Coherent Spectroscopy of Optically Gated Charged Single InGaAs Quantum Dots

The excited states of neutral and charged single InGaAs/GaAs quantum dots are studied using a confocal microspectroscopy technique. Because of their different Coulomb energy shifts, the charged and neutral states of the same quantum dot can be selectively excited. The charge of the quantum dot is controlled by a photo-depletion mechanism. Time-resolved coherent spectroscopy shows that the dephasing time of the excited states is longer when the quantum dot is charged. Rabi oscillation of the excited state of a singly charged quantum dot is demonstrated.

Figure 1

PL and PLE spectra for four individual QDs (labeled 1,2,3,4). PL spectra are obtained under resonant (excitation energy indicated by arrows) or nonresonant (${\lambda }_{\mathrm{e}\mathrm{x}\mathrm{c}}=532\mathrm{n}\mathrm{m}$) excitation. PLE spectra are obtained for detection at the energies of $X$ and $X^{*}$ lines observed in resonant PL. Inset (QD1): diagram presenting the optical transition discussed in the text. Inset (QD2): complete $s$-shell luminescence of QD2 as a function of detection and excitation energy.

Figure 2

QD1 $s$-shell PL spectra as a function of excitation intensity for (a) nonresonant and (b) resonant excitation at $e_1$. Inset: extracted intensity of $X$, $X^2$, and $X^{*}$ as a function of excitation intensity.

Figure 3

QD1 PL for fixed resonant excitation intensity on $e_2$ (${\lambda }_{\mathrm{e}\mathrm{x}\mathrm{c}}=895.2\mathrm{n}\mathrm{m}$) with varying nonresonant excitation intensity. Inset: (left) QD1 PL spectra for nonresonant (${\lambda }_{\mathrm{e}\mathrm{x}\mathrm{c}}=532\mathrm{n}\mathrm{m}$), resonant on $e_1$ (${\lambda }_{\mathrm{e}\mathrm{x}\mathrm{c}}=906.3\mathrm{n}\mathrm{m}$) and simultaneous resonant and nonresonant excitation. (right) Intensity variation of $X$, $X_2$, and $X^{*}$ as a function of the nonresonant excitation intensity.

Figure 4

(a) Amplitude of the oscillations in the PL of QD2 as a function of delay ${\tau }_c$ for excitation at $e_0$ (squares) and $e_1$ (circles). The dotted line corresponds to the autocorrelation of the excitation pulses. Inset: (left) schematic experimental setup; (right) quantum interference map for QD2 PL at ${\tau }_c=10\mathrm{p}\mathrm{s}$ under resonant excitation of $e_1$, shown as a function of emission energy and pulse fine delay ${\tau }_f$. (b),(c) Evolution of the QD2 PL intensity under pulsed resonant excitation of (b) $e_0$ and (c) $e_1$, as a function of the square root of the average power density.