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- ItemPolarization manipulation of surface acoustic waves by metallization patterns on a piezoelectric substrate(Melville, NY : AIP Publishing, 2020) Weser, R.; Darinskii, A.N.; Schmidt, H.Surface acoustic waves (SAWs) with large normal (vertical) surface displacement at the surface are commonly utilized in microfluidic actuators in order to provide the desired momentum transfer to the fluid. We present an alternative concept using a SAW with comparatively small vertical displacement. Such a SAW passes underneath the microfluidic vessel walls with minimum losses but it needs to be converted inside the vessel into surface vibrations with large vertical displacements. The principal operability of the above idea is illustrated by experimental and numerical studies of the polarization conversion of a leaky SAW on 64° rotated Y-cut of lithium niobate owing to the partial metallization of the substrate surface. In particular, it is found that vertical displacements on the metallized surface can be up to 3.5 times higher as compared to their values on the free surface. Results of computations agree reasonably well with measurements carried out with a laser Doppler vibrometer and allow the clarification of some specific features of this polarization conversion by means of spatial frequency analysis. © 2020 Author(s).
- ItemSurface acoustic wave modulation of single photon emission from GaN/InGaN nanowire quantum dots(Bristol : IOP Publ., 2018) Lazić, S.; Chernysheva, E.; Hernández-Mínguez, A.; Santos, P.V.; van der Meulen, H.P.On-chip quantum information processing requires controllable quantum light sources that can be operated on-demand at high-speeds and with the possibility of in-situ control of the photon emission wavelength and its optical polarization properties. Here, we report on the dynamic control of the optical emission from core-shell GaN/InGaN nanowire (NW) heterostructures using radio frequency surface acoustic waves (SAWs). The SAWs are excited on the surface of a piezoelectric lithium niobate crystal equipped with a SAW delay line onto which the NWs were mechanically transferred. Luminescent quantum dot (QD)-like exciton localization centers induced by compositional fluctuations within the InGaN nanoshell were identified using stroboscopic micro-photoluminescence (micro-PL) spectroscopy. They exhibit narrow and almost fully linearly polarized emission lines in the micro-PL spectra and a pronounced anti-bunching signature of single photon emission in the photon correlation experiments. When the nanowire is perturbed by the propagating SAW, the embedded QD is periodically strained and its excitonic transitions are modulated by the acousto-mechanical coupling, giving rise to a spectral fine-tuning within a ~1.5 meV bandwidth at the acoustic frequency of ~330 MHz. This outcome can be further combined with spectral detection filtering for temporal control of the emitted photons. The effect of the SAW piezoelectric field on the QD charge population and on the optical polarization degree is also observed. The advantage of the acousto-optoelectric over other control schemes is that it allows in-situ manipulation of the optical emission properties over a wide frequency range (up to GHz frequencies).
- ItemIn situ surface acoustic wave field probing in microfluidic structures using optical transmission interferometry(Melville, NY : American Inst. of Physics, 2021) Weser, R.; Schmidt, H.The generation of mechanical driving forces in fluids at the microscale can be efficiently realized using acoustic actuators. For this purpose, bulk or surface acoustic waves (SAWs) are typically excited by an electroacoustic transducer, and the acoustic energy is subsequently coupled to the fluid. The resultant acoustic pressure field in the fluid allows for precise manipulation of immersed objects and also for the agitation of the fluid itself. In general, the fluidic actuation capability is mainly determined by the mechanical displacement amplitude at the interface between the fluid and the acoustically active surface. In the case of SAW-based actuators, the fluid most often is directly attached to the substrate surface along which the surface waves propagate. Hence, the lateral distribution of surface displacement amplitude, i.e., the surface acoustic wave field, at the fluid–substrate interface is of particular interest in order to achieve full control of the fluidic actuation. Here, we present a reliable experimental method for the in situ determination of the SAW field on fluid loaded substrate surfaces based on laser interferometry. The optical accessibility of the fluid–substrate interface is realized via transmission through the anisotropic, piezoelectric substrate material requiring only an additional calibration procedure in order to compensate the parasitic influence of effects based on different indices of refraction as well as on complex acousto-optic effects. Finally, the proposed method is demonstrated to yield reliable results of displacement amplitude on the fluid loaded surface and thus, to provide a valuable insight into acoustofluidic coupling that was not available so far.
- ItemDynamics of indirect exciton transport by moving acoustic fields(Bristol : IOP, 2014) Violante, A.; Cohen, K.; Lazić, S.; Hey, R.; Rapaport, R.; Santos, P.V.We report on the modulation of indirect excitons (IXs) as well as their transport by moving periodic potentials produced by surface acoustic waves (SAWs). The potential modulation induced by the SAW strain modifies both the band gap and the electrostatic field in the quantum wells confining the IXs, leading to changes in their energy. In addition, this potential captures and transports IXs over several hundreds of μm. While the IX packets keep to a great extent their spatial shape during transport by the moving potential, the effective transport velocity is lower than the SAW group velocity and increases with the SAW amplitude. This behavior is attributed to the capture of IXs by traps along the transport path, thereby increasing the IX transit time. The experimental results are well-reproduced by an analytical model for the interaction between trapping centers and IXs during transport.
- ItemSlowness curve surface acoustic wave transducers for optimized acoustic streaming(Cambridge : Royal Society of Chemistry, 2020) O'Rorke, R.; Winkler, A.; Collins, D.; Ai, Y.Surface acoustic waves can induce force gradients on the length scales of micro- and nanoparticles, allowing precise manipulation for particle capture, alignment and sorting activities. These waves typically occupy a spatial region much larger than a single particle, resulting in batch manipulation. Circular arc transducers can focus a SAW into a narrow beam on the order of the particle diameter for highly localised, single-particle manipulation by exciting wavelets which propagate to a common focal point. The anisotropic nature of SAW substrates, however, elongates and shifts the focal region. Acousto-microfluidic applications are highly dependent on the morphology of the underlying substrate displacement and, thus, become dependent on the microchannel position relative to the circular arc transducer. This requires either direct measurement or computational modelling of the SAW displacement field. We show that the directly measured elongation and shift in the focal region are recapitulated by an analytical model of beam steering, derived from a simulated slowness curve for 128° Y-cut lithium niobate. We show how the negative effects of beam steering can be negated by adjusting the curvature of arced transducers according to the slowness curve of the substrate, for which we present a simple function for convenient implementation in computational design software. Slowness-curve adjusted transducers do not require direct measurement of the SAW displacement field for microchannel placement and can capture smaller particles within the streaming vortices than can circular arc IDTs.