  We review recent works on optomechanics of optically trapped microspheres and
nanoparticles in vacuum, which provide an ideal system for studying macroscopic
quantum mechanics and ultrasensitive force detection. An optically trapped
particle in vacuum has an ultrahigh mechanical quality factor as it is
well-isolated from the thermal environment. Its oscillation frequency can be
tuned in real time by changing the power of the trapping laser. Furthermore, an
optically trapped particle in vacuum may rotate freely, a unique property that
does not exist in clamped mechanical oscillators. In this review, we will
introduce the current status of optical trapping of dielectric particles in air
and vacuum, Brownian motion of an optically trapped particle at room
temperature, Feedback cooling and cavity cooling of the Brownian motion. We
will also discuss about using optically trapped dielectric particles for
studying macroscopic quantum mechanics and ultrasensitive force detection.
Applications range from creating macroscopic Schr{\"{o}}dinger's cat state,
testing objective collapse models of quantum wavefunctions, measuring Casimir
force, searching short-range non-Newtonian gravity, to detecting gravitational
waves.
