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 ΞOD = 1 \times 10^{-9}/\sqrt{\mbox{Hz}} for averaging
times as long as 30 s per delay point (ΞODminβ=2Γ10β10).
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 β10β8, or column densities <1010 molecules/cm2. This high sensitivity enables new directions for
ultrafast spectroscopy