Exciting Whispering Gallery Modes in liquid microdrops using sub-micron size tapered fibers

We report here the observations of high Q (up to 7 x 107) Whispering Gallery Modes in liquid microdrops of a glycerol-water mixture and three different silicone oils, which are excited and detected using high transmission tapered fibers of sub-micron waist sizes. For measuring the real Q values, which are not thermally limited, the laser frequency is scanned fast enough such that the broadband thermal oscillations of microdrops, with frequencies from kHz to 100s of kHz, do not affect the resonance signal. The two standard measurement methods `linewidth measurement' and `cavity ring-down' are used - the first one for resonances with Q <107 and the second one for Q >~ 107. We describe our analysis to show that for the tested liquids, absorption loss plays the dominant role in deciding the maximum value of the measured intrinsic Q of the microdrops. Using the analysis we show that the measured optical Q can be used to estimate a reliable upper limit of the absorption coefficient of liquids.

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