Sisyphus Laser Cooling of a Polyatomic Molecule

Abstract

We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH). This is achieved with principal optical cycling in the rotationally closed $P(N^{\prime \prime }=1)$ branch of either the ${\tilde{X}}^2{\mathrm{\Sigma }}^{+}(000)\leftrightarrow {\tilde{A}}^2{\mathrm{\Pi }}_{1/2}(000)$ or the ${\tilde{X}}^2{\mathrm{\Sigma }}^{+}(000)\leftrightarrow {\tilde{B}}^2{\mathrm{\Sigma }}^{+}(000)$ vibronic transitions. Molecules lost into the excited vibrational states during the cooling process are repumped back through the $\tilde{B}(000)$ state for both the (100) level of the Sr-O stretching mode and the $(02^{0}0)$ level of the bending mode. The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to $\sim 750\mu \mathrm{K}$. This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.