# Velocity-selective two-photon absorption induced by a diode laser in combination with a train of ultrashort pulses

## TL;DR

This paper demonstrates velocity-selective two-photon excitation in rubidium vapor using a combination of a cw diode laser and a train of ultrashort pulses, enabling detailed spectroscopy and hyperfine resolution.

## Contribution

It introduces a novel method combining cw diode lasers and ultrashort pulse trains for velocity-selective two-photon spectroscopy in rubidium vapor.

## Key findings

- Velocity groups are well characterized within the Doppler profile.
- Hyperfine levels are clearly resolved.
- Optical pumping effects are identified in the two-photon process.

## Abstract

The two-photon transition $5S-5P-5D$ in rubidium vapor is investigated by detecting the fluorescence from the $6P_{3/2}$ state when the atomic system is excited by the combined action of a cw diode laser and a train of ultrashort pulses. The cw-laser plays a role as a velocity-selective filter and allows for a spectroscopy over a large spectral range including the $5D_{3/2}$ and $5D_{5/2}$ states. For a counterpropagating beam configuration, the response of each atomic velocity group is well characterized within the Doppler profile, and the excited hyperfine levels are clearly resolved. The contribution of the optical pumping to the direct two-photon process is also revealed. The results are well described in a frequency domain picture by considering the interaction of each velocity group with the cw laser and the modes of the frequency comb.