Morphology-dependent resonances associated with stimulated processes in microspheres

The density of morphology dependent resonances (MDRs) of microspheres is much larger than the density observed in typical resonance spectra. It is large enough to be consistent with observations1 of multiorder Stokes emission in the stimulated Raman scattering from microspheres. The question of whether MDRs having extremely large cavity Q (> 1010 if the spheres are lossless) contribute to the optical feedback in real spheres is discussed. Only the low-order very narrow linewidth MDRs have angle-averaged intensities that are consistent with photographs2 of the light emission from lasing spheres. Although the integrated area under the scattered field coefficients tends to zero as the cavity Q increases, the area under the internal field coefficients (which are proportional to the intensities of stimulated processes) gradually increases as the cavity Q increases. Hence it would appear that the very high Q MDRs provide the optical feedback for stimulated processes in microspheres. When a sphere is illuminated with a high intensity laser beam a nonuniform refractive index may be induced which may so perturb or couple the low-order MDRs that it may not be appropriate to ascribe the optical feedback to the MDRs of homogeneous spheres.