Electro-optic sampling (EOS) has established itself as one of the main techniques for coherent THz spectroscopy. In recent years, EOS has been pushed towards recording infrared or even optical waveforms, opening up new possibilities for spectroscopy in these spectral regions. In this contribution, we demonstrate the potential of electric-field-resolved detection via EOS for broadband, mid-infrared molecular vibrational spectroscopy. We show that field-resolved spectroscopy can achieve detection sensitivities orders of magnitude higher than state-of-the art Fourier-transform infrared spectroscopy. This is achieved by high-quantum-efficiency non-linear temporal “piercing” and, therefore, isolation of the molecular signal from the orders-of-magnitude stronger impulsive excitation. Thereby limitations due to detector dynamic range and source excitation noise can be avoided. This promises a new level of molecular sensitivity and molecular coverage for probing complex biological samples.
Traditionally, infrared molecular spectroscopy has been performed with frequency-domain measurement techniques. Recent experiments have exploited the outstanding temporal coherence of state-of-the-art femtosecond lasers to overcome long-standing sensitivity and dynamic range limitations of these traditional techniques, with time-domain measurements. Here, we show how state-of-the-art 2-µm femtosecond technology provides (i) Watt-level infrared sources covering the entire molecular fingerprint region, with a spectral brightness exceeding even that of synchrotrons, (ii) background-free, high-sensitivity and high-dynamic range time-domain detection of molecular vibrations via electro-optical sampling with (iii) attosecond temporal accuracy. These advances herald a new regime for time-, frequency- and space-resolved molecular vibrational metrology.