Introduction
The anisotropy of materials is associated, for example, with direction dependent optical, mechanical, physical and chemical properties1-7. Anisotropy plays a key role in optoelectronic, photonic, polymer, catalytic and bio-related research and applications. Specific examples are the design and engineering of optical devices1-4, two-dimensional (2D) materials3, and touch-spun nanofibers for nerve regeneration7, the latter of which are studied in this work. For the analysis of anisotropic material properties, infrared (IR) methods are prominently used, as these probe material and structural properties in a contact-less manner in various environments with high sensitivity.
Driven by the tremendous application potential in scientific and industrial applications, numerous IR spectroscopies were developed in recent years. Classical IR spectroscopies are workhorses in many labs. However, it is not possible to simultaneously measure the real and imaginary part of complex transmissions or reflections. To satisfy this demand, polarimetric or ellipsometric IR spectroscopic methods have to be developed for the respective application fields. This challenge can be solved by introducing innovative measurement concepts, implementing new radiation sources, and realizing novel hyperspectral or imaging measurement schemes.
Recent rapidly developing IR spectroscopic methods based on quantum cascade lasers (QCLs) are heralding a new era in IR spectroscopic analytics with a plethora of new applications far beyond the possibilities of FTIR spectroscopy8–17. These methods nowadays offer high spectral resolutions in laboratory and field applications while providing high optical throughput, sub-mm spot sizes, and sub-s temporal resolution for sensing applications, time-resolved studies, as well as hyperspectral imaging. Of particular interest for structural and chemical analysis of interfaces, aggregates, thin films, and structured materials are polarization dependent QCL-based methods such as AFM-IR18,19, IR nanoscopy20, antenna-assisted IR nano-spectroscopy21, IR microscopy22,23, polarimetric IR-ATR24, vibrational circular dichroism25, far-field optical photothermal IR (O-PTIR)26, and IR spectroscopic ellipsometry/polarimetry10,11,27-29.
In 2016/2017, the dual-comb spectroscopic technique30was combined with polarization dependent measurements31,32. So-called dual-comb ellipsometry32 in the near-IR spectral range of 1514–1595 nm became available. The authors32 already anticipated a transfer of the method to the mid-IR and far-IR region, which would enable future applications for studies of materials with vibrational transitions. In this work, we fulfill these expectations and introduce QCL-based IR dual-comb polarimetry (IR-DCP) as a novel technique for temporally (sub-ms) and spectrally (1.4 cm–1) highly resolved investigations of anisotropic sample properties in the mid-IR spectral range.