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 intensi- y, 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.