  We present the optical and mechanical properties of high-Q fused silica
microtoroidal resonators at cryogenic temperatures (down to 1.6 K). A thermally
induced optical multistability is observed and theoretically described; it
serves to characterize quantitatively the static heating induced by light
absorption. Moreover the influence of structural defect states in glass on the
toroid mechanical properties is observed and the resulting implications of
cavity optomechanical systems on the study of mechanical dissipation discussed.
