Optical manipulation of bubbles in water using solid-state laser technology

For some experiments on the acoustics of bubbles in water, it may be desirable to counter buoyancy using optical radiation pressure. A previous demonstration of the optical trapping of small bubbles that would have otherwise been freely rising used a green continuous downward-directed beam from an Ar-ion gas laser having an irradiance minimum on the beam axis [B. T. Unger and P. L. Marston, J. Acoust. Soc. Am. 83, 970–975 (1988)]. Some of the difficulties in using solid-state laser technology for this purpose are that the beam should be green (to minimize absorption) and have an axial irradiance minimum (for lateral stability). We have generated the required beam using a diode-pumped Nd:YVO4 laser crystal with intracavity-second-harmonic-generation in a lithium-tri-borate crystal. The 5.5-W external green beam power levels achievable were more than sufficient for suspension of gas bubbles having 20- and 40-μm radii generated by electrolysis. The bubble size is measured using forward scattering. Occasionally, two vertically offset bubbles can be trapped along the axis of the downward directed beam. [Work supported by NASA.]