Cascaded exciton emission of an individual strain-induced quantum dot

Single strain-induced quantum dots are isolated for optical experiments by selective removal of the inducing InP islands from the sample surface. Unpolarized emission of single, bi- and triexciton transitions are identified by power-dependent photoluminescence spectroscopy. Employing time-resolved experiments performed at different excitation powers we find a pronounced shift of the rise and decay times of these different transitions as expected from cascaded emission. Good agreement is found for a rate equation model for a three step cascade

Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves

Individual self-assembled Quantum Dots and Quantum Posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For Quantum Posts, switching occurs continuously from predominantely charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent on whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is non-monotonic and shows a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of positively charged and neutral excitons is observed at high surface acoustic wave amplitudes. These findings are explained by carrier trapping and localization in the thin and disordered two-dimensional wetting layer on top of which Quantum Dots nucleate. This limitation can be overcome for Quantum Posts where acoustically induced charge transport is highly efficient in a wide lateral Matrix-Quantum Well.Comment: 11 pages, 5 figure

Direct Observation of Controlled Coupling in an Individual Quantum Dot Molecule

We report the direct observation of quantum coupling in individual quantum dot molecules and its manipulation using static electric fields. A pronounced anti-crossing of different excitonic transitions is observed as the electric field is tuned. Comparison of our experimental results with theory shows that the observed anti-crossing occurs between excitons with predominant spatially \emph{direct} and \emph{indirect} character. The electron component of the exciton wavefunction is shown to have molecular character at the anti-crossing and the quantum coupling strength is deduced optically. In addition, we determine the dependence of the coupling strength on the inter-dot separation and identify a field driven transition of the nature of the molecular ground state.Comment: 11 pages, 4 figures submitted to Physical Review Letter

Direct observation of dynamic surface acoustic wave controlled carrier injection into single quantum posts using phase-resolved optical spectroscopy

A versatile stroboscopic technique based on active phase-locking of a surface acoustic wave to picosecond laser pulses is used to monitor dynamic acoustoelectric effects. Time-integrated multi-channel detection is applied to probe the modulation of the emission of a quantum well for different frequencies of the surface acoustic wave. For quantum posts we resolve dynamically controlled generation of neutral and charged excitons and preferential injection of holes into localized states within the nanostructure.Comment: 10 pages, 4 figure

Dynamic modulation of photonic crystal nanocavities using gigahertz acoustic phonons

Photonic crystal membranes (PCM) provide a versatile planar platform for on-chip implementations of photonic quantum circuits. One prominent quantum element is a coupled system consisting of a nanocavity and a single quantum dot (QD) which forms a fundamental building block for elaborate quantum information networks and a cavity quantum electrodynamic (cQED) system controlled by single photons. So far no fast tuning mechanism is available to achieve control within the system coherence time. Here we demonstrate dynamic tuning by monochromatic coherent acoustic phonons formed by a surface acoustic wave (SAW) with frequencies exceeding 1.7 gigahertz, one order of magnitude faster than alternative approaches. We resolve a periodic modulation of the optical mode exceeding eight times its linewidth, preserving both the spatial mode profile and a high quality factor. Since PCMs confine photonic and phononic excitations, coupling optical to acoustic frequencies, our technique opens ways towards coherent acoustic control of optomechanical crystals.Comment: 11 pages 4 figure

Combined electrical transport and capacitance spectroscopy of a MoS2−LiNbO3{\mathrm{MoS_2-LiNbO_3}} field effect transistor

We have measured both the current-voltage ($I_\mathrm{SD}$-$V_\mathrm{GS}$) and capacitance-voltage ($C$-$V_\mathrm{GS}$) characteristics of a $\mathrm{MoS_2-LiNbO_3}$ field effect transistor. From the measured capacitance we calculate the electron surface density and show that its gate voltage dependence follows the theoretical prediction resulting from the two-dimensional free electron model. This model allows us to fit the measured $I_\mathrm{SD}$-$V_\mathrm{GS}$ characteristics over the \emph{entire range} of $V_\mathrm{GS}$. Combining this experimental result with the measured current-voltage characteristics, we determine the field effect mobility as a function of gate voltage. We show that for our device this improved combined approach yields significantly smaller values (more than a factor of 4) of the electron mobility than the conventional analysis of the current-voltage characteristics only.Comment: to appear in Applied Physics Letter

Non-invasive probing of random local potential fluctuations in ZnCdSe/ZnSe quantum wells

Temperature dependence and recombination behavior of trapped charge carriers in ZnCdSe/ZnSe multiple quantum wells are investigated employing surface acoustic waves. These weakly perturb the carrier system, but remain highly sensitive even at small conductivities. Using this non-invasive probe we are able to detect persistent photoconductivity minutes after optical excitation. Measurement of exciting photon energies, the temperature dependence and ability to quench the conductivity with energies lower than the bandgap, support the notion of spatial separation of electrons and holes in the wells, due to random local potential fluctuations possibly induced by compositional fluctuations

Electrical control of inter-dot electron tunneling in a quantum dot molecule

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photo-generated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (< 5 ps) inter-dot tunneling is shown to occur over a surprisingly wide bandwidth, up to ~8 meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system