Ultrafast few-fermion optoelectronics in a single self-assembled $\text{In}\text{Ga}\text{As}/\text{GaAs}$ quantum dot

We report a comprehensive study of the ultrafast optoelectronic response of a single self-assembled InGaAs/GaAs quantum dot embedded in a $n\text{−}i$-Schottky photodiode device. While manipulation of the artificial atom relies on two independently tunable picosecond pulse trains, sensitive all electrical readout is achieved via photocurrent. We apply our methods to probe the temporal evolution of the quantum dot absorption spectrum following coherent optical generation of a $s$-shell exciton. Our measurements reveal the biexciton absorption as well as previously unobserved $p$-shell transitions that appear in the presence of $s$-shell population. Furthermore, time-resolved measurements allow us to directly monitor the picosecond tunneling times of electrons and holes out of the dot. Beyond these incoherent phenomena, we also demonstrate the potential of our techniques for coherent quantum dot manipulations. Beginning with excitonic Rabi oscillations driven by a single pulse tuned to the neutral exciton we move on to demonstrate the coherent generation of the biexciton via a two-photon nonlinearity, before realizing conditional Rabi oscillations in the exciton-biexciton state manifold. The results provide significant potential for realizing and utilizing conditional coherent dynamics in quantum dot nanostructures for all optical, ultrafast quantum information processing.

Figure 1

(Color online) Schematic of the experimental setup: picosecond pump and probe pulses are spectrally sliced out of a broadband femtosecond laser using two 4f-PS. The time delay $t_D$ between the pulses is adjusted via a motorized D. A MO focuses the two pulse trains on a micron shadow mask on the sample mounted in a microscope cryostat. The sample is electrically biased and the probe-induced photocurrent is analyzed by a LI.

Figure 2

(Color online) (a) Pump off: photocurrent spectrum as detected with the probe pulse only. Pump on: photocurrent spectrum detected $t_D=3\text{ps}$ after exciting the $\text{cgs}\to {\text{X}}^0$ transition. (b) Difference of the two photocurrent spectra with same abscissa scaling. Positive (negative) signals indicate pump-induced bleaching (absorption). The arrows indicate the excitonic transitions discussed in the main text.

Figure 3

(Color online) $s$-shell population dynamics: pump-triggered changes in the probe-induced photocurrent for various delay times after resonant excitation of the $\text{cgs}\to {\text{X}}^0$ transition. [(a) and (b)] Spectral scans at different time delays. (c) Temporal scans at the fixed photon energies in resonance with the transitions labeled in the spectra. For better visibility the signal amplitudes have been rescaled by the factors given in the graph. Solid lines represent the result of a fit with a rate-equation model introduced in the main text. (d) Schematic level diagram visualizing the probed optical transitions (arrows pointing upwards) and the tunneling rates $R_e$ and $R_h$ used in the rate equation model for the decay of the ${\text{X}}^0$ population.

Figure 4

(Color online) Optical gain for intense excitation: (a) spectral scan of the probe-induced photocurrent for the dot in the cgs (red) and $t_D=10\text{ps}$ after excitation with a $\text{cgs}\to {\text{X}}^0$ pump pulse (green). Pump and probe pulse areas are close to $\pi$ pulses. (b) Temporal scan of the $\text{cgs}\to {\text{X}}^0$ transition. The scattered data points around $t_D=0$ originate from an interference of pump and probe fields.

Figure 5

(Color online) (a) Photocurrent spectra close to the $\text{cgs}\to {\text{X}}^0$ transition for various relative incident powers given in the graph. The spectra are shifted by 10 pA for visibility. (b) Squares: photocurrent detected with a central photon energy of 1.3378 eV, i.e., the mean value of the $\text{cgs}\to {\text{X}}^0$ and the ${\text{X}}^0\to 2{\text{X}}^0$ transition energies. The solid line is a quadratic fit to the data. (c) Photocurrent amplitude detected at the $\text{cgs}\to {\text{X}}^0$ resonance as a function of the relative pulse intensity. Squares are measured current values. The solid line corresponds to a simulation using the Rabi oscillation model described in the text.

Figure 6

(Color online) Photocurrent detection of probe-induced Rabi oscillations of the ${\text{X}}^0\to 2{\text{X}}^0$ transition probed 5 ps after pumping the $\text{cgs}\to {\text{X}}^0$ transition. (a) Optically driven transitions between QD states shown in single-particle energy diagrams. (b) Results of probe power scans shown for different powers of the pump pulse. For better visibility the scans are shifted in steps of 2.5 pA along the ordinate.