Broadband Fivefold reduction of vacuum fluctuations probed by dyes in photonic crystals Koenderink, A.F.; Bechger, L.; Schriemer, H.P.; Lagendijk, A.; Vos, W.L.

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to 3000 km wide separated regularly by -6800 km alolipside a dark belt at 50"s that probably pertained to a wave phenomena resulting from the disturbance. T h e u ~norphologic evolution of this disturbance resembled, o n a smaller scale, previous GWS disturba~lces (3), and it was probably a transient convective phenomenon rapidly disnersed bv the zonal winds.This obserl.ation, together with other transient snots ohserl~ed in recent times ( l o ) , suggests that mid-scale storms o n Saturn could be Inore frequent than previously thought. Theii detection is a cluestion of the telnnoral coveraee of the observations. " the techniques used, and the contrast of the features against the background of ~lorlnal clouds.

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The character of wave transport through a strongly scattering medium, excited by a pulsed plane-wave source, is investigated as a function of sample thickness over the range from about one to 13 mean free paths. To examine the behavior theoretically, we perform a first-principles calculation of both the frequency correlation function of the transmitted field and the time-domain profile of the transmitted intensity. These quantities are investigated experimentally using an ultrasonic technique, which allows us to separate the ballistic and scattered components of the total transmitted field, and hence to measure the scattered component unambiguously in thin samples. For sample thicknesses greater than about four mean free paths, we find good agreement between our theory, the diffusion approximation, and our experimental data for both the frequency correlation function and the intensity time profile. In thinner samples, there are systematic differences between theory and experiment. To characterize the transition from ballistic to diffusive behavior in thin samples, we focus on the arrival time of the peak in the scattered component of the transmitted intensity; unexpectedly we find that the scattered peak arrival time exhibits an abrupt crossover between ballistic and diffusive behavior when the ratio of sample thickness to mean free path, L/l , is approximately equal to 3. Excellent agreement is obtained between our theory and experiment for this crossover behavior over the entire range of sample thicknesses investigated. ͓S1063-651X͑99͒12009-9͔

Measurements of the diffusive transport of multiply scattered ultrasonic waves show that the energy velocity is very similar in magnitude and frequency dependence to the group velocity. Our data are accurately described using a theoretical model that accounts for the renormalization of scattering by the coupling between neighboring scatterers, quantitatively predicting the scattering delay that causes the strong frequency dependence of these velocities seen in our experiments. This gives a unified physical picture of the velocities of energy transport by both diffusive and ballistic waves.[ S0031-9007(97) PACS numbers: 43.35. + d, 43.20. + g, 62.30. + d In recent years, there has been a tremendous revival of interest in the propagation of classical waves through inhomogeneous media containing random scatterers [1,2]. When the scattering is strong, the propagation is typically very well described using the diffusion approximation, and this has successfully facilitated the interpretation of a wide range of fascinating wave phenomena. Even though the transport is diffusive, it is, nevertheless, still essential to define several propagation velocities. These include the group ͑y g ͒ and phase ͑y p ͒ velocities which characterize the ballistic, or unscattered, component of the incident waves. For diffusive waves, the relevant velocity is the energy velocity y e which is defined as the ratio of the energy flux to the energy density [3], and which corresponds to the average local velocity of energy transport in the diffusion process, since y e is related to the wave diffusion coefficient D by D y e l ‫ء‬ ͞3. Here l ‫ء‬ is the transport mean free path, or the distance the waves must propagate until their direction is randomized. Within this scenario, the energy velocity represents a distinctly different type of velocity, as wave coherence is no longer relevant. The first calculations for diffusive waves by the Amsterdam group [4], carried out for low volume fractions of scatterers f, correctly accounted for the extremely low value of y e found in light scattering experiments; they also predicted a strong frequency dependence for y e in the vicinity of Mie resonances as a result of the temporary storage of wave energy inside the scatterer [4][5][6][7]. However, these calculations fail as f is increased. Following an idea proposed by Sheng [2], recent Coherent Potential Approximation (CPA) calculations [8,9] have attempted to rectify this deficiency for higher volume fractions, and have suggested that the large variations of y e with frequency are washed out with increasing f; these results are purported to be in excellent agreement [8,9] with the limited experimental data that currently exist for y e [10,11]. However, in the intermediate frequency regime where resonant scattering occurs, such CPA theories become suspect [12], raising questions about their reliability. In addition to these questions about the behavior of y e , there have been conflicting ideas about the possible connection between the energy velocity ...

Comment on "Spontaneous emission of organic molecules embedded in a photonic crystal", Megens, M.; Schriemer, H.P.; Lagendijk, A.; Vos, W.L.

The transport of classical waves in strongly scattering media is investigated using ultrasonic techniques, allowing us to measure both the ballistic and scattered components of the wave field. We find that the ballistic propagation is dramatically slowed down by scattering resonances, although the group velocity remains well-defined. The propagation of the scattered waves is also strongly affected by resonant scattering, and is shown to be well described by using the diffusion approximation. A model based on the generalized coherent potential approximation gives a quantitative explanation of the experimental data.