Parametric feedback cooling of a rotating birefringent microsphere in a vacuum optical trap

We demonstrate parametric feedback cooling of a levitated birefringent microsphere held in a circularly polarised Gaussian trap. We show efficient cooling of the birefringent microsphere to the temperature less than a millikelvin, which is orders of magnitude lower than that of a silica counterpart. We reveal that the transverse spin momentum present in circularly polarised light plays a role in the trapped particle dynamics, which exhibits a range of nonequilibrium phenomena, namely, (i) biased stochastic motion, (ii) coupling of mechanical modes, (iii) nonconservative rotation-translation coupling. We further discuss the temperature of the trapped particle in a nonconservative vacuum optical trap.