Abstract

Optical cavity quality factors (Q’s) of ethanol microdroplets with internal bubbles were estimated from cavity mode emission efficiencies as a function of the dissolved gas content. Droplets were generated with a vibrating orifice aerosol generator driven by pressurized gas. With helium- or nitrogen-gas pressurization, Q’s were ∼2 × 108, a value expected to be near a practical upper limit for this size of cavity. Surface capillary waves with a rms amplitude of 1.4 nm are conjectured to limit Q values under these conditions. However, two regimes dominated by bubbles were observed when carbon dioxide–gas pressurization was used. In one regime, nominal 10.25-μm-radius hollow droplets, consisting of a concentric outer ethanol layer surrounding a 7.75-μm-radius gaseous central core, were formed when the vibrating orifice frequency was tuned to a particular value. These hollow sphere droplets displayed a higher Q of 4 × 108, a value that is consistent with the increase in outer radius. In the second regime, at higher vibrating orifice frequencies, smaller homogeneous microdroplets were formed. We conjecture that these microdroplets contain a dispersion of submicrometer-sized bubbles, leading to nonnegligible internal elastic scattering losses and reducing the highest observable Q values to as low as 3 × 105. Sound waves associated with the vibrating orifice must also contribute to the bubble formation because these effects were frequency dependent.

© 1996 Optical Society of America

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