Narrow-line Laser Cooling by Adiabatic Transfer

TL;DR

This paper introduces a new laser cooling method using adiabatic frequency sweeps in narrow-line transitions, effectively cooling particles with minimal spontaneous emission, demonstrated on strontium atoms.

Contribution

The authors propose and experimentally validate a novel adiabatic transfer-based laser cooling technique applicable to narrow-linewidth transitions, reducing spontaneous emission dependence.

Findings

Effective cooling demonstrated on $^{88}$Sr atoms.

Increased phase-space density achieved.

Robust cooling force with weak transitions.

Abstract

We propose and demonstrate a novel laser cooling mechanism applicable to particles with narrow-linewidth optical transitions. By sweeping the frequency of counter-propagating laser beams in a sawtooth manner, we cause adiabatic transfer back and forth between the ground state and a long-lived optically excited state. The time-ordering of these adiabatic transfers is determined by Doppler shifts, which ensures that the associated photon recoils are in the opposite direction to the particle's motion. This ultimately leads to a robust cooling mechanism capable of exerting large forces via a weak transition and with reduced reliance on spontaneous emission. We present a simple intuitive model for the resulting frictional force, and directly demonstrate its efficacy for increasing the total phase-space density of an atomic ensemble. We rely on both simulation and experimental studies using the 7.5~kHz linewidth $^1$S$_0$ to $^3$P$_1$ transition in $^{88}$Sr. The reduced reliance on spontaneous emission may allow this adiabatic sweep method to be a useful tool for cooling particles that lack closed cycling transitions, such as molecules.