Photon correlations in the collective emission of hybrid gold-(CdSe/CdS/CdZnS) nanocrystal supraparticles.

We investigate the photon statistics of the light emitted by single self-assembled hybrid gold-CdSe/CdS/CdZnS colloidal nanocrystal supraparticles through the detailed analysis of the intensity autocorrelation functiong(2)(τ). We first reveal that, despite the large number of nanocrystals involved in the supraparticle emission, antibunching can be observed. We then present a model based on non-coherent Förster energy transfer and Auger recombination that well captures photon antibunching. Finally, we demonstrate that some supraparticles exhibit a bunching effect at short time scales corresponding to coherent collective emission.

Fluorescence decay enhancement and FRET inhibition in self-assembled hybrid gold CdSe/CdS/CdZnS colloidal nanocrystal supraparticles.

We report on the synthesis of hybrid light emitting particles with a diameter ranging between 100 and 500 nm, consisting in a compact semiconductor CdSe/CdS/CdZnS nanocrystal aggregate encapsulated by a controlled nanometric size silica and gold layers. We first characterize the Purcell decay rate enhancement corresponding to the addition of the gold nanoshell as a function of the particle size and find a good agreement with the predictions of numerical simulations. Then, we show that the contribution corresponding to Förster resonance energy transfer is inhibited.

Localization of plasmon modes in a 2D photonic nanostructure with a controlled disorder.

In this paper, we focus on the optical properties of disordered hole arrays etched in a gold thin film. The disorder is induced and controlled using hole displacements following a Gaussian distribution and starting from a periodic array. The nanostructures present a transition from ordered arrays to short-range ordered arrays and random arrays by increasing the disorder amount. The associated optical properties are characterized in far and near fields by complementary approaches (absorption spectroscopy, classical scanning near field optical microscopy (SNOM) and Finite Difference Time Domain (FDTD) simulations). By increasing the disorder, a broadened absorption up to 30% in the far-field is achieved. Experiments in agreement with FDTD simulations point out the energy localization induced by the disorder and the dependence on the amount of disorder and on the excitation wavelength. By using a controlled disorder, we also show that the effect of these two parameters is also closely linked.

CdSe/CdS Dot-in-Rods Nanocrystals Fast Blinking Dynamics.

Analyzing the autocorrelation function of the fluorescence intensity, we demonstrate that these nanoemitters are characterized by a short value of the mean duration of bright periods (ten to a few hundreds of microseconds). The comparison of the results obtained for samples with different geometries shows that not only the shell thickness is crucial but also the shape of the dot-in-rods. Increasing the shell aspect ratio results in shorter bright periods suggesting that surface traps impact the stability of the fluorescence intensity.

Measuring the orientation of a single CdSe/CdS nanocrystal at the end of a near-field tip for the realization of a versatile active SNOM probe.

The orientation of a CdSe/CdS nanocrystal attached at the end of a scanning near field optical microscope (SNOM) tip is analyzed by its coupling with a flat gold layer. The Purcell factors for a set of distances to the gold surface are measured after a NC is caught by a SNOM tip. These measurements are compared with the modeling of the emission of a 2D dipole on a gold layer taking into account the layer of polymer serving as a glue for the NC. The 2D dipole is perpendicular to the c-axis of the NC, which is the growth axis. The behavior of the Purcell factor as a function of the distance to the gold layer depends on the angle made by this axis and the surface. The adjustment of the experimental results and the modelization gives the orientation of the NC at the end of the SNOM tip. Different orientations of the c-axis are determined.

Unraveling the time cross correlations of an emitter switching between two states with the same fluorescence intensity.

The autocorrelation function of the fluorescence intensity of a nanoemitter is measured with the standard Hanbury-Brown and Twiss setup. Time-tagging of the photodetection events during all the experiment has opened new possibilities in terms of post-selection techniques that enable to go beyond the blinking and antibunching characterization. Here, we first present a new method developed to investigate in detail the antibunching of a fluorophore switching between two emitting states. Even if they exhibit the same fluorescence intensity, their respective amount of antibunching can be measured using the gap between their respective decay rates. The method is then applied to a nanoemitter consisting in a colloidal quantum dot coupled to a plasmonic resonator. The relative quantum efficiency of the charged and neutral biexcitons are determined.

Thermal activation of non-radiative Auger recombination in charged colloidal nanocrystals.

