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We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots on strong piezoelectric and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III-V compound semiconductor membranes onto the host substrate. The superconducting thin film is employed as a zero-resistivity electrode material for a surface acoustic wave resonator with internal quality factors $Q \approx 17000$ representing a three-fold enhancement compared to identical devices with normal conducting electrodes. Superconducting operation of $\approx 400\,\mathrm{MHz}$ resonators is achieved to temperatures $T>7\,\mathrm{K}$ and electrical radio frequency powers $P_{\mathrm{rf}}>+9\,\mathrm{dBm}$. Heterogeneously integrated single quantum dots couple to the resonant phononic field of the surface acoustic wave resonator operated in the superconducting regime. Position and frequency selective coupling mediated by deformation potential coupling is validated using time-integrated and time-resolved optical spectroscopy. Furthermore, acoustoelectric charge state control is achieved in a modified device geometry harnessing large piezoelectric fields inside the resonator. The hybrid quantum dot - surface acoustic wave resonator can be scaled to higher operation frequencies and smaller mode volumes for quantum phase modulation and transduction between photons and phonons via the quantum dot. Finally, the employed materials allow for the realization of other types of optoelectronic devices, including superconducting single photon detectors and integrated photonic and phononic circuits., submitted manuscript

The authors combine acousto-optoelectric and multichannel photon correlation spectroscopy to probe spatiotemporal carrier dynamics induced by a piezoelectric surface acoustic wave (SAW). The technique is implemented by combining phase-locked optical microphotoluminescence spectroscopy and simultaneous three-channel time-resolved detection. From the recorded time-correlated single-photon-counting data, the time transients of individual channels and the second- and third-order correlation functions are obtained with subnanosecond resolution. The method is validated by probing the correlations of SAW-driven carrier dynamics between three decay channels of a single polytypic semiconductor nanowire on a conventional ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_{3}$ SAW delay line chip. The method can be readily applied to other types of nanosystems and probes SAW-regulated charge-state preparation in quantum dots, charge-transfer processes in van der Waals heterostructures, or other types of hybrid nanoarchitectures.

Quantum technologies that rely on photonic qubits require a precise controllability of their properties. For this purpose hybrid approaches are particularly attractive because they offer a large flexibility to address different aspects of the photonic degrees of freedom. When combining photonics with other quantum platforms like phonons, quantum transducers have to be realized that convert between the mechanical and optical domain. Here, we realize this interface between phonons in the form of surface acoustic waves (SAWs) and single photons, mediated by a single semiconductor quantum dot exciton. In this combined theoretical and experimental study, we show that the different sidebands exhibit characteristic blinking dynamics that can be controlled by detuning the laser from the exciton transition. By developing analytical approximations we gain a better understanding of the involved internal dynamics. Our specific SAW approach allows us to reach the ideal frequency range of around 1 GHz that enables simultaneous temporal and spectral phonon sideband resolution close to the combined fundamental time-bandwidth limit.

We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickes...

Wave mixing is an archetypical phenomenon in bosonic systems. In optomechanics, the bidirectional conversion between electromagnetic waves or photons at optical frequencies and elastic waves or phonons at radio frequencies is building on precisely this fundamental principle. Surface acoustic waves (SAWs) provide a versatile interconnect on a chip and thus enable the optomechanical control of remote systems. Here we report on the coherent nonlinear three-wave mixing between the coherent fields of two radio frequency SAWs and optical laser photons via the dipole transition of a single quantum dot exciton. In the resolved sideband regime, we demonstrate fundamental acoustic analogues of sum and difference frequency generation between the two SAWs and employ phase matching to deterministically enhance or suppress individual sidebands. This transfer between the acoustic and optical domains is described by theory that fully takes into account direct and virtual multiphonon processes. Finally, we show that the precision of the wave mixing is limited by the frequency accuracy of modern radio frequency electronics.

