QCL-based infrared spectroscopic ellipsometer rapidly acquires data

Ellipsometry, in which changes in polarization are measured when light interacts with a sample, is an important aid to understanding materials for industry and science. Now, scientists at the Research Center for Non-Destructive Testing (RECENDT) and Johannes Kepler Universität (both in Linz, Austria) have for the first time demonstrated laser-based infrared (IR) spectroscopic ellipsometry, whose spectral data will greatly increase this understanding. The technique uses a bright mid-IR quantum-cascade laser (QCL) to capture high-resolution spectral information in less than a second, potentially leading to new insights into quickly changing properties of samples from plastics to biological materials. The QCL laser is at least 10,000X brighter than traditional light sources used for spectroscopic ellipsometry and is brighter even than synchrotron light sources.

The QCL, which is made by Daylight Solutions (San Diego, CA), is rapidly tunable over a 900 to 1204 cm^-1 (11.1 to 8.3 µm) spectral range at a 1000 cm^-1/s rate and is combined with phase-modulated polarization in the instrument. The researchers showed that the QCL greatly improved the signal quality of the spectroscopic measurements, resulting in a high resolution of 1 cm^-1, and shortened the spectral acquisition time from several hours to less than a second, with further improvements possible as the new laser technology progresses. They also demonstrated that the technique can be used for real-time monitoring of the realignment of molecular chains as a polypropylene film was stretched.

The laser’s brightness means it can be used for mid-IR spectroscopic ellipsometry of highly absorbing materials or substances, including those dissolved in water. Another advantage is that the instrument can be used for spectroscopic measurements without relying on expensive optical components such as monochromators or interferometers. Tests against a conventional Fourier-transform spectrometer-based IR ellipsometer showed an improvement in signal-to-noise ratio of at least 290. The researchers plan to further develop the instrument and want to fully exploit the possibility of diffraction-limited laser spots to acquire hyperspectral mid-IR ellipsometry images with reasonable acquisition times. Reference: A. Ebner et al., Opt. Lett., 44, 14 (2019); https://doi.org/10.1364/ol.44.003426.