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.
We introduce the concept of broadband near-infrared molecular fieldoscopy. In this scheme, molecules are excited by femtosecond pulses in near-infrared spectral range and the complex electric field of their free induction decay is directly measured by means of electro-optic sampling. Few-cycle pulses centered at 2 µm and 1 µm are generated from a 5 kHz, Yb:YAG regenerative amplifier and employed for femtosecond excitation and electro-optic sampling, respectively. We chose water in an acetic acid solvent to demonstrate the first proof of principle measurement with the novel technique. The complex electric field of the combination bond of water molecules at 1930 nm at different molecular concentrations is detected and presented. We show the detection sensitivity of our time- domain technique is comparable to conventional specral-domain techniques. However, by employing a laser frontend with higher repetition rates, the detection sensitivity can be drastically enhanced. To the best of our knowledge, this is the first detection of the complex electric field of the molecular response in near-infrared spectral range. The new method holds promise for high-resolution overtone spectroscopy and microscopy with unparalleled sensitivity and specificity over the entire molecular fingerprint region.