  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 $\Delta$OD $ = 1 \times 10^{-9}/\sqrt{\mbox{Hz}}$ for averaging
times as long as 30 s per delay point ($\Delta$OD$_{min} = 2 \times 10^{-10}$).
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 $\approx 10^{-8}$, or column densities $<
10^{10}$ molecules/cm$^2$. This high sensitivity enables new directions for
ultrafast spectroscopy.
