Clémentine Javaux

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.

Strong Purcell effect observed in single thick-shell CdSe/CdS nanocrystals coupled to localized surface plasmons

High quality factor dielectric cavities designed to a nanoscale accuracy are mostly used to increase the spontaneous emission rate of a single emitter. Here we show that the coupling, at room temperature, between thick shell CdSe/CdS nanocrystals and random metallic films offers a very promising alternative approach. Optical modes confined at the nanoscale induce strong Purcell factors reaching values as high as 60. Moreover the quantum emission properties can be tailored: strong antibunching or radiative biexcitonic cascades can be obtained with high photon collection efficiency and extremely reduced blinking.

Influence of the cluster's size of random gold nanostructures on the fluorescence of single CdSe-CdS nanocrystals

It is well known that coupling a single emitter to metallic structures modifies drastically its fluorescence properties compared to single emitter in vacuum. Depending on various parameters such as the nature of the metal or the geometry of the metallic structure, quenching or intensity enhancement as well as radiative processes acceleration are obtained through the creation of new desexcitation channels. The use of metallic random structures gives the opportunity to magnify the effect of the coupling by strongly confined electromagnetic fields. A gold film at the percolation threshold is an interesting illustration of that effect. Here, we study the influence of the method used to realize these films through two different examples. First, we show that the mean size of the gold clusters constituting the film depends on the deposition method. Even if similar optical properties (in particular far-field absorption) are exhibited by the structures, crucial differences appear in the fluorescence of single emitters when coupled to the two kinds of random gold film. Especially, we focus our attention on the creation of desexcitation channels and show that they are cluster size dependent.

Fluorescence properties of thick shell CdSe/CdS quantum dots at cryogenic temperature

The blinking of fluorophore, that means the switch between bright and dark states, is a well-known phenomenon for single emitters. In the case of standard CdSe/ZnS colloidal quantum dots (QDs), this was considered as their main drawback for experiments at the single molecule level. Statistical analysis of these intensity fluctuations has demonstrated that the dark states duration exhibits a universal heavy-tailed power law distribution. Long off-periods, of the order of the time experiment, are always observed.

Enhancing the fluorescence of thick-shell single CdSe-CdS nanocrystals through their coupling with plasmon resonances of gold films

Summary form only given. Metallic structures open the possibility to optimize the emission of a single emitter for the generation of single photons. High collection efficiency can be achieved and the radiative decay rate can be increased through the well-known Purcell effect. For emitters operating at room temperature and thus showing a large linewidth, plasmonic structures are specially well-suited since they exhibit a very broad bandwidth that ensures the tuning between the cavity and the emitter fluorescence.In this presentation, the possibility to enhance and control the emission of individual CdSe-CdS nanocrystals (NCs) directly deposited on a semi-continuous or a semi-continuous film is studied. An original method of data analysis is used to characterize in detail the quantum and classical properties of the NCs emission for the two kinds of structures. To start with,, the gold film is made by evaporation under ultra-high vacuum (109 torr). Controlling the deposition duration, a continuous film or a semicontinuous film just below the percolation threshold can be prepared. The delocalized plasmon modes of the continuous gold film have been widely studied. Concerning the random gold film, subwavelength-sized resonances are observed and the absorbance spectrum shows a very broad plateau from the visible to the near-infrared. NCs (λ=660 nm) diluted in a mixture of hexane octance (9:1) are spin-coated directly on the films. The NCs are optically excited at the single object level by a pulse laser diode. Their fluorescence is collected with an inverted confocal microscope. The time-statistic of the photons is analyzed with a high-sensitivity HanburyBrown and Twiss set-up associated to a standalone Time-Correlated Single Photon Counting module (Picoquant, PicoHarp 300) that enables to record the absolute time of arrival of each photon (resolution of 64 ps) with respect to the beginning of the whole experiment. From the same data, the time trace of the intensity, the autocorrelation function over a time scale ranging from 100 ns to 1s, the Mandel factor and the fluorescence lifetime can be investigated. Moreover, when biexcitonic cascades are collected, in constrast with a standard setup that only provides the delays between photons, the photons corresponding to the emissions of the biexcitonic and monoexcitonic states can be separated unambiguously. First, the number of radiative and non radiative channels opened by the coupling between the plasmons and the NCs are investigated in detail. Purcell factors as high as 60 are calculated. Strong antibunching or high efficient radiative biexcitonic recombinations can be detected. The blinking properties are characterized from the nanosecond time scale to the ms one through the autocorrelation function and the Mandel factor. Perfect suppression of the blinking is reported. Finally, the rate of collected photons is analyzed. High efficiency of the photon collection (up to 37 %) is demonstrated and high increase of the radiative decay rate can be simultaneously obtained. In the field of quantum plasmonics, our results [1,2] emphasizes the great interest of associating metallic nanostructures and NCs with a very thick shell that acts as a spacer and prevents fluorescence quenching. The data analysis is original and of general interest. It could be usefull to characterize the emission of other fluorophores.

Controlling the emission of single giant CdSe/CdS nanocrystals through their coupling to random gold films

In the field of quantum optics, the control of the radiative properties of single emitter through its coupling to a photonic structure has attracted great attention. In particular, optical cavities enable to reduce the radiative lifetime through the effect first discribed by Purcell. The Purcell factor, which is equal to the increase of the radiative decay rate, is proportional to the cavity quality factor Q and to the inverse of the modal volume V. Dielectric cavities characterized by high quality factor Q are widely used. However if the quality factor of the emitter is lower than Q then it replaces it in the Purcell formula. Thus increasing Q is useless. Another possibility is to use structures which exhibit very small modal volume such as plasmonic structures. They have low Q but, due to electromagnetic field localization at subwavelength scale, they are well suited for the control of semiconductor nanocrystals (NCs) radiative lifetimes since these NCs exhibit rather small quality factor at room temperature.

CdSe/CdS nanocrystals coupled to a self-assembled plasmonic crystal

Enhancing and extracting light emission from a fluorescent system is a challenging problem, with applications ranging from light-emitting diodes to quantum optics. In particular, efficient single photon sources with enhanced decay rate and well-defined emission direction and polarization are of great interest for quantum cryptography devices. Coupling the emitter to the plasmon mode of a metallic surface is a way to increase decay rate and ensure emission redirection over a broad spectral range. This was demonstrated in a previous paper with colloidal CdSe/ZnS nanocrystals considered at the single-emitter level and coupled to a plane gold surface [1]. The fluorescence decay rate was increased by a factor 7 at 20 nm from a gold surface, and the overall detected fluorescence per nanocrystal was increased by a factor 2.4 at 80 nm from a gold surface. These results can be dramatically improved as most of the emission is lost in non-radiative surface plasmons. For this purpose, we developed and characterised an hybrid metal-dielectric photonic crystal showing a periodically nanostructured metallic surface to couple the surface plasmon mode to the far-field radiation.