Field-resolved spectroscopy in the molecular fingerprint region

Summary form only given. In this contribution we introduce field-resolved spectroscopy (FRS) in the molecular fingerprint region, as a time-domain method able to mitigate the above-mentioned limitations and providing several major advantages over state-of-the-art broadband infrared vibrational spectroscopy schemes. FRS takes advantage of recently developed sources of phase-coherent, broadband femtosecond MIR pulses [1] and the ability to sample their electric fields in time with electro-optical sampling (EOS) [1-3], see setup in Fig. 1a. The phase-coherent superposition of all MIR frequency components to an ultrashort pulse enables the confinement of the vibration excitation event to a time window of at most a few hundred femtoseconds, which is significantly shorter than the typical decay times of the excited vibrations under scrutiny. Sampling the resulting electric field therefore allows for an efficient temporal separation of the response emerging from the sample (free induction decay, FID) from the high-power ultrashort excitation, see Fig. 1b. Thus, the FID is detected in a MIR-background-free manner such that source intensity noise does not affect the instrument's ability to detect small absorptions. Furthermore, the dynamic range of detection is adjustable to the strength of the time-domain signal, rendering the scheme power-scalable up to the deformation/damage of the sample. Finally, the heterodyne detection via EOS readily provides the full phase information of the FID and, furthermore, represents one of the most sensitive detection schemes for infrared electric fields [3]. With the above-mentioned