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# Acoustic charge transport induced by the surface acoustic wave in chemical doped graphene

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## Abstract

A graphene/LiNbO3 hybrid device is used to investigate the acoustic induced charge transport in chemical doped graphene. The chemical doping of graphene via its physisorption of gas molecules affects the surface acoustic wave (SAW) charge carrier transport in a manner different from electric field drift. That transport induces doping dependent macroscopic acoustoelectric current. The chemical doping can manipulate majority carriers and induces unique acoustoelectric features. The observation is explained by a classical relaxation model. Eventually the device based on acoustoelectric current is proved to outperform the common chemiresistor for chemicals. Our finding provides insight into acoustic charge carrier transport during chemical doping. The doping affects interaction of carriers with SAW phonon and facilitates the understanding of nanoscale acoustoelectric effect. The exploration inspires potential acoustoelectric application for chemical detection involving emerging 2D nanomaterials.

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... To demonstrate the special dependence of the AE current on the electric conductivity, the graphene carrier density has been modulated by several means, e.g. photoexcitation of electron-hole pairs [81,82] and chemical doping [83]. shows an example of the last case, where a p-doped graphene layer was exposed to different concentrations of nitric acid (NO 2 ) vapor. ...
... As a consequence, the amplitude of the conventional current reduced, while the AE current increased with the concentration of NH 3 , in agreement with σ approaching σ M (not shown here). Remarkably, for low gas concentrations, the relative response changes of the AE current outperformed that of the common drift current [83], thus demonstrating the potential application of graphene-based acousto-electric devices for chemical detection. ...
... Amplitude of the AE current (blue points) and conventional direct current (red points) measured under exposition of graphene to several concentrations of a NO 2 gas dopant. Reprinted from[83] with the permission of AIP Publishing. ...
Article
This article reviews the main theoretical and experimental advances regarding the interaction between surface acoustic waves (SAWs) and electronic excitations in graphene. The coupling of the graphene electron gas to the SAW piezoelectric field can modify the propagation properties of the SAW, and even amplify the intensity of SAWs traveling along the graphene layer. Conversely, the periodic electric and strain fields of the SAW can be used to modify the graphene Dirac cone and to couple light into graphene plasmons. Finally, SAWs can generate acousto-electric currents in graphene. These increase linearly with the SAW frequency and power but, in contrast to conventional currents, they depend non-monotonously on the graphene electric conductivity. Most of these functionalities have been reported in graphene transferred to the surface of strong piezoelectric insulators. The recent observation of acousto-electric currents in epitaxial graphene on SiC opens the way to the large-scale fabrication of graphene-based acousto-electric devices patterned directly on a semi-insulating wafer.
... The effect has been observed in different classes of materials such as semiconductors, quantum wires, two-dimensional (2D) electron gas, and heterostructures [3][4][5][6][7][8][9][10][11]. More recently, it has been recognized that the coupling between the surface acoustic wave (SAW) and electrons in 2D Dirac materials provides an exciting opportunity to investigate charge transport driven by the strain fields associated with propagating SAW [12][13][14][15][16][17][18][19]. In particular, the AE effect of single-layer graphene has been investigated experimentally, and the AE current has been shown to be tunable by the application of a gate voltage [18]. ...
... Despite many studies on the piezoelectric mechanism of the AE effect in the 2D electron gas and in graphene [12][13][14][15][16][17][18][19] the analysis of the acoustoelectric effect in 2D Dirac materials is not yet complete. In particular, the relevance of gauge phonon for the AE effect in graphene has not been discussed to the best of our knowledge. ...
Preprint
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Using a diagrammatic scheme, we study the acoustoelectric effects in 2D Dirac materials due to the sound-induced pseudo-gauge field. We analyze both uniform and {\em spatially dispersive} currents in response to co-propagating and counter-propagating sound waves, respectively. In addition to the longitudinal acoustoelectric current, we obtain an exotic {\em transverse} charge current flowing perpendicular to the sound propagation direction owing to the interplay of transverse and longitudinal gauge field components $j_T\propto A_L A^\ast_T$. In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component $j_T\sim\sin(6\theta)$ is achieved that stems from the inherited three-fold symmetry of the hexagonal lattice. However, both longitudinal and transverse parts of the dispersive current are predicted to be strongly anisotropic $\sim\sin^2(3\theta)$ or $\cos^2(3\theta)$. We quantitatively estimate the pseudo-gauge field contribution to the acoustoelectric current that can be probed in future experiments in graphene and other 2D Dirac materials.
