Photon spin molasses for laser cooling molecular rotation

TL;DR

This paper proposes a theoretical method to cool molecular rotation using photon spin in laser cooling, enabling rotational cooling of molecules in regimes where traditional methods fail.

Contribution

It introduces a novel rotational cooling mechanism based on photon spin transfer, applicable when rotational splitting exceeds laser linewidth.

Findings

Rotational cooling to Doppler limit achieved theoretically.

Photon spin transfer can exert damping torque on molecules.

Method applicable to large molecules and wide bandwidth lasers.

Abstract

Laser cooling of translational motion of small molecules is performed by addressing transitions that ensure spontaneous emission cannot cause net rotational excitation. This will not be possible once the rotational splitting becomes comparable to the operational excitation linewidth, as will occur for large molecules or wide bandwidth lasers. We show theoretically that in this regime, angular momentum transfer from red-detuned Doppler cooling light can also exert a damping torque on linear molecules, cooling rotation to the same Doppler limit (typically $\approx$ 500 $\mu$K for molecules with $\approx$ 10 ns excited-state lifetimes). This cooling process is derived from photon spin, and indicates that standard optical molasses can also cool molecular rotation with no additional experimental resources.