Electron cycloids reveal the spin of a surface acoustic wave predicted by Lord Rayleigh in 1885., Spin-momentum locking is a universal wave phenomenon promising for applications in electronics and photonics. In acoustics, Lord Rayleigh showed that surface acoustic waves exhibit a characteristic elliptical particle motion strikingly similar to spin-momentum locking. Although these waves have become one of the few phononic technologies of industrial relevance, the observation of their transverse spin remained an open challenge. Here, we observe the full spin dynamics by detecting ultrafast electron cycloids driven by the gyrating electric field produced by a surface acoustic wave propagating on a slab of lithium niobate. A tubular quantum well wrapped around a nanowire serves as an ultrafast sensor tracking the full cyclic motion of electrons. Our acousto-optoelectrical approach opens previously unknown directions in the merged fields of nanoacoustics, nanophotonics, and nanoelectronics for future exploration.

We calculate the resonance fluorescence signal of a two-level system coupled to a quantized phonon mode. By treating the phonons in the independent boson model and not performing any approximations in their description, we also have access to the state evolution of the phonons. We confirm the validity of our model by simulating the limit of an initial quasi-classical coherent phonon state, which can be compared to experimentally confirmed results in the semiclassical limit. In addition we predict photon scattering spectra in the limit of purely quantum mechanical phonon states by approaching the phononic vacuum. Our method further allows us to simulate the impact of the light scattering process on the phonon state by calculating Wigner functions. We show that the phonon mode is brought into characteristic quantum states by the optical excitation process.

Today, surface acoustic waves (SAWs) and bulk acoustic waves are already two of the very few phononic technologies of industrial relevance and can been found in a myriad of devices employing these nanoscale earthquakes on a chip. Acoustic radio frequency filters, for instance, are integral parts of wireless devices. SAWs in particular find applications in life sciences and microfluidics for sensing and mixing of tiny amounts of liquids. In addition to this continuously growing number of applications, SAWs are ideally suited to probe and control elementary excitations in condensed matter at the limit of single quantum excitations. Even collective excitations, classical or quantum are nowadays coherently interfaced by SAWs. This wide, highly diverse, interdisciplinary and continuously expanding spectrum literally unites advanced sensing and manipulation applications. Remarkably, SAW technology is inherently multiscale and spans from single atomic or nanoscopic units up even to the millimeter scale. The aim of this Roadmap is to present a snapshot of the present state of surface acoustic wave science and technology in 2019 and provide an opinion on the challenges and opportunities that the future holds from a group of renown experts, covering the interdisciplinary key areas, ranging from fundamental quantum effects to practical applications of acoustic devices in life science.

In fundamental research, surface acoustic waves (SAWs), nanoscale earthquakes on a chip are an extremely versatile tool to dynamically probe and manipulate charge and spin excitations condensed matter systems. When excited on piezoelectric materials both the oscillating mechanical stress of the SAW and its accompanying electric fields can be harnessed to establish strong interactions between single quantum emitters and sound on the nanoscale [1–4]. Here, we report on the integration of single quantum emitters in suspended phononic waveguides. Figure 1 (a) shows an electron micrograph of a 50 μm long and 2 μm wide suspended beam patterned on an approximately 150 nm thick (Al)GaAs heterostructure containing a single layer of GaAs quantum dots (QDs). In our experiment a Rayleigh SAW is excited by a multiharmonic frequency-chirped transducer [5] excited on the unpatterned region and coupled into the suspended beam where it propagates as a Lamb mode of identical frequency [6]. Figure 1 (b) and (c) compare the optomechanical response of a single QD in the unpatterend region and in the suspended beam, respectively. Clearly, the optomechanical of the quantum emitter inside the suspended phononic waveguide is enhanced by a factor of 3.5, compared to that of the emitter strained by the Rayleigh SAW. This finding may arise from an enhancement of the optomechanical coupling between the emitter and the elastic wave in the one-dimensional phononic waveguide. Employing phase resolved spectroscopy on an ensemble of quantum emitters we can unambiguously show, that the observed optomechanical response arises from the coupling between the low phase velocity, anti-symmetric (flexural) Lamb-Mode and the QD. Moreover, our fabricated phononic waveguides exhibit low propagation losses.

