Xavier Quélin

Towards non-blinking colloidal quantum|[nbsp]|dots

At a single-molecule level, fluorophore emission intensity fluctuates between bright and dark states. These fluctuations, known as blinking, limit the use of fluorophores in single-molecule experiments. The dark-state duration shows a universal heavy-tailed power-law distribution characterized by the occurrence of long non-emissive periods. Here we have synthesized novel CdSe-CdS core-shell quantum dots with thick crystalline shells, 68% of which do not blink when observed individually at 33 Hz for 5 min. We have established a direct correlation between shell thickness and blinking occurrences. Importantly, the statistics of dark periods that appear at high acquisition rates (1 kHz) are not heavy tailed, in striking contrast with previous observations. Blinking statistics are thus not as universal as thought so far. We anticipate that our results will help to better understand the physico-chemistry of single-fluorophore emission and rationalize the design of other fluorophores that do not blink.

Nonclassical emission from single colloidal nanocrystals in a microcavity: a route towards room temperature single photon sources

Secure quantum communication systems (QCS) based on the transmission of crucial information through single photons are among the most appealing frontiers for telecommunications, though their development is still hindered by the lack of cheap and bright single photon sources (SPSs) operating at room temperature (RT). In this paper, we show the occurrence of photon antibunching at RT from single colloidal CdSe/ZnS nanocrystals (NCs) inserted in a vertical microcavity. Moreover, by using high-resolution lithographic techniques, we conceived a general route for positioning single colloidal quantum dots in the microcavity. The findings and the technique presented here can be considered a first step towards the development of SPS devices operating at RT.

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.

Non‐Blinking Semiconductor Colloidal Quantum Dots for Biology, Optoelectronics and Quantum Optics

Twinkle, twinkle: The blinking of semiconductor colloidal nanocrystals is the main inconvenience of these bright nanoemitters. There are various approaches for obtaining non-blinking nanocrystals, one of which is to grow a thick coat of CdS on the CdSe core (see picture). Applications of this method in the fields of optoelectronic devices, biologic labelling and quantum information processing are discussed.The blinking of semiconductor colloidal nanocrystals is the main inconvenience of these bright nanoemitters. For some years, research on this phenomenon has demonstrated the possibility to progress beyond this problem by suppressing this fluorescence intermittency in various ways. After a brief overview on the microscopic mechanism of blinking, we review the various approaches used to obtain non-blinking nanocrystals and discuss the commitment of this crucial improvement to applications in the fields of optoelectronic devices, biologic labelling and quantum information processing.

Plasmon assisted single photon emission of CdSe/CdS nanocrystals deposited on random gold film

We have investigated the modifications of the fluorescence properties of single CdSe/CdS nanocrystals deposited close to a semicontinuous gold film. We report a reduction by a factor of ten of the monoexcitonic state decay rate. Due to the structure of the plasmon resonances, the decay depends on the nanocrystal position. Through the detailed study of the collection efficiency, we show that a large fraction of the plasmons are converted in far field single photons. These results demonstrate the great interest of plasmonic devices to control the single photon emission of colloidal nanocrystals.

Enhancing the fluorescence of individual thick shell CdSe/CdS nanocrystals by coupling to gold structures

The fluorescence properties of individual CdSe/CdS nanocrystals (NCs) with a very thick shell and deposited on metallic structures are analyzed in detail. The results obtained for two metallic structures consisting of a continuous or a semi-continuous gold film are compared. Under low pulsed excitation, a strong acceleration of radiative processes is observed. The probability of electron–hole pair recombinations through Auger processes dramatically decreases, resulting in a suppression of blinking and the appearance of biexcitonic cascades. An original method of photons postselection also enables us to determine the decay rate corresponding to biexcitonic recombinations. Finally, a detailed analysis of the excitation process and the photon collection efficiency enables us to discriminate the effect of the gold structure in terms of excitation and fluorescence acceleration. It is found that high collection percentages can be achieved through the modification of NC emission with plasmonic structures.

Spatially uniform enhancement of single quantum dot emission using plasmonic grating decoupler

We demonstrate a spatially uniform enhancement of individual quantum dot (QD) fluorescence emission using plasmonic grating decouplers on thin gold or silver films. Individual QDs are deposited within the grating in a controlled way to investigate the position dependency on both the radiation pattern and emission enhancement. We also describe the optimization of the grating decoupler. We achieve a fluorescence enhancement ~3 times higher than using flat plasmon film, for any QD position in the grating.

Quantum cascades of photons in colloidal core-shell quantum dots

Individual colloidal core-shell nanocrystals represent very promising single photon emitters. Recently, progress concerning the synthesis of their shell enabled strong reduction of their fluorescence intermittency, which was their main drawback. We show that the synthesis of thick shells also enables us to modify the Auger effect efficiency which is a crucial parameter to control the quantum optical properties of an individual nanocrystal fluorescence. We demonstrate that these improvements open the possibility of generating quantum cascades of photons.

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.