Microspheres act as high Q resonators in optical regime. The curved surface of a microshere leads to efficient confinement of light waves. The light waves totally reflect at the surface and propagate along the circumference. If they round in phase, resonant standing waves are produced near the surface. Such resonances are called “morphology dependent resonances (MDRs)” because the resonance
frequencies strongly depend on the size parameter x = 2π a , (where a is the radius of λ
microstructure and λ is the light wavelength). Alternatively , the resonant modes are often called “Whispering Gallery Modes (WGMs)”. The WGMs are named because of the similarity with acoustic waves traveling around the inside wall of a gallery. Early this century, L.Rayleigh [46] first observed and analyzed the “whispers” propagating around the dome of St.Catherine’s cathedral in England. Optical processes associated with WGMs have been studied extensively in recent years [45].
WGMs are characterized by three numbers, n, l and m, for both polarizations corresponding to TE (transverse electric) and TM (transverse magnetic) modes. TE and TM modes have no radial components of electric and magnetic fields, respectively. These integers distinguish intensity distribution of the resonant mode inside a microsphere (a simple model system of Micro resonators). The order number n indicates the number of peaks in the radial intensity distribution inside the sphere and the mode number l is the number of waves of resonant light along the circumference of the sphere. The azimuthal mode number m describes azimuthal spatial distribution of the mode. For the perfect sphere, modes of WGMs are degenerate in respect to m.
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