Effect of bubble formation on microdroplet cavity quality factors

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 similar to 2 x 10(8), 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 mn 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-mu m-radius hollow droplets, consisting of a concentric outer ethanol layer surrounding a 7.75-mu m-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 x 10(8), a value that is consistent with the increase in outer radius. In the second regime, at higher vibrating orifice frequencies, smaller submicrometer-sized bubbles, leading to nonnegligible internal elastic scattering losses and reducing the highest observable Q values to as low as 3 x 10(5). Sound waves associated with the vibrating orifice must also contribute to the bubble formation because these effects were frequency dependent. (C) 1996 Optical Society of America