2018, Lazić, S., Chernysheva, E., Hernández-Mínguez, A., Santos, P.V., van der Meulen, H.P.

On-chip quantum information processing requires controllable quantum light sources that can be operated on-demand at high-speeds and with the possibility of in-situ control of the photon emission wavelength and its optical polarization properties. Here, we report on the dynamic control of the optical emission from core-shell GaN/InGaN nanowire (NW) heterostructures using radio frequency surface acoustic waves (SAWs). The SAWs are excited on the surface of a piezoelectric lithium niobate crystal equipped with a SAW delay line onto which the NWs were mechanically transferred. Luminescent quantum dot (QD)-like exciton localization centers induced by compositional fluctuations within the InGaN nanoshell were identified using stroboscopic micro-photoluminescence (micro-PL) spectroscopy. They exhibit narrow and almost fully linearly polarized emission lines in the micro-PL spectra and a pronounced anti-bunching signature of single photon emission in the photon correlation experiments. When the nanowire is perturbed by the propagating SAW, the embedded QD is periodically strained and its excitonic transitions are modulated by the acousto-mechanical coupling, giving rise to a spectral fine-tuning within a ~1.5 meV bandwidth at the acoustic frequency of ~330 MHz. This outcome can be further combined with spectral detection filtering for temporal control of the emitted photons. The effect of the SAW piezoelectric field on the QD charge population and on the optical polarization degree is also observed. The advantage of the acousto-optoelectric over other control schemes is that it allows in-situ manipulation of the optical emission properties over a wide frequency range (up to GHz frequencies).

2008, Stotz, J.A.H., Hey, R., Santos, P.V., Ploog, K.H.

Long coherence lifetimes of electron spins transported using moving potential dots are shown to result from the mesoscopic confinement of the spin vector. The confinement condition to control electron spin dephasing is governed by the relation between the characteristic spin–orbit length of the electron spins and the dimensions of the dot potential, which governs the electron spin coherence lifetime. The spin–orbit length is a sample-dependent parameter determined by the Dresselhaus contribution to the spin–orbit coupling and can be predictably controlled by varying the sample geometry. We further show that the coherence lifetime of the electron spins is independent of the local carrier densities within each potential dot, which suggests the possibility of coherent, long-range transport of single electron spins.

2012, Lazić, S., Hey, R., Santos, P.V.

We employ surface acoustic waves (SAWs) to control the transfer of photo-generated carriers between interconnected quantum wells and quantum wires (QWRs) grown on pre-patterned (311)A GaAs substrates. Optical studies, carried out under remote acoustic excitation of a single QWR, have shown sharp photoluminescence lines and antibunched photons with tunable emission energy. These features are attributed to recombination of acoustically transported carriers in potential inhomogeneities within the wire. The origin of the photon antibunching is discussed in terms of a 'bottleneck' in the number of carriers trapped in the QWR, which restricts the number of recombination events per SAW cycle. We propose a model for antibunching based on the trapping of carriers induced by the SAW piezoelectric field in states at the interface between the GaAs QWR and the AlGaAs barriers. Non-classical light is emitted during the subsequent release of the trapped carriers into the recombination centers within the wire. The spatial distribution of the emitting recombination centers is estimated using time-resolved measurements.

2018-6-18, Gradl, C., Winkler, R., Kempf, M., Holler, J., Schuh, D., Bougeard, D., Hernández-Mínguez, A., Biermann, K., Santos, P.V., Schüller, C., Korn, T.

The complex structure of the valence band in many semiconductors leads to multifaceted and unusual properties for spin-3/2 hole systems compared to common spin-1/2 electron systems. In particular, two-dimensional hole systems show a highly anisotropic Zeeman interaction. We have investigated this anisotropy in GaAs/AlAs quantum well structures both experimentally and theoretically. By performing time-resolved Kerr rotation measurements, we found a nondiagonal tensor g that manifests itself in unusual precessional motion, as well as distinct dependencies of hole-spin dynamics on the direction of the magnetic field B. We quantify the individual components of the tensor g for [113]-, [111]-, and [110]-grown samples. We complement the experiments by a comprehensive theoretical study of Zeeman coupling in in-plane and out-of-plane fields B. To this end, we develop a detailed multiband theory for the tensor g. Using perturbation theory, we derive transparent analytical expressions for the components of the tensor g that we complement with accurate numerical calculations based on our theoretical framework. We obtain very good agreement between experiment and theory. Our study demonstrates that the tensor g is neither symmetric nor antisymmetric. Opposite off-diagonal components can differ in size by up to an order of magnitude. The tensor g encodes not only the Zeeman energy splitting but also the direction of the axis about which the spins precess in the external field B. In general, this axis is not aligned with B. Hence our study extends the general concept of optical orientation to the regime of nontrivial Zeeman coupling.

2016, Buller, J.V.T., Cerda-Méndez, E.A., Balderas-Navarro, R.E., Biermann, K., Santos, P.V.

Exciton-polariton systems can sustain macroscopic quantum states (MQSs) under a periodic potential modulation. In this paper, we investigate the structure of these states in acoustic square lattices by probing their wave functions in real and momentum spaces using spectral tomography. We show that the polariton MQSs, when excited by a Gaussian laser beam, self-organize in a concentric structure, consisting of a single, two-dimensional gap-soliton (GS) state surrounded by one dimensional (1D) MQSs with lower energy. The latter form at hyperbolical points of the modulated polariton dispersion. While the size of the GS tends to saturate with increasing particle density, the emission region of the surrounding 1D states increases. The existence of these MQSs in acoustic lattices is quantitatively supported by a theoretical model based on the variational solution of the Gross–Pitaevskii equation. The formation of the 1D states in a ring around the central GS is attributed to the energy gradient in this region, which reduces the overall symmetry of the lattice. The results broaden the experimental understanding of self-localized polariton states, which may prove relevant for functionalities exploiting solitonic objects.

2012, Cerda-Méndez, E.A., Krizhanovskii, D.N., Biermann, K., Hey, R., Skolnick, M.S., Santos, P.V.

We study the spatial coherence of polariton condensates subjected to coherent modulation by a one-dimensional tunable acoustic potential.We use an interferometric technique to measure the amplitude and phase of the macroscopic condensate wavefunction. By increasing the acoustic modulation amplitude, we track the transition from the extended wavefunction of the unperturbed condensate to a regime where the wavefunction is spatially modulated and then to a fully confined regime, where independent condensates form at the minima of the potential with negligible particle tunneling between adjacent sites.

2014, Violante, A., Cohen, K., Lazić, S., Hey, R., Rapaport, R., Santos, P.V.

We report on the modulation of indirect excitons (IXs) as well as their transport by moving periodic potentials produced by surface acoustic waves (SAWs). The potential modulation induced by the SAW strain modifies both the band gap and the electrostatic field in the quantum wells confining the IXs, leading to changes in their energy. In addition, this potential captures and transports IXs over several hundreds of μm. While the IX packets keep to a great extent their spatial shape during transport by the moving potential, the effective transport velocity is lower than the SAW group velocity and increases with the SAW amplitude. This behavior is attributed to the capture of IXs by traps along the transport path, thereby increasing the IX transit time. The experimental results are well-reproduced by an analytical model for the interaction between trapping centers and IXs during transport.