Direct laser cooling of calcium monohydride molecules

TL;DR

This paper reports the successful laser cooling and sub-Doppler cooling of calcium monohydride molecules, demonstrating optical cycling, vibrational branching ratios, and temperature reduction, paving the way for a molecular MOT.

Contribution

It introduces the first demonstration of optical cycling and sub-Doppler cooling of CaH molecules, including detailed measurements and modeling, enabling future ultracold molecule experiments.

Findings

Achieved photon scattering rate of ~1.6 million photons/sec.

Lowered transverse temperature from 12.2 mK to 5.7 mK.

Demonstrated sub-Doppler cooling via magnetically assisted Sisyphus effect.

Abstract

We demonstrate optical cycling and sub-Doppler laser cooling of a cryogenic buffer-gas beam of calcium monohydride (CaH) molecules. We measure vibrational branching ratios for laser cooling transitions for both excited electronic states A and B. We measure further that repeated photon scattering via the $A\leftarrow X$ transition is achievable at a rate of $\sim$ $1.6\times10^6$ photons/s and demonstrate the interaction-time limited scattering of $\sim$ $200$ photons by repumping the largest vibrational decay channel. We also demonstrate the ability to sub-Doppler cool a molecular beam of CaH through the magnetically assisted Sisyphus effect. Using a standing wave of light, we lower the molecular beam's transverse temperature from $12.2(1.2)$ mK to $5.7(1.1)$ mK. We compare these results to sub-Doppler forces modeled using optical Bloch equations and Monte Carlo simulations of the molecular beam trajectories. This work establishes a clear pathway for creating a magneto-optical trap (MOT) of CaH molecules. Such a MOT could serve as a starting point for production of ultracold hydrogen gas via dissociation of a trapped CaH cloud.