Modelling spectra of hot alkali vapour in the saturation regime

TL;DR

This paper introduces a comprehensive model for simulating the spectra of hot alkali vapours at various intensities, including beyond the weak-probe regime, incorporating optical pumping and transit-time effects, validated with rubidium spectroscopy.

Contribution

The paper presents a novel model capable of simulating alkali vapour spectra at high intensities, extending beyond weak-probe approximations, and validated against experimental data.

Findings

Excellent agreement between model and experimental spectra for Rb-87.

Model differentiates saturation behaviors of open and closed transitions.

Applicable to multiple alkali metals and both D lines.

Abstract

Laser spectroscopy of hot atomic vapours has been studied extensively. Theoretical models that predict the absolute value of the electric susceptibility are crucial for optimising the design of photonic devices that use hot vapours, and for extracting parameters, such as external fields, when these devices are used as sensors. To date, most of the models developed have been restricted to the weak-probe regime. However, fulfilling the weak-probe power constraint may not always be easy, desired or necessary. Here we present a model for simulating the spectra of alkali-metal vapours for a variety of experimental parameters, most distinctly at intensities beyond weak laser fields. The model incorporates optical pumping effects and transit-time broadening. We test the performance of the model by performing spectroscopy of Rb-87 in a magnetic field of 0.6 T, where isolated atomic resonances can be addressed. We find very good agreement between the model and data for three different beam diameters and a variation of intensity of over five orders of magnitude. The non-overlapping absorption lines allow us to differentiate the saturation behaviour of open and closed transitions. While our model was only experimentally verified for the D2 line of rubidium, the software is also capable of simulating spectra of rubidium, sodium, potassium and caesium over both D lines.