Cavity cooling of levitated nanospheres by coherent scattering

Although cavity cooling of levitated nanospheres has been demonstrated in recent years, regime of strong optomechanical cooperativity C>1 is yet to be reached, leading to full quantum control of nanosphere motion. A common obstacle in many experiments is stable levitation of nanospheres in ultra-high vacuum (UHV). However, stable trapping has been achieved in an optical dipole trap in several experiments through the use of three-dimensional parametric feedback. We exploit this by combining such a trap with an optical cavity and demonstrate cavity optomechanics with a silica nanosphere in UHV. We confirm the trapping of nanospheres of nominal radius through a novel method using the variable coupling to a cavity mode. We achieve C=0.02, showing a five orders of magnitude improvement of cooperativity from our previous work. We then modify this setup to drive the cavity mode solely by scattered photons from tweezer laser. In addition, cavity enhanced scattered photons provide a more effective, three-dimensional cooling of the nanosphere motion, immediately allowing us to reach strong cooperativity in high vacuum.