STIRAP in a sodium gas using picosecond lasers

We demonstrate for the first time with picosecond lasers, the STIRAP technique on sodium vapor with a ladder scheme using the 3p ($^{2}$P$_{1/2}) \quad \leftarrow $ 3s ($^{2}$S$_{1/2})$ and 5s ($^{2}$S$_{1/2}) \quad \leftarrow $ 3p ($^{2}$P$_{1/2})$ transitions. Light to couple the states was produced by two synchronously pumped OPGs and further amplified in two OPAs (pumped by the 355 nm light from a picosecond YAG). Fluorescence from the 5s state to both 3p states ($^{2}$P$_{1/2}$, $^{2}$P$_{3/2})$ and from both 3p states to the 3s state was measured with a monochromator using a gated CCD to eliminate Rayleigh scattered light. Our results indicate a five-fold increase in the transfer efficiency to the 5s state when the laser pulse that couples the 3p and 5s states temporally precedes the laser pulse tuned to the 3p $\leftarrow $ 3s transition.