Applications of semiconductor nanocrystals such as biomarkers and light-emitting optoelectronic devices require that their fluorescence quantum yield be close to 100%. However, such quantum yields have not been obtained yet, in part, because non-radiative Auger recombination in charged nanocrystals could not be suppressed completely. Here, we synthesize colloidal core/thick-shell CdSe/CdS nanocrystals with 100% quantum yield and completely quenched Auger processes at low temperatures, although the nanocrystals are negatively photocharged. Single particle and ensemble spectroscopy in the temperature range 30-300 K shows that the non-radiative Auger recombination is thermally activated around 200 K. Experimental results are well described by a model suggesting a temperature-dependent delocalization of one of the trion electrons from the CdSe core and enhanced Auger recombination at the abrupt CdS outer surface. These results point to a route for the design of core/shell structures with 100% quantum yield at room temperature.

Bright and grey states in CdSe-CdS nanocrystals exhibiting strongly reduced blinking.

When compared to standard colloidal nanocrystals, individual CdSe-CdS core-shell nanocrystals with thick shells exhibit strongly reduced blinking. Analyzing the photon statistics and lifetime of the on state, we first demonstrate that bright periods correspond to single photon emission with a fluorescence quantum efficiency of the monoexcitonic state greater than 95%. We also show that low intensity emitting periods are not dark but correspond to a grey state, with a fluorescence quantum efficiency of 19%. From these measurements, we deduce the radiative lifetime (45 ns) and the Auger lifetime (10.5 ns) of the grey state.

Emission characterization of a single CdSe-ZnS nanocrystal with high temporal and spectral resolution by photon-correlation Fourier spectroscopy.

We report a spectroscopic study of single colloidal CdSe/ZnS nanocrystals at low temperature. We use photon-correlation Fourier spectroscopy, a technique based on measuring the correlations of the intensities detected at the outputs of a Michelson interferometer. Spectral diffusion over a few microeV is evidenced, on a typical time scale of 200 micros. A time resolution as high as 20 micros is obtained, and an upper limit of 6.5 microeV emission linewidth is measured, corresponding to a coherence time of at least 200 ps, similar to the values for epitaxial quantum dots.

Measurement of the radiative and nonradiative decay rates of single CdSe nanocrystals through a controlled modification of their spontaneous emission.

We present a simple method to measure the radiative and nonradiative recombination rates of individual fluorescent emitters at room temperature. By placing a single molecule successively close and far from a dielectric interface and simultaneously measuring its photoluminescence decay and its orientation, both the radiative and nonradiative recombination rates can be determined. For CdSe nanocrystals, our results demonstrate that the fluorescence quantum efficiency, determined at the single-molecule level, is 98% in average, far above the value expected from conventional ensemble experiments. The bidimensional nature of the transition dipole is also directly evidenced from a single-particle measurement.

Statistical aging and nonergodicity in the fluorescence of single nanocrystals.

The relation between single particle and ensemble measurements is addressed for semiconductor CdSe nanocrystals. We record their fluorescence at the single molecule level and analyze their emission intermittency, which is governed by unusual random processes known as Lévy statistics. We report the observation of statistical aging and ergodicity breaking, both related to the occurrence of Lévy statistics. Our results show that the behavior of ensemble quantities, such as the total fluorescence of an ensemble of nanocrystals, can differ from the time-averaged individual quantities, and must be interpreted with care.

Bunching and antibunching in the fluorescence of semiconductor nanocrystals.

The fluorescence of single-colloidal CdSe quantum dots is investigated at room temperature by means of the autocorrelation function over a time scale of almost 12 orders of magnitude. Over a short time scale, the autocorrelation function shows complete antibunching, indicating single-photon emission and atomiclike behavior. Over longer time scales (up to tens of seconds), we measure a bunching effect that is due to fluorescence intermittency and that cannot be described by fluctuations between two states with constant rates. The autocorrelation function also exhibits nonstationary behavior related to power-law distributions of On and Off times.

Spatial distribution of the intensity noise of a vertical-cavity surface-emitting semiconductor laser.

We studied anticorrelated quantum fluctuations between the TEM(00) and the TEM(01) transverse modes of a vertical-cavity surface-emitting semiconductor laser by measuring the transverse spatial distribution of the laser beam intensity noise. Our experimental results are found to be in good agreement with the predictions of a phenomenological model that accounts for quantum correlations between transverse modes in a light beam.