... One of the unique electronic properties of graphene, the ability to tune its conductivity over orders of magnitude, also allows the AE current to be tuned dynamically [26][27][28][29][30], where the ambipolar nature of graphene allows the sign of the AE current to be reversed [26][27][28][29]. In addition, using an ion gel gate to modulate the conductivity of graphene, Bandhu and Nash [30] not only reversed the direction of the current, but also demonstrated that graphene can be used to control the SAW amplitude and velocity. ...
... One of the unique electronic properties of graphene, the ability to tune its conductivity over orders of magnitude, also allows the AE current to be tuned dynamically [26][27][28][29][30], where the ambipolar nature of graphene allows the sign of the AE current to be reversed [26][27][28][29]. In addition, using an ion gel gate to modulate the conductivity of graphene, Bandhu and Nash [30] not only reversed the direction of the current, but also demonstrated that graphene can be used to control the SAW amplitude and velocity. ...
Article
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The acoustoelectric current in graphene nanoribbons, with widths ranging between 350 nm and 600 nm, has been investigated as a function of illumination. For all nanoribbon widths, the acoustoelectric current was observed to decrease on illumination, in contrast to the increase in acoustoelectric current measured in unpatterned graphene sheet devices. This is thought to be due to the higher initial conductivities of the nanoribbons compared to unpatterned devices.
... The AE effect is at its strongest when a 2D semiconduct­ ing layer with high mobility and low carrier density is placed in close proximity with a high K 2 acoustic wave [14][15][16][17]. Fundamental investigations on AE have focused on surface acoustic waves (SAW, or Rayleigh waves) in the GaAs/AlGaAs heterostructure due to the intrinsic piezoelectricity of GaAs and the high quality of the modulation doping induced 2DEG [18,19]. ...
Article
We report on the acoustoelectric (AE) interaction in heterogeneously integrated thin-film lithium niobate on standard resistivity and high resistivity silicon substrates (LNOSs). The monolithic LNOS platform delivers acoustic waves with a large electromechanical coupling coefficient (K ²) and draws on standard metal-oxide semiconductor field effect techniques to maximise AE interaction by impedance matching the acoustic wave with the semiconductor carriers. Preliminary results are obtained on AE attenuation (4 dB cm⁻¹) and AE gain (6 dB cm⁻¹). With further improvement of the LN/Si interface, the LNOS platform can be expected to give rise to an era of non-reciprocal silicon acoustoelectronics.
... These have been measured in micron-scale graphene monolayers 7,8 , and we have previously studied the acoustoelectric response of large-area graphene sheets produced by chemical vapour deposition (CVD) transferred to LiNbO 3 SAW devices at room temperature 9 , low temperature 10 , and under illumination 11 . Theoretical and experimental reports have examined DC-biased mono-and multi-layer graphene sheets for SAW amplification 12,13 , while others have studied the use of electrolytic solutions 14 , and gases 15 to control the carrier concentration in graphene films, opening up possibilities for low-power chemical sensors. In contrast, by using an ion gel gate to modulate the conductivity of graphene, Bandhu and Nash 16 demonstrated that graphene can be used to control the SAW amplitude and velocity shift. ...
Article
Surface acoustic waves (SAWs) propagating on piezoelectric substrates offer a convenient, contactless approach to probing the electronic properties of low-dimensional charge carrier systems such as graphene nanoribbons (GNRs). SAWs can also be used to transport and manipulate charge for applications such as metrology and quantum information. In this work, we investigate the acoustoelectric effect in GNRs, and show that an acoustoelectric current can be generated in GNRs with physical widths as small as 200 nm at room temperature. The positive current in the direction of the SAWs, which corresponds to the transportation of holes, exhibits a linear dependence on SAW intensity and frequency. This is consistent with the description of the interaction between the charge carriers in the GNRs and the piezoelectric fields associated with the SAWs being described by a relatively simple classical relaxation model. Somewhat counter-intuitively, as the GNR width is decreased, the measured acoustoelectric current increases. This is thought to be caused by an increase of the carrier mobility due to increased doping arising from damage to the GNR edges.
... LiNbO 3 SAW devices also have found their usage in solid state physics, for example, SAW-driven quantized charge transport [53,54], the use of SAWs to control phonon angular momentum [55], the strong optomechanical coupling of individual quantum emitters and a surface acoustic wave [56], and quantum control of surface acoustic-wave phonons [57]. In integrated photonic systems, LiNbO 3 SAW resonators can be used to confine surface phonons [58]. ...
