$\Lambda$-enhanced sub-Doppler cooling in a grating magneto-optical trap

We report our observation of sub-Doppler cooling of lithium using a tetrahedral laser beam arrangement, which is produced by a nanofabricated diffraction grating. We are able to capture $10~\%$ of the lithium atoms from a grating magneto-optical trap into $\Lambda$-enhanced $D_1$ gray molasses. The molasses cools the captured atoms to approximately $30~\mu$K. In contrast to results from conventional counterpropagating beam configurations, we do not observe cooling when our optical fields are detuned from Raman resonance. Our results show that grating magneto-optical traps can serve as a robust source of cold atoms for tweezer-array and atom-chip experiments, even when the atomic species is not amenable to sub-Doppler cooling in bright optical molasses.