Anisotropic sub-Doppler laser cooling in dysprosium magneto-optical traps

TL;DR

This paper reports an unusual anisotropic sub-Doppler laser cooling mechanism in dysprosium MOTs, where cooling cores flip orientation at a critical laser intensity ratio, linked to Dy's large magnetic moment and near degenerate g factors.

Contribution

It introduces a novel intra-MOT sub-Doppler cooling phenomenon in dysprosium, revealing orientation flips driven by laser intensity ratios and magnetic properties.

Findings

Anisotropic cooled cores appear beyond critical laser intensities.

Orientation of cooled cores flips at a specific intensity ratio.

The phenomenon is linked to Dy's large magnetic moment and g factor degeneracy.

Abstract

Magneto-optical traps (MOTs) of Er and Dy have recently been shown to exhibit population-wide sub-Doppler cooling due to their near degeneracy of excited and ground state Lande g factors. We discuss here an additional, unusual intra-MOT sub-Doppler cooling mechanism that appears when the total Dy MOT cooling laser intensity and magnetic quadrupole gradient increase beyond critical values. Specifically, anisotropically sub-Doppler-cooled cores appear, and their orientation with respect to the quadrupole axis flips at a critical ratio of the MOT laser intensity along the quadrupole axis versus that in the plane of symmetry. This phenomenon can be traced to a loss of the velocity-selective resonance at zero velocity in the cooling force along directions in which the atomic polarization is oriented by the quadrupole field. We present data characterizing this anisotropic laser cooling phenomenon and discuss a qualitative model for its origin based on the extraordinarily large Dy magnetic moment and Dy's near degenerate g factors.