The optomechanical coupling of quantum dots and flexural mechanical modes is studied in suspended nanophononic strings. The investigated devices are designed and monolithically fabricated on an (Al)GaAs heterostructure. Radio frequency elastic waves with frequencies ranging between $f$=250 MHz to 400 MHz are generated as Rayleigh surface acoustic waves on the unpatterned substrate and injected as Lamb waves in the nanophononic string. Quantum dots inside the nanophononic string exhibit a 15-fold enhanced optomechanical modulation compared to those dynamically strained by the Rayleigh surface acoustic wave. Detailed finite element simulations of the phononic mode spectrum of the nanophononic string confirm, that the observed modulation arises from valence band deformation potential coupling via shear strain. The corresponding optomechanical coupling parameter is quantified to $0.15 \mathrm{meV nm^{-1}}$. This value exceeds that reported for vibrating nanorods by approximately one order of magnitude at 100 times higher frequencies. Using this value, a derive vertical displacements in the range of 10 nm is deduced from the experimentally observed modulation. The results represent an important step towards the creation of large scale optomechanical circuits interfacing single optically active quantum dots with optical and mechanical waves., Comment: Submitted manuscript

This paper presents a surface acoustic wave (SAW) technique that provides a contact-less and massively parallel probe for the intrinsic mobilities of electrons and holes in otherwise undoped surface-passivated GaAs/AlGaAs core-shell NWs. To resolve the SAW-driven carrier dynamics, stroboscopic time-correlated single photon counting (s-TCSPC) of the photoluminescence (PL) emission of individual NWs was performed. When subject to a SAW (fSAW = 194 MHz, PSAW = +5 dBm), a characteristic beating in the PL transient (blue), matching TSAW = 5.1 ns is observed. As expected, this beating shifts in time by TSAW/2 as the local acoustic phase is tuned by 180° (green). This observation is a direct fingerprint of the charge carrier dynamics and acoustic charge conveyance within the SAW-induced type-II band edge modulation. Electrons transfer from the position of the excitation laser (x0) into the energetically favourable stable points at the minimum of the conduction band. In contrast to electrons, holes remain stationary at the point of photogeneration due to their reduced mobility. TSAW/2 later these electrons are conveyed to the position of the holes giving rise to the observed beating.

500 MHz surface acoustic waves travel across a commensurate plasmonic grating coupler. A stroboscopic optical technique shows that the dynamic surface deformation deliberately modulates the coupler's efficiency by +/-2\% during the ~2ns acoustic cycle

Single-domain VO2 nanocrystals in a fused silica matrix exhibit an extremely large thermally and/or optically induced hysteresis around room temperature. We demonstrate optical imprinting of persistent diffraction gratings and repetitive (> 104) cycling through the phase transition.

Surface acoustic waves (SAWs) have proven a useful tool for dynamically manipulating the optical properties of semiconductor nanostructures. Recently we have shown that the mechanical deformation induced by a SAW on a two-dimensional photonic crystal membrane (PCM) spectrally tunes the resonance frequency of nanocavities within the PCM employing phase locked stroboscopic excitation.

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...

A hybrid device comprising a (Al)GaAs quantum dot heterostructure and a LiNbO$_3$ surface acoustic wave resonator is fabricated by heterointegration. High acoustic quality factors $Q>4000$ are demonstrated for an operation frequency $f\approx 300$ MHz. The measured large quality factor-frequency products $Q\times f>10^{12}$ ensures the suppression of decoherence due to thermal noise for temperatures exceeding $T>50\,\mathrm{K}$. Frequency and position dependent optomechanical coupling of single quantum dots and the resonator modes is observed., Accepted manuscript

