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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.

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.

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.

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

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.

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.

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 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.

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.

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

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.

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., Time-resolved coherent X-ray diffraction experiments of standing surface acoustic waves, illuminated under grazing incidence by a nanofocused synchrotron beam, are reported. The data have been recorded in stroboscopic mode at controlled and varied phase between the acoustic frequency generator and the synchrotron bunch train. At each time delay (phase angle), the coherent far-field diffraction pattern in the small-angle regime is inverted by an iterative algorithm to yield the local instantaneous surface height profile along the optical axis. The results show that periodic nanoscale dynamics can be imaged at high temporal resolution in the range of 50 ps (pulse length).

The concept of Fourier synthesis is heavily employed in both consumer electronic products and fundamental research. In the latter, pulse shaping is key to dynamically initialize, probe and manipulate the state of classical or quantum systems. In nuclear magnetic resonance, 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 implement 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 $ f_1 \sim 150$ MHz using a superposition of up to three discrete harmonics $f_n$. We employ these shaped pulses to interact with an individual sensor quantum dot and detect their deliberately and temporally modulated strain component via the opto-mechanical quantum dot response. Importantly, and in contrast to the direct mechanical actuation by bulk piezoactuators, surface acoustic waves provide much higher frequencies (> 20 GHz) 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., 18 pages - final manuscript and supporting material

We use standing surface acoustic waves to induce coherent phonons in model lipid multilayers deposited on a piezoelectric surface. Probing the structure by phase-controlled stroboscopic x-ray pulses we find that the internal lipid bilayer electron density profile oscillates in response to the externally driven motion of the lipid film. The structural response to the well-controlled motion is a strong indication that bilayer structure and membrane fluctuations are intrinsically coupled, even though these structural changes are averaged out in equilibrium and time integrating measurements. Here the effects are revealed by a timing scheme with temporal resolution on the picosecond scale in combination with the sub-nm spatial resolution, enabled by high brilliance synchrotron x-ray reflectivity. peerReviewed

Nanothermochromic diffraction gratings based on the metal-insulator transition of VO 2 are fabricated by site- selective ion-beam implantation in a SiO 2 matrix. The studied diffraction gratings were defined (i) directly by spatially selective ion-beam synthesis or (ii) by site-selective deactivation of the metal-insulator transition by ion-beam induced structural defects. The strongest increase of the diffraction efficient was observed at a wavelength of 1550 nm exceeding one order of magnitude for the selectively deactivated gratings. The observed pronounced thermal hysteresis extends down close to room temperature and makes these optical elements well suited for optical memory devices.

We show that the photoluminescence intensity and decay dynamics of semiconducting single-walled carbon nanotube films can be remotely controlled by surface acoustic waves (SAW) launched on the piezoelectric substrate LiNbO3. Time-resolved measurements in the picosecond regime reveal that photoluminescence quenching results from a decrease of the radiative recombination rate by up to 25% for the accessible SAW amplitudes. The SAW-induced piezoelectric field acts as a quasi-static perturbation that polarizes the luminescent exciton state reducing the oscillator strength of the radiative transition following a quadratic field dependence. Surface acoustic waves could be used for the remote and contact-free electrical control of high-speed electronic and optoelectronic nanotube-based devices.

Surface Acoustic Waves (SAW) are employed to deliberately modify the resonator properties of a high Q photonic crystal membrane (PCM) on a semiconductor nano structure. The SAW periodically deforms the PCM, giving rise to a periodically modulated resonance position of the photonic crystal. While achieving resonance shifts exceeding Δλ = 2 nm, the quality factor of the PCM resonance remains basically unchanged. Combination of the acousto-photonic resonance modulation with single photon generation of acoustically charged quantum dots and quantum posts are discussed in the context of single photon generation of non-classical light.

