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.