Observation of photon recoil effects in single-beam absorption spectroscopy with an ultracold strontium gas

TL;DR

This paper demonstrates the observation of photon recoil effects in ultracold strontium gas absorption spectroscopy, revealing asymmetries in the lineshape caused by recoil shifts comparable to the natural linewidth, modeled with extended optical Bloch equations.

Contribution

It provides the first detailed observation and modeling of photon recoil effects in single-beam absorption spectroscopy with ultracold strontium.

Findings

Recoil effects cause asymmetry in absorption lineshape.

Recoil shifts are comparable to the natural linewidth.

Extended optical Bloch equations accurately describe the observations.

Abstract

We report on observing photon recoil effects in the absorption of a single monochromatic light at 689~nm through an ultracold $^{88}$Sr gas, where the recoil frequency is comparable to natural linewidth of the narrow-line transition $5\mathrm{s^2}\, ^1\textrm{S}_0 - 5\mathrm{s}5\mathrm{p}\, ^3\textrm{P}_1$ in strontium. In the regime of high-saturation, the absorption profile becomes asymmetric due to the photon-recoil shift, which is of the same order as the natural linewidth. The lineshape is described by an extension of the optical Bloch equations including the momentum transfers to atoms during emission and absorption of photons. Our work reveals the photon recoil effects in a simplest single-beam absorption setting, which is of significant relevance to other applications such as saturation spectroscopy, Ramsey interferometry, and absorption imaging.