We employ surface acoustic waves to probe and manipulate optically active nanosystems. We demonstrate that the attenuation of a SAW provides a high sensitivity probe of the electrical conductivity by measuring the persistent photoconductivity in a multi quantum well structure. Moreover we apply SAW to dissociate photogenerated electron-hole pairs and by acoustic transport convey these at the speed of sound over macroscopic distances to a remote quantum emitter. After inherently sequential injection of electrons and holes, excitons are formed in these artificial atoms which recombine and emit single photons. Our SAW-based approach provides a low-jitter acoustically pumped single photon source with programmable exciton configuration.

Nanothermochromic diffraction gratings based on the metal-insulator transition of $\mathrm{VO_2}$ are fabricated by site-selective ion beam implantation in a $\mathrm{SiO_2}$ matrix. Gratings were defined either (i) directly by spatially selective ion beam synthesis or (ii) by site-selective deactivation of the phase transition by ion beam induced defects. The strongest increase of the diffracted light intensities was observed at a wavelength of 1550\,nm exceeding a factor of 20 for the selectively deactivated gratings. The observed pronounced thermal hysteresis extending down close to room temperature makes this system ideally suited for optical memory applications., Comment: 4 pages, 3 figures, 1 table - corrected Figure 3

We report on optical experiments performed on individual GaAs nanowires and the ma- nipulation of their temporal emission characteristics using a surface acoustic wave. We find a pronounced, characteristic suppression of the emission intensity for the surface acoustic wave propagation aligned with the axis of the nanowire. Furthermore, we demonstrate that this quenching is dynamical as it shows a pronounced modulation as the local phase of the surface acoustic wave is tuned. These effects are strongly reduced for a SAW applied in the direction perpendicular to the axis of the nanowire due to their inherent one-dimensional geometry. We resolve a fully dynamic modulation of the nanowire emission up to 678 MHz not limited by the physical properties of the nanowires., This document is the Accepted Manuscript version of a Published Work that appeared in final form in Nano Letters, copyright \c{opyright} American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/nl1042775

We report on recent progress in the acousto-electrical control of self-assembled quantum dot and quantum post using radio frequency surface acoustic waves (SAWs). We show that the occupancy state of these optically active nanostructures can be controlled via the SAW-induced dissociation of photogenerated excitons and the resulting sequential bipolar carrier injection which strongly favors the formation of neutral excitons for quantum posts in contrast to conventional quantum dots. We demonstrate high fidelity preparation of the neutral biexciton which makes this approach suitable for deterministic entangled photon pair generation. The SAW driven acoustic charge conveyance is found to be highly efficient within the wide quantum well surrounding the quantum posts. Finally we present the direct observation of acoustically triggered carrier injection into remotely positioned, individual quantum posts which is required for a low-jitter SAW-triggered single photon source., Comment: Proceedings of ISCS 2011; to appear in physics status solidi (c)

We report on recent progress towards single photon sources based on quantum dot and quantum post nanostructures which are manipulated using surface acoustic waves. For this concept acoustic charge conveyance in a quantum well is used to spatially separate electron and hole pairs and transport these in the plane of the quantum well. When conveyed to the location of a quantum dot or quantum post these carriers are sequentially captured into the confined levels. Their radiative decays gives rise to the emission of a train of single photons. Three different approaches using (i) strain- induced and (ii) self-assembled quantum dots, and (iii) self-assembled quantum posts are discussed and their application potential is discussed. First devices and initial experiments towards the realization of such an acoustically driven single photon source are presented and remote acoustically triggered injection into few individual emitters is demonstrated., 11 pages, 7 figures

We study the influence of the mechanical deformation induced by a surface acoustic wave (SAW) on the resonance frequency of a defect cavity in a 2D photonic crystal membrane. Using FDTD-simulations we determine the resonance frequency and quality factor of a nanocavity of a GaAs based structure with embedded InAs quantum dots. Under the influence of a SAW, we find a periodic modulation of the cavity resonance wavelength of Δλ >2 nm accompanied by only a weak < 0.5× reduction of the Q-factor. Initial experiments for a SAW wavelength of ~ 1.8μm show a pronounced broadening of the time-integrated cavity emission line corresponding to a shift of ≥ 1 nm.

