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Abstract
Whispering gallery modes (WGMs) are frequently observed in the emission spectrum of a fluorophore coupled to a dielectric microsphere. If the fluorophore is excited strongly enough the WGMs can become lasing modes, producing a much higher signal-to-noise ratio and higher -factors. These favorable properties have led to recent demonstrations of biochemical sensing with lasing WGMs; however, as we show here, the underlying cavity structure that leads to lasing can be highly complicated and the lasing spectrum can result from a large number of closely spaced or overlapping modes. In silica spheres coated with a dye-doped polymer bilayer, hints of underlying complexity are already indicated by the skewed shape of the fluorescence WGMs. Under lasing conditions, a more complicated structure was observed with -factors over 10 times higher than observed in fluorescence, while transmission measurements showed a dense forest of resonances due to -order degeneracy breaking from a lack of perfect spherical symmetry. Lasing preferentially occurs at wavelengths where these modes tended to be most densely spaced. The tapered part of the fiber to which the microsphere is attached can also have a strong effect on the lasing spectrum, leading to significant differences between the fluorescence and lasing resonances.
© 2017 Optical Society of America
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