Brilliant mid-infrared ellipsometry and polarimetry of thin films: Toward laboratory applications with laser based techniques

Abstract

Tunable quantum cascade lasers (QCLs) have recently been introduced as mid-infrared (mid-IR) sources for spectroscopic ellipsometric and polarimetric setups. QCLs, with their unique properties with respect to coherence and brilliance in either pulsed or continuous-wave operation, are opening up numerous new possibilities for laboratory and industrial applications. In this review, the authors will focus on thin-film characterization techniques like ellipsometric and nanopolarimetric methods and summarize related state-of-the-art techniques in this rapidly developing field. These methods are highly relevant for optical, electronical, and biomedical applications and allow detailed structural analyses regarding band properties, spectra–structure correlations, and material anisotropy. Compared to classical Fourier-transform-IR spectroscopy, thin-film sensitivity can be achieved at high spectral and spatial resolution (<0.5 cm−1, <150 μm). Measurement times are reducible by several orders of magnitude into the millisecond and microsecond range with laser-based polarimetric setups involving modulation or single-shot concepts. Thus, mid-IR ellipsometric and polarimetric hyperspectral imaging can be performed on the time scale of minutes. For mid-IR ellipsometric imaging, thickness and structure information become simultaneously accessible at spatial resolutions of a few 100 μm and possibly even at the micrometer scale by the integration of microscopic concepts. With the atomic force microscopy-infrared spectroscopy based nanopolarimetric approach, anisotropy in the absorption properties can be investigated with lateral resolutions beyond the diffraction limit, reaching a few 10 nm.