Laurent Coolen

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.

Controlling spontaneous emission with plasmonic optical patch antennas.

We experimentally demonstrate the control of the spontaneous emission rate and the radiation pattern of colloidal quantum dots deterministically positioned in a plasmonic patch antenna. The antenna consists of a thin gold microdisk separated from a planar gold layer by a few tens of nanometers thick dielectric layer. The emitters are shown to radiate through the entire patch antenna in a highly directional and vertical radiation pattern. Strong acceleration of spontaneous emission is observed, depending on the antenna geometry. Considering the double dipole structure of the emitters, this corresponds to a Purcell factor up to 80 for dipoles perpendicular to the disk.

Controlled modification of single colloidal CdSe/ZnS nanocrystal fluorescence through interactions with a gold surface.

Single colloidal CdSe/ZnS nanocrystals are deposited at various distances from a gold film in order to improve their performance as single photon sources. Photon antibunching is demonstrated and the experimental curves are accurately fitted by theoretical equations. Emission lifetime and intensity are measured and found in excellent agreement with theoretical values. The various effects of a neighbouring gold film are discussed : interferences of the excitation beam, interferences of the fluorescence light, opening of plasmon and lossy-surface-wave modes, modification of the radiation pattern leading to a modified objective collection efficiency. At 80 nm from the gold film, when using an objective with 0.75 numerical aperture, about a 2.4-fold increase of the detected intensity is evidenced.

Manipulating emission of CdTeSe nanocrystals embedded in three-dimensional photonic crystals

We report experimental and theoretical results on the photoluminescence of CdTeSe nanocrystals, embedded in a silica opaline structure by infiltration of a highly diluted solution. Strong modification of emission diagrams of embedded nanocrystals have been observed in good agreement with theoretical models. At macroscopic scale, we measured the difference of nanocrystals emission lifetime embedded either in an opal for which the emission is in the gap, or in an opal of smaller balls diameter for which the emission is outside the gap. The photonic bandgap effect leads to a lifetime increase of the order of 10%. These lifetime variations are shown to be in good agreement with the calculated local density of states modification due to the pseudogap.

Photon-correlation Fourier spectroscopy

We describe a method to probe the spectral fluctuations of a transition over broad ranges of frequencies and timescales with the high spectral resolution of Fourier spectroscopy, and a temporal resolution as high as the excited state lifetime, even in the limit of very low photocounting rates. The method derives from a simple relation between the fluorescence spectral dynamics of a single radi-ating dipole and its fluorescence intensity correlations at the outputs of a continuously scanning Michelson interferometer. These findings define an approach to investigate the fast fluorescence spectral dynamics of single molecules and other faint light sources beyond the time-resolution capabilities of standard spectroscopy experiments.

Experimental Determination of the Fluorescence Quantum Yield of Semiconductor Nanocrystals.

Many studies have considered the luminescence of colloidal II–VI nanocrystals, both in solution at a collective scale and at an individual scale by confocal microscopy. The quantum yield is an important figure of merit for the optical quality of a fluorophore. We detail here a simple method to determine the quantum yield of nanocrystals in solution as a function of the absorption. For this purpose, we choose rhodamine 101 as a reference dye to measure the nanocrystal fluorescence quantum yield. The influence of the concentration on quantum yield is therefore studied for both the reference and the solutions of nanocrystals and is found to be critical for the acuity of the method. Different types of nanocrystals are studied to illustrate different quantum yield evolutions with the concentration.

Polarimetry-based analysis of dipolar transitions of single colloidal CdSe/CdS dot-in-rods

We use polarization resolved micro-photoluminescence to analyze the dipolar nature of single core/shell cadmium selenide/cadmium sulfide (CdSe/CdS) dot-in-rods. Polarization analysis, anisotropy measurements on more than 400 nanoparticles, and defocused imaging suggest that these nanoparticles behave as linear dipoles. The same methods were also used to determine the three-dimensional orientation of the emission dipole, which proved to be consistent with the hypothesis of a linear dipole tilted with respect to the rod axis. Moreover, we observe that for high-energy pumping, the excitation transition of the dot-in-rod cannot be approximated by a single linear dipole, contrary to the emission transition.

Isotropic broadband absorption by a macroscopic self-organized plasmonic crystal.

We describe the plasmonic properties of a two-dimensional periodic metallic grating of macroscopic size obtained by gold deposition on a self-assembled silica opal. Structural characterization shows a transition from microscopic order to isotropy at macroscopic scale. Optical reflection spectra exhibit a dip of almost complete absorption due to coupling to surface-plasmon-polaritons (SPP). This is explained by theoretical calculations introducing a density of coupled SPP modes. We demonstrate, at a given incidence angle, a broad continuum of coupled wavelengths over the visible spectrum. This opens new possibilities in fields where light-plasmon coupling is required over a broad range of wavelengths and incidence orientations.

A sputtered-silica defect layer between two artificial silica opals: an efficient way to engineer well-ordered sandwich structures

We demonstrate a low cost and efficient method to engineer deterministically a defect between two well-ordered silica opals by sputtering silica on the top of the first one. The optical response of these sandwich structures is studied in terms of specular reflection and transmission spectroscopy. For a given thickness of sputtered silica, tailored highly transmitted and reflected optical modes are evidenced. The very good agreement between the experimental results and the simulations, run without fitting parameters, demonstrates the good control of the geometry of our structures. The macroscopic realization of well-controlled 3D photonic crystals with planar defects, presented in this paper, opens the way for an efficient manipulation of embedded nano-emitter fluorescence with applications in the fields of light-emitting devices and high-sensitivity pollutant optical detection.

Determination of the Surface Plasmon Polariton Extraction Efficiency from a Self-Assembled Plasmonic Crystal

We experimentally measure and analytically describe the fluorescence enhancement obtained by depositing CdSe/CdS nanocrystals onto a gold plasmonic crystal, a two-dimensional grating of macroscopic size obtained by gold deposition on a self-assembled opal. We show evidences of nanocrystals near-field coupling to the gold surface plasmon polaritons (SPP) followed by grating-induced SPP reemission to far-field. We develop a theoretical framework and an original method in order to evaluate, from photoluminescence experiments, the SPP extraction efficiency of a grating.