Hybrid systems consisting of a quantum emitter coupled to a mechanical oscillator are receiving increasing attention for fundamental science and potential applications in quantum technologies. In contrast to most of the presented works in this field, in which the oscillator eigenfrequencies are irreversibly determined by the fabrication process, we present here a simple approach to obtain frequency-tunable mechanical resonators based on suspended nanomembranes. The method relies on a micromachined piezoelectric actuator, which we use both to drive resonant oscillations of a suspended Ga(Al)As membrane with embedded quantum dots and to fine-tune their mechanical eigenfrequencies. Specifically, we excite oscillations with frequencies of at least 60 MHz by applying an AC voltage to the actuator and tune the eigenfrequencies by at least 25 times their linewidth by continuously varying the elastic stress state in the membranes through a DC voltage. The light emitted by optically excited quantum dots is used as a sensitive local strain gauge to monitor the oscillation frequency and amplitude. We expect that our method has the potential to be applicable to other optomechanical systems based on dielectric and semiconductor membranes possibly operating in the quantum regime., This work was supported by the FWF (P 29603), the Linz Institute of Technology (LIT), the LIT Secure and Correct Systems Lab funded by the state of Upper Austria, the EU project HANAS (No. 601126210), AWS Austria Wirtschaftsservice (PRIZE Programme, Grant No. P1308457), the European Research council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (SPQRel, Grant Agreement No. 679183), and the German Excellence Initiative via the Cluster of Excellence Nanosystems Initiative Munich (NIM). X. Yuan acknowledges support of the China Scholarship Council (CSC, No. 201306090010). Y. Huo thanks support of NSFC (No. 11774326) and STCSM (Nos. 17ZR1443900 and 17PJ1409900). J.M.-S. acknowledges support through the Clarín Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND European grant (No. PA-18-ACB17-29).

Wide passband interdigital transducers are employed to establish a stable phase-lock between a train of laser pulses emitted by a mode-locked laser and a surface acoustic wave generated electrically by the transducer. The transducer design is based on a multi-harmonic split-finger architecture for the excitation of a fundamental surface acoustic wave and a discrete number of its overtones. Simply by introducing a variation of the transducer's periodicity $p$, a frequency chirp is added. This combination results in wide frequency bands for each harmonic. The transducer's conversion efficiency from the electrical to the acoustic domain was characterized optomechanically using single quantum dots acting as nanoscale pressure sensors. The ability to generate surface acoustic waves over a wide band of frequencies enables advanced acousto-optic spectroscopy using mode-locked lasers with fixed repetition rate. Stable phase-locking between the electrically generated acoustic wave and the train of laser pulses was confirmed by performing stroboscopic spectroscopy on a single quantum dot at a frequency of 320 MHz. Finally, the dynamic spectral modulation of the quantum dot was directly monitored in the time domain combining stable phase-locked optical excitation and time-correlated single photon counting. The demonstrated scheme will be particularly useful for the experimental implementation of surface acoustic wave-driven quantum gates of optically addressable qubits or collective quantum states or for multi-component Fourier synthesis of tailored nanomechanical waveforms., Comment: revised manuscript

In this topical review, we report on recent advances on the coupling of single semiconductor quantum emitters, quantum dots, to the dynamic strain and electric fields of surface acoustic waves. Quantum dots are atom-like optically addressable two-level systems embedded in semiconductor matrices. On the one hand, the occupancy states of these 'artificial atoms' can be programmed by spatio-temporal carrier dynamics driven by sound waves. On the other hand, the quantized energy levels of electrons and holes couple strongly to the mechanical strain of these waves. We present an overview of the fundamental coupling mechanisms, experimental techniques to probe these systems in the time domain, and recent hallmark experiments. We discuss emerging research themes including hybrid architectures comprising advanced LiNbO3 SAW devices and single quantum dot devices of nanowire-based quantum emitters, and sound-driven control of light-matter interaction between single photons in nanophotonic resonators and two-level quantum dot systems.

We report the fully-scalable fabrication of a large array of hybrid molybdenum disulfide (MoS$_2$) - silicon dioxide (SiO$_2$) one-dimensional, free-standing photonic-crystal cavities capable of enhancement of the MoS$_2$ photoluminescence at the narrow cavity resonance. We demonstrate continuous tunability of the cavity resonance wavelength across the entire emission band of MoS$_2$ simply by variation of the photonic crystal periodicity. Device fabrication started by substrate-scale growth of MoS$_2$ using chemical vapor deposition (CVD) on non-birefringent thermal oxide on a silicon wafer; it was followed by lithographic fabrication of a photon crystal nanocavity array on the same substrate at more than 50% yield of functional devices. Our cavities exhibit three dominant modes with measured linewidths less than 0.2 nm, corresponding to quality factors exceeding 4000. All experimental findings are found to be in excellent agreement with finite difference time domain simulations., accepted for Scientific Reports (2017)

Monolayer Transition Metal Dichalcogenides (TMDs) have recently attracted great interest in the field of photonics because of their distinctive optical and spin properties. In contrast to bulk TMD materials, which are indirect bandgap semiconductors, monolayer TMDs are highly optically active due to a direct bandgap ranging between 1 and 2 eV [1]. The coupling of TMD excitons to optical fields can be dramatically enhanced by integration with nanophotonic resonators. For instance, enhanced light extraction, signatures of Purcell enhanced emission and even lasing in coupled systems of TMD monolayers transferred onto two-dimensional GaP-based photonic crystal cavities (PCCs) [2,3] as well as microdisk cavities [4] was reported.

