Sisyphus Laser Cooling of a Polyatomic Molecule

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 = 1) branch of either the (X) over tilde (2)Sigma(+)(000) <-> (A) over tilde (II1/2)-I-2(000) or the (X) over tilde (2)Sigma(+) (000) <-> (B) over tilde (2)Sigma(+)(000) vibronic transitions. Molecules lost into the excited vibrational states during the cooling process are repumped back through the (B) over tilde (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 similar to 750 mu K. This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.