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Several in-situ chemical reduction methods were systematically evaluated in view of the formation of silver−latex composites, and in particular with respect to the task of coating colloidal latex spheres with uniform thin layers of silver. Such nanocomposite materials are of profound interest, due to their expected novel optical properties. The samples were investigated by transmission electron microscopy and UV−vis spectroscopy. A range of silver particle features was obtained, including even and uniform silver coatings in the nanometer size regime on the latex particles.

Optical microcavities that confine the propagation of light in all three dimensions (3D) are fascinating research objects to study 3D-confined photon states, low-threshold microlasers, or cavity quantum electrodynamics of quantum dots in 3D microcavities. A challenge is the combination of complete electronic confinement with photon confinement, e.g., by linking a single quantum dot to a single photonic dot. Here we report on the interplay of 3D-confined cavity modes of single microspheres (the photonic dot states) with photons emitted from quantized electronic levels of single semiconductor nanocrystals (the quantum dot states). We show how cavity modes of high cavity finesse are switched by single, blinking quantum dots. A concept for a quantum-dot microlaser operating at room temperature in the visible spectral range is demonstrated. We observe an enhancement in the spontaneous emission rate; i.e., the Purcell effect is found for quantum dots inside a photonic dot.Spherical microcavities of a few micrometers in diameter show sharp, spectrally well-separated cavity modes in the visible spectral range. These eigenmodes of a 3D microcavity are characterized by the angular quantum number l and radial quantum number n for the transverse electric (TE l n ) andtransverse magnetic (TM l n ) field modes. The optical quality of a microcavity is defined by Q ) ω cav /∆ω cav , the ratio between resonance frequency ω cav and bandwidth ∆ω cav of a cavity mode. High Q values mean narrow modes and efficient light trapping inside the microcavity. When a lightemitting dipole, for example, a semiconductor nanocrystal (quantum dot (QD)), is inserted into a 3D microcavity and its eigenfrequency ω QD is resonant with a high-Q cavity mode ω cav , then the confined photonic and electronic states interact and a new coupled quantum-mechanical system evolves with a coupling strength defined by the Rabi splitting Ω R (see, e.g., refs 1-6). Semiconductor quantum dots inside a 3D microcavity (photonic dot (PD)), a structure we will designate from now on as QDs@PD, provide a fascinating artificial system to study light-matter interaction in confined systems. The examples presented here apply to the regime of weak coupling, i.e., Ω R < ∆ω cav . Figure 1 shows a representative QDs@PD structure based on a glass microsphere. The emission characteristics of a luminescent CdSe nanocrystal will completely change when it is incorporated into a 3D microcavity, as illustrated by the theoretical emission pattern plotted in Figure 1. As can be seen in the single mode pattern of a so-called whispering gallery mode, the electromagnetic field is highest close to the surface of the sphere. Therefore, to study the combined 3D confinement of electrons and photons, most promising are concepts with QDs attached to the surface of a bulk microsphere or embedded in the thin shell of hollow microspheres doped with QDs. 7-9 We choose as lightemitting dipoles highly luminescent ZnS-coated CdSe nanocrystals 10 (diameter ∼4.5 nm). These nanocrystals are chemically bond...

J-aggregation has been shown to be beneficial for light amplification in single crystals of π-conjugated organic molecules. In the case of H-aggregation, the criteria for such processes are still under debate. It has also been shown that H-aggregate arrangements with considerable π-π overlap are detrimental for light amplification. We show here that a proper alignment of the molecules in the crystal lattice, which minimizes π-π overlap between adjacent molecules, gives rise to (random) stimulated emission from cofacial arrangements similar to that of the herringbone aggregates.

Time-resolved spectral hole-burning experiments have been performed to probe the dynamics of the S 1 r S 0 0-0 transition of bacteriochlorophyll-a at low concentration (1 × 10 -5 M) in four different glasses (2-methyltetrahydrofuran, protonated and deuterated ethanol, diethyl ether, and triethylamine) as a function of delay time t d (from 10 -6 to 10 3 s) and temperature T (1.2-4.2 K). It is shown that spectral diffusion, the broadening of the optical linewidth followed here over nine orders of magnitude in time, increases with temperature as T 1.3(0.1 and strongly depends on the glass structure. The functional dependence, however, is not influenced by the specific glass. The variation of the "effective" homogeneous linewidth (Γ′ hom ) with T and t d is described by a function Γ′ hom (T,t d ) derived by modifying the standard model of two-level systems (TLS). This revised TLS model, in which the distribution functions of the TLS tunneling parameters are different from those in the standard model, takes into account the common origin of the dependence of Γ′ hom on t d and T. It is shown that other hole-burning and photon-echo data reported in the literature can also be fitted by the same function Γ′ hom (T,t d ). In ethanol glass, the number of TLSs and the amount of spectral diffusion appear to be independent of the probe molecule.