Trapping and cooling mechanisms in blue-detuned magneto-optical traps of molecules

TL;DR

This paper investigates the trapping and cooling mechanisms in blue-detuned magneto-optical traps for molecules, highlighting the roles of Zeeman-induced dark states, moving lattices, and gray molasses cooling in achieving effective molecular cooling and trapping.

Contribution

It identifies the trapping force in blue-detuned molecular MOTs and explains how dark states and moving lattices contribute to molecule transport and cooling.

Findings

Zeeman-induced dark states create restoring forces.

Moving lattices generate velocity-dependent forces.

Gray molasses cooling achieves near-zero velocities.

Abstract

In red-detuned magneto-optical traps (MOTs) of molecules, sub-Doppler heating competes with Doppler cooling, resulting in high temperature and low density. A solution is offered by the blue-detuned MOT where sub-Doppler cooling dominates and the cloud is compressed. Several blue-detuned molecular MOTs have been implemented. A recent implementation relies on a pair of orthogonally polarized components whose frequency separation is smaller than the transition linewidth. We identify the trapping force in these MOTs. At a certain magnetic field, there is a state that is dark to the laser propagating in one direction, but not to the counter-propagating one. This Zeeman-induced dark state (ZIDS) sets up an imbalance in the photon scattering rate, leading to a restoring force. We also study the role of the moving lattices generated by the closely-spaced frequency components of the light. We show that there is a velocity-dependent force that drives the molecules towards the speeds of these moving lattices, and that over a relevant range of magnetic fields this combines with the ZIDS force to transport molecules towards the centre of the MOT. Here, gray molasses cooling, assisted by non-adiabatic transitions driven by the time-varying polarization of the light field, cools the molecules towards zero velocity. We study these mechanisms for model systems with simple level structures, then extend them to molecules with ground state hyperfine structure.