Florian J. R. Schülein

Dynamic Acoustic Control of Individual Optically Active Quantum Dot-like Emission Centers in Heterostructure Nanowires

We probe and control the optical properties of emission centers forming in radial heterostructure GaAs-Al0.3Ga0.7As nanowires and show that these emitters, located in Al0.3Ga0.7As layers, can exhibit quantum-dot like characteristics. We employ a radio frequency surface acoustic wave to dynamically control their emission energy, and occupancy state on a nanosecond time scale. In the spectral oscillations, we identify unambiguous signatures arising from both the mechanical and electrical component of the surface acoustic wave. In addition, different emission lines of a single emission center exhibit pronounced anticorrelated intensity oscillations during the acoustic cycle. These arise from a dynamically triggered carrier extraction out of the emission center to a continuum in the radial heterostructure. Using finite element modeling and Wentzel-Kramers-Brillouin theory we identify quantum tunneling as the underlying mechanism. These simulation results quantitatively reproduce the observed switching and show that in our systems these emission centers are spatially separated from the continuum by >10.5 nm.

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 predominantly charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent of whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is nonmonotonic 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.

Scalable fabrication of a hybrid field-effect and acousto-electric device by direct growth of monolayer MoS2/LiNbO3

Lithium niobate is the archetypical ferroelectric material and the substrate of choice for numerous applications including surface acoustic wave radio frequencies devices and integrated optics. It offers a unique combination of substantial piezoelectric and birefringent properties, yet its lack of optical activity and semiconducting transport hamper application in optoelectronics. Here we fabricate and characterize a hybrid MoS2/LiNbO3 acousto-electric device via a scalable route that uses millimetre-scale direct chemical vapour deposition of MoS2 followed by lithographic definition of a field-effect transistor structure on top. The prototypical device exhibits electrical characteristics competitive with MoS2 devices on silicon. Surface acoustic waves excited on the substrate can manipulate and probe the electrical transport in the monolayer device in a contact-free manner. We realize both a sound-driven battery and an acoustic photodetector. Our findings open directions to non-invasive investigation of electrical properties of monolayer films.

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 multichannel 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.

Fourier synthesis of radiofrequency nanomechanical pulses with different shapes

The concept of Fourier synthesis is heavily used in both consumer electronic products and fundamental research. In the latter, pulse shaping is key to dynamically initializing, probing and manipulating the state of classical or quantum systems. In NMR, for instance, shaped pulses have a long-standing tradition and the underlying fundamental concepts have subsequently been successfully extended to optical frequencies and even to the implementation of quantum gate operations. Transferring these paradigms to nanomechanical systems requires tailored nanomechanical waveforms. Here, we report on an additive Fourier synthesizer for nanomechanical waveforms based on monochromatic surface acoustic waves. As a proof of concept, we electrically synthesize four different elementary nanomechanical waveforms from a fundamental surface acoustic wave at f1 ≈ 150 MHz using a superposition of up to three discrete harmonics. We use these shaped pulses to interact with an individual sensor quantum dot and detect their deliberately and temporally modulated strain component via the optomechanical quantum dot response. Importantly, and in contrast to direct mechanical actuation by bulk piezoactuators, surface acoustic waves provide much higher frequencies (>20 GHz; ref. 10) to resonantly drive mechanical motion. Thus, our technique uniquely allows coherent mechanical control of localized vibronic modes of optomechanical crystals, even in the quantum limit when cooled to the vibrational ground state.

Radio frequency occupancy state control of a single nanowire quantum dot

The excitonic occupancy state of a single, nanowire-based, heterostructure quantum dot is dynamically programmed by a surface acoustic wave. The quantum dot is formed by an interface or thickness fluctuation of a GaAs QW embedded in a AlGaAs shell of a GaAs–AlGaAs core–shell nanowire. As we tune the time at which carriers are photogenerated during the acoustic cycle, we find pronounced intensity oscillations of neutral and negatively charged excitons. At high acoustic power levels these oscillations become anticorrelated which enables direct acoustic programming of the dots charge configuration, emission intensity and emission wavelength. Numerical simulations confirm that the observed modulations arise from acoustically controlled modulations of the electron and electron–hole pair concentrations at the position of the quantum dot.

Surface acoustic wave mediated carrier injection into individual quantum post nano emitters

Acousto-electric charge conveyance induced by a surface acoustic wave (SAW) is employed to dissociate photogenerated excitons. Over macroscopic distances, both electrons and holes are injected sequentially into a remotely positioned, isolated and high quality quantum emitter, a self-assembled quantum post. This process is found to be highly efficient and to exhibit improved stability at high acoustic powers when compared to direct optical pumping at the position of the quantum post. These characteristics are attributed to the wide matrix quantum well in which charge conveyance occurs and to the larger number of carriers available for injection in the remote configuration, respectively. The emission of such pumped quantum posts is dominated by recombination of neutral excitons and fully directional when the propagation direction of the SAW and the position of the quantum post are reversed.

The Native Material Limit of Electron and Hole Mobilities in Semiconductor Nanowires

Piezoelectric surface acoustic waves are employed to induce radio frequency spatiotemporal dynamics of photogenerated electrons and holes in the GaAs core of individual GaAs/AlGaAs core/shell semiconductor nanowires. Comparison of the time-dependent interband optical recombination to numerical simulations allow to determine the charge carrier transport mobilities of electrons, μe = 500–250+500 cm2/(V s), holes, μh = 50–30+50 cm2/(V s) and their ratio μe:μh = (20 ± 5):1. Our method probes carrier transport at low carrier density. Thus, the obtained values represent the native material limit of these nanowires, determined by their structural properties. We show that for near-pristine nanowires, individual twin defects do not significantly affect electrical transport, in strong contrast to polytypic nanowires. In the acoustoelectrically modulated emission, we observe unambiguous signatures of (i) hole localization within long wurtzite-rich segments and (ii) electrons in zinc blende regions being reflected at...

Time-resolved coherent X-ray diffraction imaging of surface acoustic waves

The time-dependent one-dimensional height profile of a standing surface acoustic wave on an LiNbO3 substrate has been reconstructed from stroboscopically recorded coherent grazing-incidence small-angle diffraction patterns.

Standing surface acoustic waves in LiNbO3 studied by time resolved X-ray diffraction at Petra III

We have carried out time resolved stroboscopic diffraction experiments on standing surface acoustic waves (SAWs) of Rayleigh type on a LiNbO3 substrate. A novel timing system has been developed and commissioned at the storage ring Petra III of Desy, allowing for phase locked stroboscopic diffraction experiments applicable to a broad range of timescales and experimental conditions. The combination of atomic structural resolution with temporal resolution on the picosecond time scale allows for the observation of the atomistic displacements for each time (or phase) point within the SAW period. A seamless transition between dynamical and kinematic scattering regimes as a function of the instantaneous surface amplitude induced by the standing SAW is observed. The interpretation and control of the experiment, in particular disentangling the diffraction effects (kinematic to dynamical diffraction regime) from possible non-linear surface effects is unambiguously enabled by the precise control of phase between the...