition of broadband (900 cm^-1-1204 cm^-1) high-resolution (1 cm^-1) hyperspectral Ψ, Δ-cubes in a scanning approach in reasonable time scales. The spatial resolution of the QCL micro-ellipsometer was experimentally characterized by the knife-edge method and measurements of a resolution test target. Furthermore, the hyperspectral ellipsometric investigation of a polymer multilayer cross section and the portrait window of a 200-euro bank note demonstrate the capabilities of diffraction-limited QCL micro-ellipsometry.

Multi-timescale infrared quantum cascade laser ellipsometry

We recently introduced a novel, to the best of our knowledge, infrared laser ellipsometer for sub-decisecond spectroscopy [Opt. Lett.44, 4387 (2019)10.1364/OL.44.004387] and 0.03 mm² spot-sized hyperspectral imaging [Opt. Lett.44, 4893 (2019)10.1364/OL.44.004893]. 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 µs 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.

Sensing and structure analysis byin situIR spectroscopy: from mL flow cells to microfluidic applications

In situmid-infrared (MIR) spectroscopy in liquids is an emerging field for the analysis of functional surfaces and chemical reactions. Different basic geometries exist forin situMIR spectroscopy in milliliter (mL) and microfluidic flow cells, such as attenuated total reflection (ATR), simple reflection, transmission and fiber waveguides. After a general introduction of linear opticalin situMIR techniques, the methodology of ATR, ellipsometric and microfluidic applications in single-reflection geometries is presented. Selected examples focusing on thin layers relevant to optical, electronical, polymer, biomedical, sensing and silicon technology are discussed. The development of an optofluidic platform translates IR spectroscopy to the world of micro- and nanofluidics. With the implementation of SEIRA (surface enhanced infrared absorption) interfaces, the sensitivity of optofluidic analyses of biomolecules can be improved significantly. A large variety of enhancement surfaces ranging from tailored nanostructures to metal-island film substrates are promising for this purpose. Meanwhile, time-resolved studies, such as sub-monolayer formation of organic molecules in nL volumes, become available in microscopic or laser-based set-ups. With the adaption of modern brilliant IR sources, such as tunable and broadband IR lasers as well as frequency comb sources, possible applications of far-field IR spectroscopy inin situsensing with high lateral (sub-mm) and time (sub-s) resolution are considerably extended.