High purcell effect and directional emission for semi-conductor nanocrystals deterministically positionned in a plasmonic patch antenna

Abstract

Plasmonic nano-antennas provide broadband spontaneous emission control by confining light on highly sub-wavelength volumes. This property assures efficient coupling and spectral matching of spectrally broad emitters like defects in diamonds or colloidal quantum dots with nanostructures. Precise positioning of emitters inside the nanostructures has been a limitation for efficient coupling and remains a key parameter to control for efficient light nanostructures interaction. In this paper, we propose to optimise the interaction of nanoemittors with antennas by assuring spatial and spectral matching. Although spontaneous emission acceleration ensuring large coupling to the mode has been evidenced in antennas, this property has not been combined with high emission directionnality.Here, we control both the spontaneous emission rate and radiation pattern of nanocrystals in a plasmonic antenna. We use a patch antenna, as proposed in [1], consisting in a thin gold microdisk 30 nm above a thick gold layer (fig. 1a), with a emitter positioned in the dielectric spacer. The small 30 nm separation between the disk and the gold film provide a large confinement of the electromagnetic field. The emitters are clusters of CdSe/CdS colloidal nanocrystals. A deterministic positioning of clusters inside each antenna with a precision of 25nm is provided by an optical in situ lithography technique as demonstrated in [2] for quantum dots in micropillars. The emitters are shown to radiate through the entire patch antenna in a highly directional mode. The theory predicts an acceleration of dipole emission up to 70 for a dipole perpendicular to the gold layers, and much lower for a dipole parallel (fig. 2a). For an antenna with a gold disk diameter ranging from 1.4 to 2.2. m, emission is directive in accordance with the theory (fig 1b-1c). The average cluster lifetime is reduced by a factor ranging from 5 to 15 (fig. 2b) which correspond, taking into account the 2D dipole transition of the emitters, their orientation and distribution of lifetime inside the cluster, to accelerations consistent with an acceleration of 70 for vertical dipoles. These measurements, in good agreement with our theoretical calculations, evidence high directionality and acceleration of spontaneous emission. Our work demonstrates the potential of plasmonic patch antennas to fabricate efficient single photon sources.