Optical levitation is a method developed by Arthur Ashkin whereby a material is levitated against the downward force of gravity by an upward force stemming from photon momentum transfer. Typically photon radiation pressure of a vertical upwardly directed and focused laser beam of enough intensity counters the downward force of gravity to allow for a stable optical trap capable of holding small particles in suspension.
Micrometer sized (from several to 50 micrometer in diameter) transparent dielectric spheres such as fused silica spheres, oil or water droplets, are used in this type of experiment. The laser radiation can be fixed in wavelength such as that of an argon ion laser or that of a tunable dye laser. Laser power required is of the order of 1 Watt focused to a spot size of several tens of micrometers. Phenomena related to morphology-dependent resonances in a spherical optical cavity have been studied by several research groups.
For a shiny object, such as a metallic micro-sphere, stable optical levitation has not been achieved. Optical levitation of a macroscopic object is also theoretically possible.