Spontaneous Spectral Diffusion in CdSe Quantum Dots.

Spectral diffusion of the emission line of single colloidal nanocrystals is generally regarded as a random process. Here, we show that each new spectral position has a finite memory of previous spectral positions, as evidenced by persistent anticorrelations in time series of spectral jumps. The anticorrelation indicates that there is an enhanced probability of the charge distribution around the nanocrystal returning to a previous configuration. We show both statistically and directly that this memory manifests as an observable spontaneous "relaxation" in the absence of a pump laser, so that spectral diffusion progresses in a manner of "two steps forward and one step back".

Detection of bright trion states using the fine structure emission of single CdSe/ZnS colloidal quantum dots.

We report direct observation of the lowest two states of the band-edge exciton fine structure in the photoluminescence from single CdSe/ZnS core/shell nanocrystals at cryogenic temperatures. The temperature dependence of this spectral fingerprint reveals exciton spin relaxation rates as low as 10 micros(-1). The fine structure is also dependent on the nanocrystal charge state facilitating the identification of a bright negatively charged trion state with a quantum yield comparable to that of neutral emission.

Fluorescent organosilica micro- and nanoparticles with controllable size.

This paper reports on the synthesis of uniformly dye-doped organosilica particles with narrow size distribution. The particle size can be controlled from tenths of nanometers up to several micrometers, whilst still maintaining monodispersity. Microparticles were observed to swell in various solvents up to approximately 2.5 times their original volume, suggesting the presence of a gel-like internal structure. As shown by confocal microscopy, this morphological control of particle swelling has important implications for the encoding of the nano/micro particles with organic dyes, such as rhodamine B isothiocyanate. Swelling allows the dye to penetrate the organosilica matrix and produce uniformly dye-doped nano- and microparticles. Finally, we suggest a coagulation model for the particle formation which significantly differs from conventional Stöber synthesis.

OH concentration and temperature measurements by use of near-resonant holographic interferometry.

Near-resonant holographic interferometry is demonstrated to measure temperature and species concentration in a two-dimensional steady premixed air-acetylene flame. A peak temperature of (2600 +/- 100) K and a peak OH number density of (9.6 +/- 0.3) x 10(22) m(-3) are obtained, consistent with the expected values for such a flame. These values are determined by recording interferograms with a laser assumed sufficiently detuned from line center so that pressure and temperature broadening can be ignored. The results are thus obtained without making prior assumptions on the temperature or pressure of the flame beyond the existence of thermal equilibrium.

Near-resonant holographic interferometry and absorption measurements of seeded atomic species in a flame.

The development of near-resonant holographic interferometry techniques for use on flows seeded with atomic species is described. A theoretical model for the refractivity that is due to the seed species is outlined, and an approximation to this model is also described that is shown to be valid for practical regimes of interest and allows the number density of the species to be determined without knowledge of line-broadening effects. The details of quantitative number density experiments performed on an air-acetylene flame are given, and a comparison with an alternative absorption-based experiment is made.