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.
Categories: physics.optics physics.atom-ph physics.chem-ph