Cavity-QED-enhanced stimulated visible emission was observed in 14-pm-diam rhodamine-66-ethanol droplets pumped by cw 514.5-nm radiation. Use of droplets provides an excellent test of cavity QED theory for spherical geometries. The mode number and order of the spherical-cavity resonances responsible for the observed emission peaks were identified and their Q values calculated from Lorenz-Mie theory. By equating stimulated gain to calculated/measured cavity losses, it was determined that spherical-cavity g's of only 10 -10 lead to cavity QED enhancements in excess of 120 in the emission cross section of rhodamine 66, consistent with theory.PACS numbers: 42.50.Wm, 12.20.Fv, 32.80.t, 42.55.f It is now generally accepted that the fluorescence from an atom or molecule may be alternatively enhanced [1][2][3] or inhibited [4,5] by its placement in a microcavity, depending on whether or not the emission spectrally coincides with the cavity resonance. This efIect was first discussed by Purcell [1], who noted that the changes in the final density of states per unit volume and unit frequency would lead to a greatly enhanced probability for spontaneous emission over that normally observed in free space. On resonance, the enhancement may be approximated by the expression 3DQX /4rr V, where D is the degeneracy of the resonance, Q is the cavity quality factor, A, is the emission wavelength, and V is the mode volume. Much of the previous experimental work on cavity enhancement at visible wavelengths was performed using Fabry-Perot cavities [6]. Recently, the importance of restricted dimensionality of the cavity, leading to greater spontaneous emission rates [7], was noted. A sphericalcavity represents a case of three-dimensional enclosure and is attractive because all fields and modes, both internal to and external to the cavity, are exactly calculable in practice. It has been known for some time that nearly transparent microdroplets act as such high-Q resonators, the feedback provided by light waves that totally internally reflect at the droplet-air interface and fold back on themselves [8]. Spherical-cavity resonances in micrometer-sized droplets occur at a series of discrete wavelengths throughout the visible. For a given droplet, resonances occur at specific values of x" I. Here x is the size parameter given by 2+a/k, where a is the droplet radius, and n and I are integers. The mode number n indicates the order of the spherical Bessei and Hankel functions describing the radial field distribution and the order l indicates the number of maxima in the radial dependence of the internal field distribution. Both discrete transverse electric (TE) and transverse magnetic (TM) resonances exist. Emission from dielectric microspheres [9] and fibers [10] containing fluorescing dyes shows sharp line structure superimposed on the normal broadband emission. These spectral features (see Fig. 1) result from cavity quantum electrodynamic (QED) enhancement of the Einstein 3 coe5cient at specific spherical-cavity-reso-nance wavelengths. Rece...

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Operation of a two-dimensional photonic bandgap optical limiter was studied at 514.5 nm for pulse durations of 0.1 to 4 ms . Photonic crystals consisted of 180- 230-nm spatial-period nanochannel glasses containing a thermal nonlinear liquid. A dynamic range in excess of 130 was observed in a single-element device.