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(Color online) (a) Sketch of the experimental setup. Light from a tunable Ti:sapphire laser is coupled into a microsphere via frustrated total internal reflection at a prism. Resonances are detected by a photomultiplier as dips in the transmitted signal or as peaks in the light scattered into a multimode fiber sensor. The scanning near-field probe can be moved at a constant height of a few nanometers above the sphere’s surface. (b) Raman lasing for a sphere with a diameter of and a factor of . The measured threshold is of absorbed pump power. The inset shows the spectrum above and below threshold.
(Color online) Mode map of the pump [(a) and (c)] and lasing [(b) and (d)] modes for WGMs with [(a) and (b)] and [(c) and (d)]. The pump powers were 20 and 130 times , respectively. Red lines are theoretical fits (see text).
(Color online) Mode maps of the lasing modes for pump powers of five and ten times for WGMs with (a) and (b). Red lines are theoretical fits. In (c) the intensity gradient as derived from the smoothed experimental data in (a) is plotted. (d) shows the theoretical curves for different pump powers and the two different modes with (top) and (bottom).
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