Subnanosecond spectral diffusion of a single quantum dot in a nanowire

We have studied spectral diffusion of the photoluminescence of a single CdSe quantum dot inserted in a ZnSe nanowire. We have measured the characteristic diffusion time as a function of pumping power and temperature using a recently developed technique [G. Sallen et al., Nat. Photon. 4, 696 (2010)] that offers subnanosecond resolution. These data are consistent with a model where only a single carrier wanders around in traps located in the vicinity of the quantum dot.

Figure 1

The experimental setup is based on a standard microphotoluminescence experiment operating at a temperature of $T=4$ K. A continuous-wave diode laser emitting at 405 nm excites the sample via beam splitter BS1 with reflection R1 $=$ 30%. The luminescence is then transmitted through BS1 and split by 50$/$50 beam splitter BS2. Each beam is sent to a monochromator (resolution of $0.2$ meV) whose output slit illuminates an avalanche photodiode (APD) connected to a time-correlated single-photon (TCSP) module that builds a histogram of the time delays between photons. This allows us to perform either autocorrelation when the two spectrometers (Spectro) are tuned at the same wavelength or a cross correlation otherwise. The output slits of the spectrometers are adjusted so that a controlled spectral window of a given line is detected. The work presented herein has been obtained with high-quantum-efficiency APDs ($\eta =60\%$ at 550 nm). With these APDs the measured timing resolution of the whole setup is 800 ps.

Figure 2

Spectral diffusion of the charged exciton (CX) line (see text). (a) Autocorrelation of one half of the profile showing the bunching due to SD (${\tau }_d=4$ ns) and the narrower single-photon antibunching. (b) Cross correlation between the two halves of the profile displaying the antibunching due to SD with the same characteristic time ${\tau }_d=4$ ns. These two plots have been obtained on the same QD with the same excitation power. The solid lines are fits with the model explained in Ref. 18.

Figure 3

(a) Scanning electron microscope image of a ZnSe nanowire containing a CdSe quantum dot. (b) Photoluminescence spectrum.

Figure 4

(a) Left axis: Diffusion rate ${\gamma }_d$ at $T=4$ K (open red circles) and $T=10$ K (solid blue circles) and the charged to neutral hopping rate ${\gamma }_C$ (open green triangles; see text) as a function of exciting power. Right axis: The black stars are proportional to the total amount of light $\Sigma$ emitted by X, CX, and XX and a fair approximation of the total number of carrier $N$ in the NW. The saturation pumping power for the charged exciton is $4\mu W$. (b) Corresponding linewidth as a function of exciting power at $T=4$ K. The insets are examples of two spectra taken at different powers, showing that not only the linewidth but also the shape is unchanged.