Field-Resolved Infrared Transmission Spectroscopy Of Strongly Absorbing Samples

Summary form only given. 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. 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. In this contribution, we demonstrate the potential of fi eld -resolved infrared spectroscopy (FRS) to overcome these long-standing limitations. FRS relies on the excitation of resonant molecular vibrations with waveform stable, broadband IR pulses, and electric-field-resolved detection of the emerging fingerprint waveforms via electro-optic sampling [4], bringing about two major advantages over other IR spectroscopies. Firstly, it directly measures the electric field, resulting in a square root scaling of the signal with the intensity attenuation. Together with quantum-efficiency maximised electro-optic sampling and sensitive photodiodes in the near -infrared, this offers an unprecedented intensity dynamic range (DR), see Fig. la. Secondly, the nature of time-domain measurements enables recording resonant fi ngerprints trailing an impulsive excitation in a background -free manner, rendering FRS mostly immune to intensity noise of the excitation.