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., to appear in Applied Physics Letters

We probe and control the optical properties of emission centers forming in radial het- erostructure 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 nanosec- ond timescale. In the spectral oscillations we identify unambiguous signatures arising from both the mechanical and electrical component of the surface acoustic wave. In addition, differ- ent emission lines of a single quantum dot exhibit pronounced anti-correlated intensity oscilla- tions during the acoustic cycle. These arise from a dynamically triggered carrier extraction out of the quantum dot to a continuum in the radial heterostructure. Using finite element modeling and Wentzel-Kramers-Brillouin theory we identify quantum tunneling as the underlying mech- anism. These simulation results quantitatively reproduce the observed switching and show that in our systems these quantum dots are spatially separated from the continuum by > 10.5 nm., This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters, copyright \c{copyright} American Chemical Society after peer review. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/nl4040434

Cadmium selenide nanocrystal quantum dots (NC-QDs) are site-selectively synthesized by sequential ion beam implantation of selenium and cadmium ions in a SiO2 matrix through sub-micron apertures followed by a rapid thermal annealing step. The size, areal density and optical emission energy of the NC-QDs are controlled by the ion fluence during implantation and the diameter of implantation aperture. For low fluences and small apertures the emission of these optically active emitters is blue-shifted compared to that of the bulk material by $>100\,{\rm meV}$ due to quantum confinement. The emission exhibits spectral diffusion and blinking on a second timescales as established also for solution synthesized NC-QDs.

Ultrafast pump-probe spectroscopy is employed to directly monitor the tunneling of charge carriers from single and vertically coupled quantum dots and probe intra-molecular dynamics. Immediately after resonant optical excitation, several peaks are observed in the pump-probe spectrum arising from Coulomb interactions between the photogenerated charge carriers. The influence of few-Fermion interactions in the photoexcited system and the temporal evolution of the optical response is directly probed in the time domain. In addition, the tunneling times for electrons and holes from the QD nanostructure are independently determined. In polarization resolved measurements, near perfect Pauli-spin blockade is observed in the spin-selective absorption spectrum as well as stimulated emission. While electron and hole tunneling from single quantum dots is shown to be well explained by the WKB formalism, for coupled quantum dots pronounced resonances in the electron tunneling rate are observed arising from elastic and inelastic electron tunneling between the different dots.

We propose and implement a new concept for thermochromic plasmonic elements. It is based on vanadium dioxide (VOsub2/sub) nanocrystals located in the near field of surface plasmon polaritons supported by an otherwise unstructured gold thin film. When the VOsub2/subundergoes the metal-insulator phase transition, the coupling conditions for conversion of light into propagating surface plasmon polaritons change markedly. In particular, we realize thermochromic plasmonic grating couplers with substantial switching contrast as well as tunable plasmonic couplers in a Kretschmann configuration. The use of VOsub2/subnanocrystals permits highly repetitive switching and room temperature operation. Simulations based on the actual dielectric function of our VOsub2/subnanocrystals agree well with the experiment.

We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantum dot molecules. By probing the optical response of an individual quantum dot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k·p calculations taking into account a realistic quantum dot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule.

A coupled quantum dot--nanocavity system in the weak coupling regime of cavity quantumelectrodynamics is dynamically tuned in and out of resonance by the coherent elastic field of a $f_{\rm SAW}\simeq800\,\mathrm{MHz}$ surface acoustic wave. When the system is brought to resonance by the sound wave, light-matter interaction is strongly increased by the Purcell effect. This leads to a precisely timed single photon emission as confirmed by the second order photon correlation function $g^{(2)}$. All relevant frequencies of our experiment are faithfully identified in the Fourier transform of $g^{(2)}$, demonstrating high fidelity regulation of the stream of single photons emitted by the system., Comment: revised manuscript - to be published in Applied Physics Letters

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs-core-shell-nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photo-thermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

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., Lithium niobate has piezoelectric and birefringent properties useful for optics, but it is not readily integrated with traditional optoelectronics. Here, the authors create a device that senses photoconductance in semiconducting molybdenum disulfide using surface acoustic waves excited in lithium niobate.