Article
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Lithium niobate (LiNbO3) crystals are important dielectric and ferroelectric materials, which are widely used in acoustics, optic, and optoelectrical devices. The physical and chemical properties of LiNbO3 are dependent on microstructures, defects, compositions, and dimensions. In this review, we first discussed the crystal and defect structures of LiNbO3, then the crystallization of LiNbO3 single crystal, and the measuring methods of Li content were introduced to reveal reason of growing congruent LiNbO3 and variable Li/Nb ratios. Afterwards, this review provides a summary about traditional and non-traditional applications of LiNbO3 crystals. The development of rare earth doped LiNbO3 used in illumination, and fluorescence temperature sensing was reviewed. In addition to radio-frequency applications, surface acoustic wave devices applied in high temperature sensor and solid-state physics were discussed. Thanks to its properties of spontaneous ferroelectric polarization, and high chemical stability, LiNbO3 crystals showed enhanced performances in photoelectric detection, electrocatalysis, and battery. Furthermore, domain engineering, memristors, sensors, and harvesters with the use of LiNbO3 crystals were formulated. The review is concluded with an outlook of challenges and potential payoff for finding novel LiNbO3 applications.
... Liang et al. 48 , Zheng et al. 49 and Okuda et al. 50 reported similar behavior, where acoustic charge transportation was induced by SAW propagation in the graphene. ...
Article
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We perform self-consistent analysis of the Boltzmann transport equation for momentum and energy in the hypersound regime i.e., $$ql \gg 1$$ q l ≫ 1 ( $$q$$ q is the acoustic wavenumber and l is the mean free path). We investigate the Landau damping of acoustic phonons ( $$LDOAP$$ LDOAP ) in graphene nanoribbons, which leads to acoustoelectric current generation. Under a non-quantized field with drift velocity, we observed an acoustic phonon energy quantization that depends on the energy gap, the width, and the sub-index of the material. An effect similar to Cerenkov emission was observed, where the electron absorbed the confined acoustic phonon energy, causing the generation of acoustoelectric current in the graphene nanoribbon. A qualitative analysis of the dependence of the absorption coefficient and the acoustoelectric current on the phonon frequency is in agreement with experimental reports. We observed a shift in the peaks when the energy gap and the drift velocity were varied. Most importantly, a transparency window appears when the absorption coefficient is zero, making graphene nanoribbons a potential candidate for use as an acoustic wave filter with applications in tunable gate-controlled quantum information devices and phonon spectrometers.
... Details of the preparation can be found in our previous work. 19 Typically, surface acoustic waves penetrate into the material to a depth comparable to the acoustic wavelength. Therefore, to keep energy from leaking into the silicon substrate, thicker AlN or smaller wavelength is preferred. ...
Article
We fabricated a hybrid on-chip acousto-electric (AE) and field-effect device to investigate the modulation of acoustic carrier transportation by gate voltage. The device fabrication exploited a surface micromachining aluminum nitride process on a silicon wafer, facilitating an integration of a surface acoustic wave(SAW) delay line and a graphene field-effect transistor. The SAW device induced an AEcurrent in graphene, which scales linearly with the input power and remains essentially constant when subtracting the offset current at different DC biases. At a constant DC bias, the AEcurrent can be modulated by the gate voltage, due to the change of the carrier mobility in graphene. A four-fold enhancement in the AEcurrent was realized when ∼35 V voltage was applied to the gate electrode. The highly integrated device proves to be a powerful tool to understand the AEcurrent in graphene, and since it supports integration for versatile functionality, it opens an avenue to explore the properties of diverse nanomaterials.
... The possibility of charge transport by SAWs in graphene and the possibility of monitoring a SAW under an electric potential applied to graphene were reported in a number of works. [7][8][9][10][11][12][13][14] The SEM method can be used to study the SAW propagation in piezoelectric crystals. This method enables the visualization of acoustic wave propagation through registration of low-energy secondary electrons sensitive to electric fields accompanying the SAW propagation in piezoelectric crystals (the electric fields between SAW minima and maxima). ...
Article
Full-text available
Acoustically stimulated charge transport in solids was studied using the scanning electron microscopy method (SEM). The surface acoustic wave on the surface of the YZ-cut of a LiNbO3 crystal was visualized by registration of low-energy secondary electrons in SEM, and the charge distribution on the crystal surface was visualized using the electron beam induced current method. To register the induced current, an interdigital transducer structure was formed from graphene on the crystal surface. It was shown that the charge distribution on the crystal surface corresponds to the distribution of the acoustic wave field on the crystal surface.
