VOLUME 85, NUMBER 1 PHYSICAL REVIEW LETTERS 3JULY 2000 Enhanced Backward-Directed Multiphoton-Excited Fluorescence from Dielectric Microcavities Steven C. Hill Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783-1197 Veronique Boutou, Jin Yu, Stephane Ramstein, and Jean-Pierre Wolf LASIM (UMR5579), Universite Claude Bernard Lyon 1, 43 bd du 11 Novembre, 69622 Villeurbanne Cedex, France Yong-le Pan Physical Science Laboratory, New Mexico State University, Las Cruces, New Mexico 88003-8002 Stephen Holler and Richard K. Chang Department of Applied Physics and Center for Laser Diagnostics, Yale University, New Haven, Connecticut 06520-8284 (Received 28 January 2000) We demonstrate theoretically and experimentally that one-, two-, and three-photon excited fluorescence from dye molecules in spherical microcavities has an asymmetrical angular distribution and is enhanced in the backward direction. The enhancement ratios (of intensities at 180 ± and 90 ± ) are 9, 5, and 1.8 for three-, two-, and one-photon excitation, respectively. Even larger ratios are expected for microspheres with an index of refraction larger than that used in the experiments. Because of the reciprocity principle and concentration of the incident wave inside particles, the backward enhancement is expected to occur even with nonspherical particles. PACS numbers: 33.80.Wz, 33.50.–j, 42.25.Fx, 87.64.Vv Fluorescence of an ensemble of freely rotating molecules is isotropic. However, when these molecules are homogeneously imbedded in a dielectric microcavity (such as a sphere, spheroid, or cylinder), the emission can become anisotropic. The microcavity can concentrate the internal field intensity I r  of the incident radiation [1 – 3], and can introduce an angular-dependent reemission efficiency [4]. Because of the reciprocity principle [5,6], fluorescence from regions of high I r  tends to return toward the illuminating source. This backward enhance- ment is expected not only for one-photon [7] but even more so for multiphoton-excited fluorescence (MPEF) that is proportional to I n r . The concentration and redirection effect for a spherical particle are analogous to epi-illumination microscopy, where the lens focuses the incident light and directs a large fraction of the fluores- cence back toward the source (here the microcavity itself, which is the sample, acts as the lens to focus the incident wave and redirect the fluorescence emission). Microscopy using MPEF has demonstrated improved contrast and resolution [8]. Backward enhancement may be useful for the detection of ultraweak one-photon and especially MPEF from microparticles. The concept should also help in detecting nanocrystals, quantum well structures [9], and possibly even single molecules [10], placed inside or on the surface of a dielectric microcavity, e.g., a glass sphere [11,12]. The effect may call to mind coherent backscattering or weak localization in random media [13]. However, MPEF is not a coherent process (it is emitted spontaneously), and it occurs over a range of wavelengths different from the incident wave. In this Letter, we demonstrate theoretically and experi- mentally, for the first time, the existence of backward enhancement of multiphoton-excited fluorescence in mi- crocavities. We show that the enhancement increases with the order n of the multiphoton-excitation process. Specif- ically, we investigate the angular dependence (from 0 ± to 180 ± ) of the fluorescence from microdroplets with the same dye, excited by one, two, and three photons. Remark- able enhancement in the backward direction (as much as a factor of 9 relative to the 90 ± emission for the three-photon excitation process) is both predicted and observed using 100 femtosecond laser pulses. We have calculated that for microspheres, the backward enhancement increases with the refractive index. The principle of reciprocity [5,6] as applied to this prob- lem implies the following: If a source at r a far from the particle generates at r b (inside the particle) an intensity that is large relative to intensities at other internal points, then a source at r b radiates preferentially toward r a (i.e., in the backscattering direction). If the input intensity at r b is relatively weak, then a source at r b tends to radiate in directions other than r a . Reciprocity relations are ex- act only when the excitation and emission wavelengths are identical. When the wavelengths are different [as when the source at r b is a fluorescent molecule that reemits longer- wavelength radiation in proportion to I n r ], the reciproc- ity principle still tends to imply the angular preferences stated above so long as the refractive index dispersion be- tween the two wavelengths is small. The total fluorescence power U n u d  collected by a de- tector at angle u d (with respect to the incident beam) 54 0031-9007 00  85(1)  54(4)$15.00 © 2000 The American Physical Society