Abstract

We detected high-frequency sound radiated by individual gas bubbles in response to modulated green light from an Ar laser. The bubbles were surrounded by clear water. They were prevented from rising by contact with a thin glass plate or with a clear Mylar film. Bubble radii were in the 20-100-μm range. Each bubble was first illuminated by short pulses, well separated in time, with a peak power ≃2 W. Sound radiated from the bubbles exhibited the ringing of a damped oscillator. This is evidence of monopole oscillations of the bubble's surface in which the surrounding water provides the inertia and the gas within impedes compression and acts like a spring.¹ The resonance frequency f_R was found by Fourier trans forming this signature; typically f_R < 150 kHz and f_R was close to the predicted monopole resonance frequency. Bubbles were subsequently illuminated with bursts of pulses with a frequency f ≃ f_R. The resulting sound was characterized by an initial buildup of its amplitude which is the expected resonance response of an oscillator. The mechanism for driving the oscillations appears to be the compression of the bubble by modulated optical radiation pressure rather than thermal expansion. The transfer of momentum is thought to occur at the total reflection region of the bubble’s surface.

© 1985 Optical Society of America