Optically Gated Resonant Emission of Single Quantum Dots

We report on the resonant emission in coherently driven single semiconductor quantum dots. We demonstrate that an ultraweak nonresonant laser acts as an optical gate for the quantum dot resonant response. We show that the gate laser suppresses Coulomb blockade at the origin of a resonant emission quenching, and that the optically gated quantum dots systematically behave as ideal two-level systems in both regimes of coherent and incoherent resonant emission.

Figure 1

(a) Resonant emission (RE) spectra of the exciton with (●, gate on) and without (○, gate off) the additional nonresonant laser, in a single QD at 10 K. The RE intensity is measured as a function of the laser detuning with the exciton energy, $\delta =E_L-E_X$. The powers of the optical gate and the resonant laser are, respectively, equal to 3 nW and $4\mu \mathrm{W}$. Lorentzian fit (solid line) of width $3.2\mu \mathrm{eV}$. (b) Time-resolved RE of the QD when the gate is switched on (○). The solid line is a fit with a decay time $T_1=330\pm 20\mathrm{ps}$.

Figure 2

Intensity (a) and linewidth (b) of the optically gated resonant emission spectrum versus the power of the resonant laser, $P$. The experimental data are fitted by using Eqs. (1, 2) corresponding to a two-level system model (solid line).

Figure 3

(a) High-resolution Fourier transform spectroscopy of the optically gated RE, at zero detuning ($\delta =0$), for $P/P_0=2.4$. The background level is indicated with a dashed line. Fit (solid line) by using Eq. (3) with $T_L=100\mathrm{ns}$, $\hslash \Omega =2.5\mu \mathrm{eV}$, $T_1=330\mathrm{ps}$ and $T_2=480\mathrm{ps}$. (b) Theoretical RE signal in the spectral domain (red [medium gray] solid line) together with the incoherent contribution (dashed line) and the radiatively-limited emission line of width $2\mu \mathrm{eV}$ (light gray solid line). (c) Intensity autocorrelation measurements under resonant excitation at $\delta =0$, for $P/P_0=0.7$, when the optical gate is switched on.

Figure 4

(a) RE intensity as a function of the energy detuning between the resonant laser and the neutral exciton $X$ (left row), and positively charged exciton $X^{+}$ (right row), when the gate laser is switched off (top) and on (bottom). (b) RE intensity as a function of time (○) when the gate laser is switched on at $t=0$. The theoretical fit (solid line, red) is the convolution of the system response function (●) with an exponential of time constant $t_{\mathrm{on}}=5.1\pm 0.2\mu \mathrm{s}$. Inset: Scheme of the electronic states ($|v⟩$: empty QD; $|+e⟩$: one hole in the QD; $|X⟩$: neutral exciton; $|X^{+}⟩$: positively charged exciton) and the transitions involved in the optical gate effect (dashed arrow: neutralization of the QD by the optical gate; solid arrows: optical transitions under resonant excitation of the neutral exciton).