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Illustration of the spectral tuning mechanism. The scattering force of an infrared laser, that is focused to near the center of the microdroplet, is in the direction. This causes a deformation in the truncated spherical shape of the microdroplet (left) toward a truncated prolate spheroid geometry (right). As a result, the equatorial radius drops from to .
Consecutive microscope images of a microdroplet under the [(a) and (c)] absence and (b) presence of infrared laser. In the presence of the infrared laser, the diameter of the microdroplet drops from (a) to (b) . When the laser is once more blocked, the diameter of the microdroplet recovers to (c) .
Emission spectra recorded from an -diameter microdroplet while the infrared laser power is first increased from and then decreased to zero. Arrows indicate the spectral drift direction of the high quality WGMs belonging to the same mode set. At the maximum infrared laser power of , total spectral drift is .
(Color online) Hysteresis curves of the spectral drift in the WGMs as a function of the infrared laser power for three different microdroplets with diameters of (microdroplet A) , (microdroplet B) , and (microdroplet C) . At the infrared laser power of , a maximum spectral drift of is observed in microdroplet C.
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