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We present a new technique using a frequency comb laser and optical cavities for performing ultrafast transient absorption spectroscopy with improved sensitivity. Resonantly enhancing the probe pulses, we demonstrate a sensitivity of ΔOD = 1 10^-9/Hz for averaging times as long as 30 s per delay point (ΔOD₌₈₍ = 2 10^-10). Resonantly enhancing the pump pulses allows us to produce a high excitation fraction at high repetition-rate, so that signals can be recorded from samples with optical densities as low as OD 10^-8, or column densities < 10^10 molecules/cm². This high sensitivity enables new directions for ultrafast spectroscopy.
Reber et al. (2015) studied this question.