Combined time and frequency spectroscopy with engineered dual-comb spectrometer

Dual-comb spectroscopy (DCS) is a powerful technique for broadband spectroscopy with high precision. High-frequency resolution requires long data acquisition times, limiting the temporal resolution in time-resolved measurements. We overcome this limitation by engineering the interaction between the sample under test and the DCS pulsed laser. The DCS interferogram is measured in steps with every step comprising a different number of pulses that interact with the sample. The sample's complex properties (absorption and phase) are extracted from the Fourier transform of the interferogram as a function of the number of pulses; this maps the temporal evolution of the excited state population. A two-dimensional spectrum is generated from which the system time evolution is deduced. We benchmark this method by measuring the two-dimensional spectrum of a room-temperature rubidium vapor. The measured population dynamics of the excited state show a square dependence on the number of interacting pulses due to the coherent accumulation of population. Rabi oscillations are observed under intense excitation conditions. This is the first demonstration of DCS with high frequency and high temporal resolutions (which is given by the inverse of the repetition rate of the comb laser) without invoking pump-probe spectroscopy, combining the pulsed laser spectral and temporal properties. This method allows one to detect simultaneously the kinetics of different chemical species and hence the pathway for chemical reactions.