Field-resolved infrared transmission spectroscopy of strongly absorbing samples

Abstract

Molecular vibrational spectroscopies provide chemically-specific information on complex samples. The label-free nature of these techniques renders them highly attractive for studies of biological processes and medical diagnosis [1,2]. Among these methods, the direct, broadband interrogation of molecular vibrations at their fundamental frequencies in the infrared (IR) molecular fingerprint region, profits from large interaction cross-sections, potentially affording a unique combination of detection sensitivity and molecular coverage. However, the strong absorption of (liquid) water in this spectral range has so far severely limited the applicability of IR vibrational spectroscopy (and microscopy) in transmission geometry. In fact, in most table-top setups, the transmission path length has to be limited to < 10 µm, or attenuated total reflection techniques with even smaller penetration depth are applied. Alternatively, the sample can be dried - however this strongly alters it. For larger thicknesses, the current approach is to use high-brightness sources like quantum cascade lasers, although their applicability is limited due to their narrowband emission and modest intensity stability [3].