... 2bshows the dependence of Γ q maximum amplification is obtained at Γ q = −0.16 at q  = 2 . It is interesting to note that, our results is in good agreed with the work in ref.[27] where acousticphonon frequencies above 10 were attained. The field in the material can be 11.5 / . ...
Preprint
Full-text available
We perform self-consistent analysis of the Boltzmann transport equation for momentum and energy in the hypersound regime i.e., ql >> 1 ( q is the acoustic wavenumber and l is the mean free path). Here, we investigate Landau damping of acoustic phonons (LDOAP) in graphene nanoribbon that leads to acoustoelectric current generation. Under a non-quantized field with drift velocity, we observed an acoustic phonon energy quantization which depends on the energy gap, the width and the sub-index of the material. An effect similar to Cerenkov emission was observed where the electron absorbs the confined acoustic phonons energy, causing the generation of acoustoelectric current in Graphene Nanoribbon. A qualitative analysis of the absorption and versus phonon frequency is in agreement with experimental reports. We observed a shift in the peaks when the energy gap and the drift velocity were varied. Most importantly, a transparency window appears when making graphene nanoribbon a potential candidate as an acoustic wave filter with applications in phonon spectrometers and also as tunable gate-controlled quantum information device.
... The SAW (more precisely Rayleigh wave) is an elastic wave traveling along the surface of piezoelectric substrate and is generated by applying an ac voltage to comb-shaped electrodes, as illustrated in Fig. 1A. The excited SAW can be coupled with the lattice system of thin films on the substrate, resulting in the modulation of electronic properties of the thin films (16)(17)(18)(19)(20)(21)(22). ...
Preprint
We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe$_2$ thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we propose that soliton-antisoliton pairs in the charge density wave of NbSe$_2$ modulated by the SAW serve as a time-dependent capacitance in the superconducting state, leading to the dc negative resistance. The present experimental result would provide a previously unexplored way to examine nonequilibrium manipulation of the superconductivity.
... The SAW (more precisely Rayleigh wave) is an elastic wave traveling along the surface of piezoelectric substrate and is generated by applying an ac voltage to comb-shaped electrodes, as illustrated in Fig. 1A. The excited SAW can be coupled with the lattice system of thin films on the substrate, resulting in the modulation of electronic properties of the thin films (16)(17)(18)(19)(20)(21)(22). ...
Article
Full-text available
We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe 2 thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we propose that soliton-antisoliton pairs in the charge density wave of NbSe 2 modulated by the SAW serve as a time-dependent capacitance in the superconducting state, leading to the dc negative resistance. The present experimental result would provide a previously unexplored way to examine nonequilibrium manipulation of the superconductivity.
... A graphene/LiNbO 3 hybrid device was used to investigate the acoustic induced charge transport in chemically doped graphene (Zheng et al. 2016). The chemical doping of graphene via physisorption of gas molecules affected the piezoelectric surface acoustic wave charge carrier transport in a manner different from that of electric field drift. ...
... As σ BP increases with a negative gate bias from −3 V to −9 V, j or gradually decreases. The inverse dependence of j or on σ BP is attributed to the partial screening of the acousto-electric field when σ BP σ M [20]. σ M is a characteristic conductivity as the few-layer BP is placed on the BN in our system which is approximately given by σ M = υε 0 · (ε γ + 1) = 1.8 × 10 −7 S according to Weinreich relations where ε 0 = 8.85 × 10 −12 F/ m is the permittivity of free space, ε γ is relative dielectric constant of BN, υ is the speed of the acoustic wave and approximately 3990 m/s [22]. ...
