Petrovet al.Reply:

Abstract

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...

Spectral and angular redistribution of photoluminescence near a photonic stop band

“…28,29 Especially the time-resolved measurements yielded quite contradictory results concerning the influence of the stop band on the radiative lifetime of the emitters and have therefore been the subject of extensive discussion. [29][30][31][32][33] More recently a report on the emission of CdSe quantum dots in strong inverse PCs by Lodahl et al 34 indeed demonstrated a variation in the lifetime of up to 30%, depending on the emission frequency. However, the question remains how this large variety of observations can be understood in terms of photonic effects and why the observed effects often differ significantly from each other.…”

Nonexponential decay of spontaneous emission from an ensemble of molecules in photonic crystals

“…These contradictory results concerning the influence of the stop band on the radiative lifetime of the emitters have been extensively discussed. [15][16][17][18][19] More recently, depending on the emission frequency, lifetime fluctuations up to 30% have been reported 20 for CdSe quantum dots in inverse PCs. Furthermore, nonexponential decay profiles of quantum dots embedded in 3D fcc inverse opals have been experimentally observed.…”

Radiation properties of sources inside photonic crystals

“…17,18 These experiments have led to different interpretations and are still subject to discussion. 19,20 On the other hand, it has been suggested that similar effects such as nonexponential decay rates, 21 inhibition of radiation, 22 enormous enhancement at the band edges 12 and cavity modes 23 can be observed at longer wavelength scales. The effect of PC's on the radiation of a localized source is encapsulated in local density of states ͑LDOS͒, which depends on position and frequency.…”