Temperature limits in laser cooling of free atoms with three-level cascade transitions

TL;DR

This paper presents a semiclassical analysis of laser cooling in three-level cascade atomic systems, revealing conditions for achieving temperatures below Doppler limits through enhanced damping at two-photon resonance.

Contribution

It introduces a new cooling scheme for group II atoms using three-level cascade transitions and characterizes the temperature limits achievable in this setup.

Findings

Temperatures below Doppler limits are achievable.

Enhanced damping and reduced diffusion occur at two-photon resonance.

Lowest temperatures are estimated by the Doppler limit with optical coherence decay rate.

Abstract

We employ semiclassical theoretical analysis to study laser cooling of free atoms using three-level cascade transitions, where the upper transition is much weaker than the lower one. This represents an alternate cooling scheme, particularly useful for group II atoms. We find that temperatures below the Doppler limits associated with each of these transitions are expected. The lowest temperatures arise from a remarkable increase in damping and reduced diffusion compared to two-level cooling. They are reached at the two-photon resonance, where there is a crossing between the narrow and the partially-dark dressed states, and can be estimated simply by the usual Doppler limit considering the decay rate of the optical coherence between these states.