We report on modification of the spontaneous emission of dye molecules embedded in a threedimensional solid-state photonic crystal exhibiting a stop band in the visible range. Molecules embedded in artificial opal filled with a polymer show a dip in the fluorescence spectrum and nonexponential spontaneous decay kinetics containing both accelerated and inhibited components compared to the dye fluorescence in a reference polymer matrix. Results are interpreted in terms of redistribution of the photon density of states in the photonic crystal. [S0031-9007(98)06494-1] PACS numbers: 42.50. -p
Abstract-Photonic crystals based on silica colloidal crystals (artificial opals) exhibit pronounced stopbands for electromagnetic wave propagation and the corresponding modification of the photon density of states in the visible range. These spectrally selective features can be enhanced by impregnating opals with higher refractive materials like, e.g., polymers. Doping of these structures with dye molecules, semiconductor nanoparticles (quantum dots), and rare-earth ions provides a possibility to examine the challenging theoretical predictions of the inhibited spontaneous emission in photonic bandgap (PBG) materials. First experiments are discussed in which pronounced modification of spontaneous emission spectra and noticeable changes in decay kinetics were observed.
Petrov et al. Reply: In our recent Letter [1], we reported on modification of the spontaneous emission of dye molecules embedded in a photonic crystal fabricated by impregnating an artificial opal with a higher-refractive polymer containing a fluorescent dye. The spontaneous emission of the dye in the inverted opal recorded at the red edge of the stop band was found to decay nonexponentially exhibiting both inhibited and accelerated components compared to a single-exponential decay of the dye fluorescence in a reference polymer film. The preceding Comment by Megens et al. [2] reduces to the three following statements: (i) Inhibition of the spontaneous emission of a dye in a photonic crystal can be observed only at the blue edge of the stop band, (ii) the dielectric contrast in our experiments was insufficient for observing a modification of the spontaneous emission rate, and (iii) their fluorescence kinetics measurements do not reveal a considerable difference in mean fluorescence decay times for dye molecules attached to silica spheres forming ordered and disordered aqueous suspensions.The homogeneous fluorescence emission band of a complex molecule is formed by numerous vibronic transitions; however, its fluorescence lifetime is constant over the homogeneous emission spectrum as a result of fast thermalization of the electronically excited state [3]. Therefore, modification of radiative transition rates relevant to certain subsets of vibronic transitions in a dye molecule (by, e.g., a photonic crystal environment) will lead to a change in the wavelength-independent fluorescence decay time of the homogeneous emission band. In our experiment [1], the upper (most pessimistic) estimate of the inhomogeneousto-homogeneous width ratio is 0.15, and thus the fluorescence emission band is essentially homogeneous and should exhibit similar decay kinetics at all emission wavelengths.The difference between our experimental results and those by Megens et al. can be explained by different topologies of the systems investigated. Theoretical calculations [4] show that for a given dielectric contrast, the network topology of the higher-refractive material (as in our experiment) provides stronger spatial variations of the local photon density of states (DOS) in a photonic crystal. (Notice that, when comparing our experimental data with the theory, Megens et al. refer to Fig. 7 of [5], presenting the total rather than local photon DOS for artificial opal.) In addition, whereas in our structure dye molecules are uniformly distributed over the entire higherrefractive network, in [2] dye molecules are located only within narrow spherical layers of higher-refractive balls, and therefore, in our structure, dye molecules should experience stronger variations of the local photon DOS and stronger position-dependent variations of fluorescence lifetimes. Unlike atoms, dye molecules in a rigid environment have fixed directions of emission dipole moments, and therefore, in this case modification of the spontaneous emission rate of a molecule...
A combined theoretical and experimental study is carried of the polarized emission of polyatomic products produced through photodissociation of polyatomic molecules. A general approach, based on the formalism of dissociation kernels and orientational correlation functions, is developed to predict anisotropy of the fluorescence of photoproducts. We consider the most general case of asymmetric top parent and product molecules. The rotational predissociation effect is taken into account. Various kinds of photoreactions are studied: those when fragments after dissociation are in the electronically excited states and those when fragments are in the ground electronic states so that additional laser pulse is necessary to excite their fluorescence. Particular attention is concentrated on those practically important extreme cases, when predissociation times and lifetimes of the electronically excited states of photoproducts are short or long as compared to the averaged period of free rotation. The steady state polarized fluorescence of radicals produced through dissociation of several disulfides into two identical radicals is measured. The results are interpreted in the framework of the free recoil model (FRM). In this model, photoproducts are assumed to experience no torque and fly apart freely, so that the only origin of the fragment rotation is rotation of the parent molecule. Predictions of the impulsive model (IM), in which fragments are supposed to suffer instantaneous torque due to the rupture of the chemical bonds of the parent molecule, are demonstrated to disagree strongly with our experimental data. This gives an additional confirmation of the validity of the FRM in describing dissociation of polyatomic molecules into polyatomic fragments. The FRM can therefore be invoked to estimate interrelation between the characteristic times, governing the processes of dissociation and emission, and the averaged period of free molecular rotation. Also, the FRM can be used for the determining orientations of the absorption and emission dipole moments in the reference frames of the parent and product molecules.
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