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.

Narrow zero-phonon emission lines are observed in single CdSe/CdZnS core/shell colloidal nanocrystals over a range of cryogenic temperatures up to 40 K. These nanocrystals display dramaticaly improved spectral stability enabling the observation of acoustic phonon sidebands accompanying most zero-phonon lines. A discrete phonon mode is attributed to the electron coupling to the l ) 0 acoustic breathing mode via the deformation coupling. The Huang-Rhys parameter, S ac , for this interaction is found to vary from 0.0016 to 0.09, demonstrating a wide dispersion in exciton-phonon coupling between different nanocrystals. Indeed, we observe single nanocrystals in which all acoustic phonon sidebands vanish, in close agreement with theoretical predictions that there should be negligible acoustic phonon coupling in an ideal spherical CdSe nanocrystal. Such nanocrystals are virtually decoupled from their environment, which is potentially useful for quantum technologies, such as single photon sources and quantum computing. In general, the ability to detect and quantify phonon interactions within single nanocrystals will provide significant insight into energy relaxation and dephasing processes in these systems.

The use of multiple radar configurations can overcome some of the geometrical limitations that exist when obtaining radar images of a target using inverse synthetic aperture radar (ISAR) techniques. It is shown here how a particular bistatic configuration can produce three view angles and three ISAR images simultaneously. A new ISAR signal model is proposed and the applicability of employing existing monostatic ISAR techniques to bistatic configurations is analytically demonstrated. An analysis of the distortion introduced by the bistatic geometry to the ISAR image point spread function (PSF) is then carried out and the limits of the applicability of ISAR techniques (without the introduction of additional signal processing) are found and discussed. Simulations and proof of concept experimental data are also provided that support the theory

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".

Spectral fluctuations observed in single CdSe/ZnS nanocrystals at 5 K are found to be entirely the result of discrete charge hops in the local environment of the nanocrystal, which occur at a rate comparable to the acquisition time of a single spectrum. We show that intervals between discrete spectral hops introduce a correlation between the successive measurements of the emission wavelength of single CdSe nanocrystals. This correlation can be recovered even in the presence of noise, but is shown to be sensitive to the experimental acquisition time, in good agreement with theory. However, we only find correlations for the smaller of the two nanocrystal sizes studied and discuss this in terms of size-dependent time scales correlated with the amount of excess energy dissipated in the nanocrystal due to hot-carrier relaxation.

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We show that the wandering of transition frequencies in colloidal quantum dots does not follow the statistics expected for ordinary diffusive processes. The trajectory of this anomalous spectral diffusion is characterized by a ffiffi t p dependence of the squared deviation. The behavior is reproduced when the electronic states of quantum dots are assumed to interact with environments such as, for example, an ensemble of two-level systems, where the correlation times are distributed according to a power law similar to the one generally attributed to the dot's luminescence intermittency.

We propose a scheme for achieving widefield coherent anti-Stokes Raman scattering (CARS) microscopy images with sub-diffraction-limited resolution. This approach adds structured illumination to the widefield CARS configuration [Applied Physics Letters 84, 816 (2004)]. By capturing a number of images at different phases of the standing wave pattern, an image with up to three times the resolution of the original can be constructed. We develop a theoretical treatment of this system and perform numerical simulations for a typical CARS system, which indicate that resolutions around 120 nm are obtainable with the present scheme. As an imaging system, this method combines the advantages of sub-diffraction-limited resolution, endogenous contrast generation, and a wide field of view.