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.