Single Photons Emitted by Nanocrystals Optically Trapped in a Deep Parabolic Mirror

We investigate the emission of single photons from $\mathrm{CdSe}/\mathrm{CdS}$ dots-in-rod which are optically trapped in the focus of a deep parabolic mirror. Thanks to this mirror, we are able to image almost the full $4\pi$ emission pattern of nanometer-sized elementary dipoles and verify the alignment of the rods within the optical trap. From the motional dynamics of the emitters in the trap, we infer that the single-photon emission occurs from clusters comprising several emitters. We demonstrate the optical trapping of rod-shaped quantum emitters in a configuration suitable for efficiently coupling an ensemble of linear dipoles with the electromagnetic field in free space.

Figure 1

(a) Normalized second-order intensity correlation $g^{(2)}(0)$ of fluorescence photons emitted by DRs against the power $P_{\min }$ at which the particles escaped the trap. Error bars show the Poissonian uncertainty. Green circles denote experiments in which the spatial intensity pattern revealed azimuthal symmetry and an obvious alignment of the rods to the axis of the PM. A single such data point with $P_{\min }=22\mathrm{mW}$ and $g^{(2)}(0)=0.19$ is off scale. Orange squares correspond to cases where the emission pattern shows a clear azimuthal asymmetry. Black triangles mark experiments with an inconclusive outcome. (Inset) Magnified view for low power levels. (b) Damping rate of axial motion ${\mathrm{\Gamma }}_z$ of trapped nanoparticles against $P_{\min }$. Error bars denote the 95% confidence interval of the fit from which ${\mathrm{\Gamma }}_z$ was obtained. The dashed line denotes the function ${\mathrm{\Gamma }}_z\propto P_{\min }^{0.48\pm 0.03}$, with the power scaling obtained from a fit. (Inset) Pictorial view of a cluster of rods aligned along the optical axis of the PM ($z$ axis). The yellow disk with radius $r_z$ denotes the effective area of the cluster relevant for damping the motion along the $z$ axis.

Figure 2

(a) Example of a spatial intensity pattern with rotational symmetry as observed in the aperture of the PM. The corresponding sample is the one with $P_{\min }=2.9\mathrm{mW}$ and $g^{(2)}(0)=0.31$ in Fig. 1. From left to right, the panels show the intensity when projected onto a vertical state of polarization, the projection onto a horizontal state of polarization, and the sum of the two, respectively. Dashed lines indicate the orientation of extinction lines as a guide for the eye. (b) Fraction of linear-dipole radiation in the fluorescence of small clusters of DRs as a function of trapping beam power. Values obtained in the same experimental run are connected by lines. The data points correspond to the ones depicted by circles in Fig. 1.