Cavity-enhanced ultrafast spectroscopy: Ultrafast meets ultrasensitive

Ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are widely used across many disciplines. However, these techniques are typically restricted to optically thick samples, such as solids and liquid solutions. Using a frequency comb laser and optical cavities, we present a technique for performing ultrafast optical spectroscopy with high sensitivity, enabling work in dilute gas-phase molecular beams. Resonantly enhancing the probe pulses, we demonstrate transient absorption measurements with a detection limit of ΔOD = 2 × 10^-10 (1 × 10^-9/[Equation Found]). Resonantly enhancing the pump pulses allows us to produce a high excitation fraction at a high repetition rate, so that signals can be recorded from samples with optical densities as low as OD ≈ 10^-8, or column densities <10¹⁰ molecules∕cm². To our knowledge, this represents a 5000-fold improvement of the state of the art.