Cavity-Enhanced Ultrafast Transient Absorption Spectroscopy

Abstract

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 $\hat{I}”$OD $ = 1 \times 10^{-9}/\sqrt{\mbox{Hz}}$ for averaging times as long as 30 s per delay point ($\hat{I}”$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.