Quantum dots as single-photon sources: Antibunching via two-photon excitation

Two-photon excitation induced photoluminescence (2PE-PL) microscopy of CdSe colloidal quantum dots at the single-entity level is demonstrated. We provide evidence for single nanoparticle microscopy in the two-photon excitation regime by varying the laser excitation average power, as well as by measuring the confocal point spread function in three dimensions with a single quantum dot. Model calculations of the point spread function are in good agreement with our experimental findings in the 2PE-PL nonsaturation regime. Ultimately, we observe photon antibunching and triggered single-photon emission at room temperature from those quantum nanostructures under two-photon excitation in a well-defined three-dimensional photonic volume.

Figure 1

Double logarithmic plot of the laser excitation power vs the PL luminescence rate showing a slope of two. Inset: Schematic of the experimental setup used. The photoluminescence of a single quantum dot sample (S) is sent through a dichroic mirror (DC) and a mirror (M) into a Hanbury Brown and Twiss detection device comprising two avalanche photo diodes (APD) connected by a 50:50 beam splitter (BS).

Figure 2

(a) and (d) show the model of the confocal point spread function (PSF); (b) and (e) show the confocal point spread function measured with a single quantum dot as the probe; (c) and (f) show the comparison (normalized) of the model PSF [solid line; line cut through center of (a) and (d)] with the experimental PSF [scattered points; cut through center of (b) and (e)].

Figure 3

(a) Photoluminescence intensity image of quantum dots via two-photon excitation at room temperature (scale bar 1 $\mu$m, typical PL count rate $500\hspace{0.5em}000$ photons/s for a single quantum dot at ambient conditions); (b) corresponding photoluminescence lifetime information image; and (c) coincidence measurement of PL photon pairs of a single quantum dot [arrow in (a)] using a Hanbury Brown and Twiss detection device (connected points). The coincidence measurement is fitted with a sum of single exponentials (solid line) yielding an apparent excited-state lifetime for this single quantum dot of $\tau =1.7$ ns. The normalized coincidence curve yields the intensity autocorrelation function $g^2({\tau }_{\mathrm{inter}})$ with numeric values above each peak.