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 figures

This article reviews the fabrication, structural and optical properties of self-assembled In(Ga)As quantum posts. We introduce the growth mechanism and present the resulting structural properties and electronic structure which provides a direct route to tunable intraband transitions. We show that in the interband spectrum, sharp optical transitions, single photon emission and tailored giant electrostatic dipole moments are observed. We demonstrate that these properties can be used for an exciton memory.

GaAs-AlxGa1-xAs (AlGaAs) core-shell nanowires show great promise for nanoscale electronic and optoelectronic devices, but the application of these nonplanar heterostructures in devices requires improved understanding and control of nanoscale alloy composition and interfaces. Multiple researchers have observed sharp emission lines of unknown origin below the AlGaAs band edge in photoluminescence (PL) spectra of core-shell nanowires; point defects, alloy composition fluctuations, and self-assembled quantum dots have been put forward as candidate structures. Here we employ laser-assisted atom probe tomography to reveal structural and compositional features that give rise to the sharp PL emission spectra. Nanoscale ellipsoidal Ga-enriched clusters resulting from random composition fluctuations are identified in the AlGaAs shell, and their compositions, size distributions, and interface characteristics are analyzed. Simulations of exciton transition energies in ellipsoidal quantum dots are used to relate the Ga nanocluster distribution with the distribution of sharp PL emission lines. We conclude that the Ga rich clusters can act as discrete emitters provided that the major diameter is ≥4 nm. Smaller clusters are under-represented in the PL spectrum, and spectral lines of larger clusters are broadened, due to quantum tunneling between clusters.

Strongly confined photonic modes can couple to quantum emitters and mechanical excitations. To harness the full potential in quantum photonic circuits, interactions between different constituents have to be precisely and dynamically controlled. Here, a prototypical coupled element, a photonic molecule defined in a photonic crystal membrane, is controlled by a radio frequency surface acoustic wave. The sound wave is tailored to deliberately switch on and off the bond of the photonic molecule on sub-nanosecond timescales. In time-resolved experiments, the acousto-optically controllable coupling is directly observed as clear anticrossings between the two nanophotonic modes. The coupling strength is determined directly from the experimental data. Both the time dependence of the tuning and the inter-cavity coupling strength are found to be in excellent agreement with numerical calculations. The demonstrated mechanical technique can be directly applied for dynamic quantum gate operations in state-of-the-art-coupled nanophotonic, quantum cavity electrodynamic and optomechanical systems., Dynamic control of components is required for large-scale quantum photonic networks. Here, Kapfinger et al. show dynamic control of the interaction between two coupled photonic crystal nanocavities forming a photonic molecule. Tuning is achieved by using an electrically generated radio frequency surface acoustic wave.

Carrier dynamics in InGaAs/GaAs quantum dots is analyzed with highly sensitive femtosecond transmission spectroscopy. In a first step, measurements on a large ensemble of nanoislands reveal the dynamical electronic filling of quantum dots from the surrounding wetting layer. Most interestingly, we find a spin-preserving phonon mediated scattering into fully localized states within a few picoseconds. Then, individual artificial atoms are isolated with metallic shadow masks. For the first time, a single self-assembled quantum dot is addressed in an ultrafast transmission experiment. We find bleaching signals in the order of 10 -5 that arise from individual interband transitions of one quantum dot. As a result, we have developed an ultrafast optical tool for both manipulation and read-out of a single self-assembled quantum dot.

We demonstrate our ability to control and manipulate the optical modes in 2D Photonic Crystal Defect cavities and investigate their coupling to InGaAs self-assembled quantum dots. Our results enable us to probe the nature of individual cavity modes and directly investigate cavity QED phenomena. For the lowest mode volume cavities investigated, consisting of a single missing air hole within a hexagonal lattice, we have measured a clear Purcell enhancement of the light-matter interaction in the weak coupling regime. For QDs on-resonance with localized cavity modes this translates to a shortening of the quantum dot spontaneous emission lifetime by a factor ∼2 when compared to off-resonance dots.

