Single quantum dot photocurrent spectroscopy in the cavity quantum electrodynamics regime

We study cavity quantum electrodynamics (cQED) in coupled quantum dot–microcavity systems under electrical readout. Strict resonant excitation of a target quantum dot (QD) allows us to monitor the photocurrent response of a single emitter in the quantum limit of light-matter interaction. We find a strong anticorrelation between radiative recombination and nonradiative tunnel escape of photoexcited carries which can be controlled by cQED effects in the Purcell regime. In fact, cavity-enhanced radiative emission from a QD results in a weaker photocurrent signal which reflects the cQED controlled competition between radiative and nonradiative recombination at the single emitter level.

Figure 1

Schematic band diagram, PL intensity and resulting PC of a QD inside the active layer of a micropillar cavity under reverse bias for different detunings between the resonator mode and the QD exciton. The photoexcited charge carriers leave the QD by either tunnel emission or radiative recombination, depending on the tunnel (${\tau }_{\text{esc}}$) and recombination lifetime (${\tau }_r\left(\Delta \right)$). In the weak coupling regime, ${\tau }_r\left(\Delta \right)$ is reduced on resonance ($\Delta =0$) due to the Purcell effect. This leads to an increase in PL intensity (red line) and a corresponding reduction of PC (blue line).

Figure 2

(a) High resolution PC spectrum of a QD micropillar with a diameter of $d_c=2.0\mu$m. The PC signals are associated with single QDs (X1 and X2) and with the fundamental cavity mode (C) of the micropillar. (b) Maximum of the PC signal from a single, off-resonant QD as a function of excitation power for a bias voltage of $V_{\text{bias}}=0$ V. Modeling (solid line) of the experimental data (bullets) yields a tunnel time of ${\tau }_{\text{esc}}=330$ ps.

Figure 3

(a) PC intensity map showing the temperature resonance tuning of a QD micropillar with a diameter of 1.8 $\mu$m and a $Q$ factor of 7000 under side excitation for $P=180$ $\mu$W and $V_{\text{bias}}=0$ V. On resonance between X and C at about 38 K, the PC decreases due to accelerated radiative emission from the QD weakly coupled to the cavity mode. (b) Normalized PC (peak value) as a function of the detuning $\Delta$ between X and C. Fitting the experimental data according to Eqs. (1) and (2) (calculated data: solid lines) yields a Purcell factor of $5.2\pm 0.5$. (c) PC resonance behavior for three excitation powers. The experimental data (bullets) is well reproduced by theory (solid lines), which allows the determination of excitation power dependent Purcell factors between 6.5 (for 30 $\mu$W) and 3.1 (for 600 $\mu$W).

Figure 4

Temperature resonance tuning of a 1.7 $\mu$m micropillar ($Q=4200$) under excitation in the normal direction. Temperature dependent set of (a) $\mu$PL and (b) $\mu$PC spectra at $V_{\text{bias}}=0$ V. At resonance with the cavity mode C, the PL and PC intensity of the single QD exciton X1 is strongly enhanced. (c) Energy dispersion of excitons (circles) and cavity mode C (stars) in PL (black symbols) and PC (red symbols). (d) Extracted intensity of X1 in dependence of detuning between X1 and C for PC and PL data, and calculated values (solid lines). Fitting the experimental data yields a Purcell factor of 8.6 (PL) and 4.3 (PC), respectively.