We propose and demonstrate a surface acoustic wave-driven converter of light into surface plasmon polaritons. An otherwise unstructured thin metal film is deformed by traveling acoustic waves on a piezoelectric substrate underneath. This spatially periodic corrugation enables to bridge the momentum gap between free-space radiation and surface-bound modes. This principle is realized with plain gold films on a ${\text{LiNbO}}_{3}$ wafer where surface acoustic waves induce surface ripples in the metal. For near-infrared light we observe efficiencies of order ${10}^{\ensuremath{-}4}$ for exciting surface plasmon polaritons.

We demonstrate storage of excitons in a single nanostructure, a self-assembled Quantum Post. After generation electron and holes forming the exciton are separated by an electric field towards opposite ends of the Quantum Post inhibiting their radiative recombination. After a defined time such spatially indirect excitons are reconverted to optically active direct excitons by switching the electric field. The emitted light of the stored exciton is detected in the limit of a single nanostructure and storage times exceeding 30 msec are demonstrated. We identify a slow tunneling of the electron out of the Quantum Post as the dominant loss mechanism by comparing the field dependent temporal decay of the storage signal to models for this process and radiative losses., Comment: 23 pages

A single InGaAs/GaAs quantum dot is addressed in a two-color femtosecond transmission experiment. We find bleaching signals arising from individual excitonic interband transitions thus providing a scheme for ultrafast optical read-out of one artificial atom.

We demonstrate tuning of single quantum dot emission lines by the combined action of the dynamic acoustic field of a radio frequency surface acoustic wave and a static electric field. Both tuning parameters are set all-electrically in a LiNbO$_{3}$-GaAs hybrid device. The surface acoustic wave is excited directly on the strong piezoelectric LiNbO$_{3}$ onto which a GaAs-based p-i-n photodiode containing a single layer of quantum dots was epitaxially transferred. We demonstrate dynamic spectral tuning with bandwidths exceeding 3 meV of single quantum dot emission lines due to deformation potential coupling. The center energy of the dynamic spectral oscillation can be independently programmed simply by setting the bias voltage applied to the diode., Comment: 13 pages, 4 figures, revised version, some typos fixed

We demonstrate the ability to control the spontaneous emission dynamics of self-assembled quantum dots via the local density of optical modes in two-dimensional s2Dd photonic crystals. We show that an incomplete 2D photonic band gap is sufficient to significantly lengthen the spontaneous emission lifetimes.2 3 d over a wide bandwidth sDl o40 nmd. For dots that are both spectrally and spatially coupled to strongly localized fVmodes, 1.5sl / nd 3 g, high Q, 2700 optical modes, we have directly measured a strong Purcell-enhanced shortening of the emission lifetime o5.63, limited only by our temporal resolution. Analysis of the spectral dependence of the recombination dynamics shows a maximum lifetime shortening of 19± 4. From the directly measured enhancement and suppression we show that the single-mode coupling efficiency for quantum dots in such structures is at least b = 92% and is estimated to be as large as ,97%.

Recent experimental developments in the field of semiconductor quantum dot spectroscopy will be discussed. First we report about single quantum dot exciton two-level systems and their coherent properties in terms of single qubit manipulations. In the second part we report on coherent quantum coupling in a prototype "two-qubit" system consisting of a vertically stacked pair of quantum dots. The interaction can be tuned in such quantum dot molecule devices using an applied voltage as external parameter., 37 pages, 15 figures, submitted to New Journal of Physics, focus issue on Solid State Quantum Information, added references

AIP Advances 3(7), 072127 - (2013). doi:10.1063/1.4816801, Published by American Inst. of Physics, New York, NY

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 figures