2, Iberian Nanotechnology Laboratory, Braga, , Portugal
3, ITODYS, Paris, , France
Quantum dots (QDs) can lead either to the enhancement or to the quenching of their photoluminescence , provided that they are coupled with metallic nanoparticles (MNPs). Such MNPs, well known to sustain Localized Surface Plasmon (LSP) resonances, may indeed affect the QDs photoluminescence. The distance between QDs and MNPs is one of the switch parameters between both regimes. The goal of this study is to control the coupling distance (different from the physical distance) between QDs and MNPs by changing the refractive index of the surrounding medium using photochromic molecules. These molecules are optical switches, which move from a transparent state to a colored one by absorbing UV light. The spectral overlap and the lifetime of each optical phenomenon are the key parameters, since the photochromic molecules can couple to LSP to induce strong coupling  or couple to QDs to quench the photoluminescence .
In this study, the Fluorescence Lifetime Imaging Microscopy (FLIM)  has been performed to record QDs photoluminescence lifetime and intensity. We fabricated silver nanoparticles arrays covered with a protective SiO2 layer and we spin-coated different mixtures on top of it. Firstly, we studied this sample spin-coated with QDs in a PMMA matrix and then, we studied the same sample spin-coated with QDs and photochromic molecules diluted in a PMMA matrix. The QDs photoluminescence lifetime and intensity have then been explored before and after the photochromic transition, above and nearby the MNPs arrays.
The analysis of the results shows a Förster Resonant Energy Transfer between the QDs (donors) and the colored form of the photochromic molecules (acceptors). In addition, it is observed an optical activation of the resonant coupling between QDs and MNPs due to the photochromic transition.
 Enhancement and quenching regimes in metal-semiconductor hybrid optical nanosources, P. Viste et al., ACS Nano, vol.4, n° 2, p. 759-764 (2010).
 Reversible strong coupling in silver nanoparticle arrays using photochromic molecules, A.-L. Baudrion et al., Nano Lett. 13, p. 282−286 (2013).
 Reversible Modulation of Quantum Dot Photoluminescence Using a Protein-Bound Photochromic Fluorescence Resonance Energy Transfer Acceptor, I. L. Medintz et al., J. Am. Chem. Soc., 126, p. 30-31 (2004).
 Munster, Erik B. van, and Theodorus W. J. Gadella. « Fluorescence Lifetime Imaging Microscopy (FLIM) ». In Microscopy Techniques, edited by Jens Rietdorf, 143 75. Berlin, Heidelberg: Springer Berlin Heidelberg, 2005. https://doi.org/10.1007/b102213.