Acoustically regulated carrier injection into a single optically active quantum dot

F. J. R. Schülein, K. Müller, M. Bichler, G. Koblmüller, J. J. Finley, A. Wixforth, H. J. Krenner

Physical Review B 88, 085307 (Aug 2013) DOI: Physical Review B

After catching the wave at the perfect time, wave riders can surf it until they reach the shore precisely at the time a wave breaks at the sandy beach bed. Sometimes catching the wave requires to swim for a short time against the incoming tide. In semiconductors, electrons and holes can surf on nanoscale sound waves. This surfing transport provides efficient transfer of quantum information encoded in the carriers on a semiconductor chip. Since these ”stationary” quantum bits (qubits) are trapped inside a solid it is crucial for long-distance quantum communication to convert them to ”flying” photonic qubits using a semiconductor quantum dot. Here we study how the nanoscale sound wave regulates the injection of single electrons and holes into an isolated quantum dot and their conversion to photons.

We generate electrons and holes by a short laser pulse at a fixed time during the cycle of the wave at the position of the dot. Every time electrons and holes simultaneously reside inside the quantum dot, they can recombine and emit a single photon whose energy directly reflects the exact number of electrons and holes. We measure the time at which photons from distinct configurations are emitted and compare them to calculated trajectories of electrons and holes catching and surfing the sound wave. From this comparison we identify injection processes which occur instantaneously with the laser pulse and time-delayed injection arising from capture of surfing electrons or holes arriving at a well-defined time at the dot’s position. Taken together, our results demonstrate that our unique mechanism not only dynamically programs the number of electrons and holes in the dot, but also sets the time at which these are converted to photons.

We study the carrier injection into a single InGaAs/GaAs quantum dot regulated by a radio frequency surface acoustic wave. We find that the time of laser excitation during the acoustic cycle programs both the emission intensities and time of formation of neutral (X0) and negatively charged (X) excitons. We identify underlying, characteristic formation pathways of both few-particle states in the time-domain experiments and show that both exciton species can be formed either with the optical pump or at later times by injection of single electrons and holes “surfing” the acoustic wave. All experimental observations are in excellent agreement with calculated electron and hole trajectories in the plane of the two-dimensional wetting layer which is dynamically modulated by the acoustically induced piezoelectric potentials. Taken together, our findings provide insight on both the onset of acoustoelectric transport of electrons and holes and their conversion into the optical domain after regulated injection into a single quantum dot emitter