EFFECT OF BUBBLE FORMATION ON MICRODROPLET CAVITY QUALITY FACTORS

Optical cavity quality factors (Q's) of ethanol microdroplets with int ernal 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 co njectured to limit Q values under these conditions. However, two regim es dominated by bubbles were observed when carbon dioxide-gas pressuri zation was used. In one regime, nominal 10.25-mu m-radius hollow dropl ets, consisting of a concentric outer ethanol layer surrounding a 7.75 -mu m-radius gaseous central core, were formed when the vibrating orif ice frequency was tuned to;a particular value. These hollow sphere dro plets displayed a higher Q of 4 x 10(8), a value that is consistent wi th the increase in outer radius. In the second regime, at higher vibra ting 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 associ ated with the vibrating orifice must also contribute to the bubble for mation because these effects were frequency dependent. (C) 1996 Optica l Society of America