Cavity-Enhanced Ultrafast Transient Absorption Spectroscopy

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 $\mathrm{\Delta }$OD $=1\times {10}^{-9}\mathrm{/}\sqrt{Hz}$ for averaging times as long as 30 s per delay point ($\mathrm{\Delta }$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\mathrm{/}cm$^2$. This high sensitivity enables new directions for ultrafast spectroscopy.