Article
We have combined a graphene field-effect transistor (GFET) and a surface acoustic wave (SAW) sensor on a LiTaO3 substrate to create a graphene surface acoustic wave (GSAW) sensor. When a SAW propagates in graphene, an acoustoelectric current (IA) flows between two attached electrodes. This current has unique electrical characteristics, having both positive and negative peak values with respect to the electrolyte-gate voltage (VEg) in solution. We found that IA is controlled by VEg and the amplitude of the SAW. It was also confirmed that the GSAW sensor detects changes of electrical charge in solution like conventional GFET sensors. Furthermore, the detection of amino-group-modified microbeads was performed by employing a GSAW sensor in a phthalate buffer solution at pH 4.1. The hole current peak shifted to the lower left in the IA–VEg characteristics. The left shift was caused by charge detection by the GFET and can be explained by an increase of amino-groups that have positive charges at pH 4.1. In contrast, the downward shift is thought to be due to a reduction in the amplitude of the propagating SAW because of an increase in the mass loading of microbeads. This mass loading was detected by the SAW sensor. Thus we have demonstrated that the GSAW sensor is a transducer capable of the simultaneous detection of charge and mass, which indicates that it is an attractive platform for highly sensitive and multifunctional solution sensing.
Chapter
This chapter reports on the state of the art of piezoelectric micro-/nano-mechanical devices in frequency control and sensing applications. Recent studies on bulk acoustic wave (BAW) devices are introduced, including investigation of high-coupling materials and filter and oscillator designs. A novel class of frequency devices based on Lamb waves is also reviewed. Micro- and nano-mechanical sensors for various sensing applications and integrated module are outlined.
Article
We demonstrate in situ gate-tunable acoustoelectric transport in exfoliated monolayer graphene by measuring the voltage created as high-frequency surface acoustic waves dynamically drive charge carriers in the graphene. We employ a flip-chip device configuration to conduct acoustoelectric measurements while simultaneously controlling the graphene carrier density with a metal-oxide back-gate. At high carrier density we observe dependence of the acoustoelectric signal on the sign of the graphene charge carriers, while at low densities we observe anomalous sign reversals of the acoustoelectrically generated voltage. We attribute these anomalous sign reversals to spatially heterogeneous conduction in the vicinity of charge neutrality.
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The strong piezoelectric fields accompanying a surface acoustic wave on a semiconductor quantum well structure are employed to dissociate optically generated excitons and efficiently trap the created electron hole pairs in the moving lateral potential superlattice of the sound wave. The resulting spatial separation of the photogenerated ambipolar charges leads to an increase of the radiative lifetime by orders of magnitude as compared to the unperturbed excitons. External and deliberate screening of the lateral piezoelectric fields triggers radiative recombination after very long storage times at a remote location on the sample. Comment: 4 PostScript figures included, Physical Review Letters, in press
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The acoustoelectric current in graphene has been investigated as a function of illumination, using blue (450 nm) and red (735 nm) light-emitting diodes (LEDs), and surface acoustic wave (SAW) intensity and frequency. The measured acoustoelectric current increases with illumination, more than the measured change in the conductivity of the graphene, whilst retaining a linear dependence on the SAW intensity. The latter is consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model suggesting that the change in the acoustoelectric current is caused by the effect of the illumination on the electronic properties of the graphene. The increase in the acoustoelectric current is greatest under illumination with the blue LED, consistent with the creation of a hot electron distribution.
Article
The acoustoelectric current in graphene has been investigated as a function of temperature, surface acoustic wave (SAW) intensity, and frequency. At high SAW frequencies, the measured acoustoelectric current decreases with decreasing temperature, but remains positive, which corresponds to the transport of holes, over the whole temperature range studied. The current also exhibits a linear dependence on the SAW intensity, consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model. At low temperatures and SAW frequencies, the measured acoustoelectric current no longer exhibits a simple linear dependence on the SAW intensity, and the direction of the acoustoelectric current is also observed to reverse under certain experimental conditions.
Article
We demonstrate macroscopic acoustoelectric transport in graphene, transferred onto piezoelectric lithium niobate substrates, between electrodes up to 500 μm apart. Using double finger interdigital transducers we have characterised the acoustoelectric current as a function of both surface acoustic wave intensity and frequency. The results are consistent with a relatively simple classical relaxation model, in which the acoustoelectric current is proportional to both the surface acoustic wave intensity and the attenuation of the wave caused by the charge transport.
Article
We report the first observation of the direct current induced by a surface acoustic wave through a quantum point contact defined in a GaAs - AlGaAs two-dimensional electron gas by means of a split gate. We have observed giant oscillations in the acoustoelectric current as a function of gate voltage, with minima corresponding to the plateaux in quantum point contact conductivity. A theoretical consideration is presented which explains the observed peaks in terms of the matching of sound velocity with electron velocity in the upper one-dimensional subband of the quantum point contact.