Ultrafast relaxation phenomena in self-organized InAs/AlGaAs quantum dots with ground state transition energies of 1.42 eV are analyzed with different methods of femtosecond spectroscopy. A time-correlated luminescence experiment of an ensemble of InAs islands reveals a picosecond relaxation mechanism of carriers within the quantum dots. Moreover, modifications of the quantum dot properties due to excitation of the surrounding wetting layer is found. Employing a shadow-mask technique, a structure containing only a few zero-dimensional objects is studied. Selectively creating electron-hole pairs in excited states of approximately ten quantum dots, we observe characteristic line shifts of their ground state absorption in a two-color femtosecond transmission experiment. This finding demonstrates the feasibility of ultrafast studies of single self-assembled quantum dots.

We fabricated an acousto-optic semiconductor hybrid device for strong optomechanical coupling of individual quantum emitters and a surface acoustic wave. Our device comprises a surface acoustic wave chip made from highly piezoelectric LiNbO$_3$ and a GaAs-based semiconductor membrane with an embedded layer of quantum dots. Employing multi-harmonic transducers, we generated sound waves on LiNbO$_3$ over a wide range of radio frequencies. We monitored their coupling to and propagation across the semiconductor membrane both in the electrical and optical domain. We demonstrate enhanced optomechanical tuning of the embedded quantum dots with increasing frequencies. This effect was verified by finite element modelling of our device geometry and attributed to an increased localization of the acoustic field within the semiconductor membrane. For moderately high acoustic frequencies, our simulations predict strong optomechanical coupling making our hybrid device ideally suited for applications in semiconductor based quantum acoustics., Comment: revised manuscript. unit conversion error fixed

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 dot's 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., Revised manuscript, 12 pages, submitted to J. Phys. D as a contribution in a Special Issue on Nanowires

We explore the possibility of entangling an excitonic two-level system in a semiconductor quantum dot with a cavity defined on a photonic crystal by sweeping the cavity frequency across its resonance with the exciton transition. The dynamic cavity detuning is established by a radio frequency surface acoustic wave (SAW). It induces Landau-Zener transitions between the excitonic and the photonic degrees of freedom and thereby creates a superposition state. We optimize this scheme by using tailored Fourier-synthesized SAW pulses with up to five harmonics. The theoretical study is performed with a master equation approach for present state-of-the-art setups. Assuming experimentally demonstrated system parameters, we show that the composed pulses increase both the maximum entanglement and its persistence. The latter is only limited by the dominant dephasing mechanism, i.e., the photon loss from the cavity. © 2014 American Physical Society., We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) via Sonderforschungsbereich SFB 631 (Projects No. A5 and No. B5) and the Emmy Noether Program (HJK, KR3790/2-1). This work was supported by the Spanish Ministry of Economy and Competitiveness via Grant No. MAT2011-24331.

Cadmium selenide nanocrystal quantum dots (NC-QDs) are site-selectively synthesized by sequential ion beam implantation of selenium and cadmium ions in a SiO2 matrix through submicrometer apertures followed by a rapid thermal annealing step. The structural and optical properties of the NC-QDs are controlled by the ion fluence during implantation and the diameter of the implantation aperture. For low fluences and small apertures the emission of these optically active emitters is blue-shifted compared to that of the bulk material by100 meV due to quantum confinement. The emission exhibits spectral diffusion and blinking on a second time scales as established also for solution-synthesized NC-QDs.

We explore the impact of ∼500 MHz surface acoustic waves traveling across a commensurable plasmonic grating coupler. A stroboscopic technique involving surface acoustic waves synchronized to a modelocked optical source allows to time-resolve the dynamical impact of the electromechanically induced perturbation. The surface acoustic wave periodically enhances or decreases the surface ripple of the static grating. Most remarkably, the dynamic surface deformation deliberately modulates the coupler’s efficiency by ±2% during the ∼2 ns acoustic cycle.