Optical levitation of bubbles in water by the radiation pressure of a laser beam: An acoustically quiet levitator

This article demonstrates the stable levitation of single small gas bubbles in water in a downward directed visible laser beam. The natural upward buoyancy of the bubbles was counteracted by the optical radiation pressure of the light beam, whose optical power ranged from 1–3.5 W. The sizes of the levitated bubbles ranged from 10–30 μm in radius. (The term ‘‘levitation’’ is synonymous to ‘‘trapping’’ even though the radiation force must be directed downward on the bubble.) For horizontal stability of the levitated bubble, the laser was operated in a mode giving an irradiance minimum at the center of the laser beam. An approximation is derived for the force on the bubble in the direction of the beam axis; it includes only the contribution to the radiation pressure due to the total-reflection region of the bubble’s surface (which appears to be the major contribution). The resulting estimate of the power required to levitate is consistent with observations. Optical levitation is a quiet alternative to acoustical levitation that could facilitate the measurement of weak acoustic emissions from isolated bubbles.