Article
The acoustoelectric interaction of surface phonons with conduction electrons in piezoelectric semiconductors is investigated in the frequency region where the piezoelectric coupling is the predominant mode of interaction. We derive the interaction explicitly taking the elastic anisotropy of the semiconductors into account. The electronic state near the semiconductor surface, due to the presence of the surface space-charge layer, is also properly considered to specify the interaction. Applying the interaction to the study of the amplification characteristics of surface acoustic waves of GHz frequencies (propagated on the basal plane of a cubic crystal), we find that the elastic anisotropy reduces the interaction significantly compared with that of the isotropic approximation. Furthermore, when the wavelengths of the surface phonons become comparable to the width of the surface-depletion layer, the frequency dependence of the amplification rates suffers modifications also qualitatively.
Article
The interaction between surface acoustic waves and quasi-two-dimensional inversion electron systems on GaAs/AlxGa1-xAs heterojunctions is investigated in high magnetic fields and at low temperatures. The interaction of the surface acoustic wave with high-mobility inversion electrons leads to strong quantum oscillations in both the transmitted surface wave intensity as well as in the sound velocity, reflecting the quantum oscillations of the magnetoconductivity as a function of an applied magnetic field. We study the dependence of this interaction on the magnetic field and on the surface-acoustic-wave power and frequency, and discuss the results using simple models. The influence of slight spatial inhomogeneities in the carrier density on the line shape of the quantum oscillations is analyzed in detail and related to their influence on the quantum Hall effect. First experimental results on the interaction of surface acoustic waves with two-dimensional electron systems in gated heterojunctions providing an adjustable carrier density are presented.
Article
The presence in a semiconductor of both signs of current carriers makes it possible to disturb the spatial distribution of these carriers appreciably without giving rise to electrical space charge. As a result, the interaction of particles with acoustic waves leads to certain effects which, at least at low frequencies, are peculiar to semiconductors; these include a (complex) addition to the elastic modulus and the "acoustoelectric effect."
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This paper reviews the interaction between coherently stimulated acoustic phonons in the form of surface acoustic waves with light beams in semiconductor based photonic structures. We address the generation of surface acoustic wave modes in these structures as well as the technological aspects related to control of the propagation and spatial distribution of the acoustic fields. The microscopic mechanisms responsible for the interaction between light and surface acoustic modes in different structures are then reviewed. Particular emphasis is given to the acousto-optical interaction in semiconductor microcavities and its application in photon control. These structures exhibit high optical modulation levels under acoustic excitation and are compatible with integrated light sources and detectors.
Article
Acoustoelectric amplification, in the linear approximation, is considered for a system containing several types of carriers in the two limiting cases of fast and slow carrier equilibration. The pertinent expressions are first derived for a piezoelectric semiconductor and then expanded to the more practical case of a layered system containing one or more semiconducting layers coupled to a piezoelectric element. An experimental example is given in the form of a guided elastic plate wave in lead zirconate titanate coupled to a plate of near‐intrinsic Ge.
Article
The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited carriers, as well as to spatially control exciton recombination in GaAs-based nanowires (NWs) on a subns time scale. The experiments are carried out in core-shell NWs transferred to a SAW delay line on a LiNbO(3) crystal. Carriers generated in the NW by a focused laser spot are acoustically transferred to a second location, leading to the remote emission of subns light pulses synchronized with the SAW phase. The dynamics of the carrier transport, investigated using spatially and time-resolved photoluminescence, is well-reproduced by computer simulations. The high-frequency contactless manipulation of carriers by SAWs opens new perspectives for applications of NWs in opto-electronic devices operating at gigahertz frequencies. The potential of this approach is demonstrated by the realization of a high-frequency source of antibunched photons based on the acoustic transport of electrons and holes in (In,Ga)As NWs.
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We report on optical experiments performed on individual GaAs nanowires and the manipulation 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 surface acoustic wave 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.
Article
The dynamic nature of the first water adlayers on solid surfaces at room temperature has made the direct detection of their microscopic structure challenging. We used graphene as an atomically flat coating for atomic force microscopy to determine the structure of the water adlayers on mica at room temperature as a function of relative humidity. Water adlayers grew epitaxially on the mica substrate in a layer-by-layer fashion. Submonolayers form atomically flat, faceted islands of height 0.37 ± 0.02 nanometers, in agreement with the height of a monolayer of ice. The second adlayers, observed at higher relative humidity, also appear icelike, and thicker layers appear liquidlike. Our results also indicate nanometer-scale surface defects serve as nucleation centers for the formation of both the first and the second adlayers.
• Lv R.