Cavity-Enhanced Ultrafast Spectroscopy*

Using the general pump-probe concept, there are various spectroscopy techniques, such as transient absorption spectroscopy and 2D spectroscopy, for studying ultrafast dynamics. These methods are typically restricted to optically thick samples, such as solids and liquid solutions. In these systems, the dynamics often have broad spectral features due to the collisions between the sample molecules and the solvent molecules.

In a molecular beam, the molecules are cold and isolated from the disturbance of solvent molecules. Measurements taken in these gas-phase molecules can be more directly compared to theoretical predictions. Using frequency comb lasers and optical cavities, we have developed an all-optical technique, Cavity-Enhanced Ultrafast Spectroscopy, to study dynamics in molecular beams with femtosecond temporal resolution. The narrow-linewidth frequency comb laser is coupled into two optical cavities that overlap at the focus. The sample molecules under study are introduced to the overlap by a molecular beam with a noble gas carrier. We have carried out a series of transient absorption measurements in I₂ and I₂-Ar clusters in a molecular beam and have demonstrated ultrafast transient absorption measurements with a detection limit of ΔOD = 2×10^-10(1×10^-9/√Hz). Such a high sensitivity enables one to take all-optical ultrafast spectroscopy measurements in samples with column densities less than 10¹⁰ molecules/cm² and overall, this represents a nearly 4 orders of magnitude improvement over the previous state of the art. In this talk, I will discuss the technical details of this spectrometer and the experiment in I₂. I will also present a widely tunable version of this spectrometer operating at probe wavelengths between 450 and 700 nm using only one set of dispersion-managed cavity mirrors.