Experimental Realization of Topological Surface Acoustic Wave Resonances under Surface Liquid Loading for Micro-volume sample Sensings

Acoustic delocalization sensing by scattering of surface acoustic waves

Micro- and nanomechanical resonators are widely used for chemical and biological detection. Mode localization sensing, based on the distribution of energy between two or more coupled resonators, has demonstrated enhanced sensitivity with respect to the conventional frequency shift method. In contrast, this work introduces an alternative sensing concept based on acoustic delocalization, where energy redistribution is induced in initially uncoupled surface acoustic wave (SAW) cavities by particle-triggered acoustic scattering. We implement this concept using two orthogonally arranged SAW resonators operating with a center frequency of around 415 MHz. Particle adsorption at the intersection region generates wave scattering that effectively couples the resonant cavities. This scattering-driven interaction leads to a redistribution of acoustic energy that was initially confined to a single cavity, enabling detection through the signal emerging in the non-driven cavity and providing information on the mechanical properties of the particles. Devices were fabricated to validate the proposed mechanism and demonstrated the detection of particle arrays. Finite element simulations agree well with experimental results and further reveal how the acoustic scattering coefficient depends on the mechanical properties of the particles and their spatial distribution. These results demonstrate the feasibility of acoustic delocalization sensing using SAW scattering and highlight its potential for robust and sensitive detection of micro- and nanoparticles.

Electromagnetic and Acoustic Fano Interference in Surface Acoustic Wave Resonators

Surface acoustic wave-based resonators are sensitive probes of condensed matter systems, as well as surface-selective sensors for chemistry and biology. Surface acoustic wave devices have also been integrated into hybrid quantum systems with qubit platforms for applications in quantum information processing and sensing. The sensitivity of piezoelectric surface wave-based resonators to investigate these various systems can be enhanced by optimizing the device architecture and electrical measurement techniques. Alternatively, tailoring the spectral symmetry, arising from interference effects, offers a promising route to further improve sensitivity. In this work, we demonstrate the simultaneous introduction of both electromagnetic and acoustic Fano interference to shape the spectral response of GHz-frequency surface acoustic wave resonators. By systematically modifying the acoustic reflectivity of the resonators, we are able to isolate and analyze each interference mechanism independently. The broad range of temperature operation, from ambient to cryogenic temperatures highlights the potential for both classical and quantum sensing applications.

An integrated platform for metamaterial-based sensing and surface acoustic wave-based acoustofluidics utilising circular interdigital transducers

Topological Skyrmion-type microparticle manipulation based on surface acoustic wave phase modulations

Surface acoustic wave (SAW) micromanipulation enables the precise, non-contact handling of microscale particles and has attracted considerable interest in microfluidics and biomedicine. However, conventional SAW platforms generally rely on simple interference fields which are susceptible to fabrication imperfections and environmental perturbations, resulting in limited trapping stability. Here, we develop a SAW-based acoustofluidic platform that generates an acoustic skyrmion lattice through the coherent interference of three SAWs. The topologically structured field provides robust phase singularities and a stable gradient-force landscape, enabling microparticles to be localized at predefined lattice sites and supporting controllable rotational manipulation. Independent modulation of the amplitude and phase of the electrical inputs allows the field strength to be tuned for particles of different sizes. Numerical simulations and proof-of-concept experiments confirm particle trapping and ordered lattice assembly in the acoustic skyrmion field, demonstrating the feasibility of translating topological acoustic textures into practical on-chip manipulation functions. This reconfigurable strategy offers a route to robust SAW manipulation and may support applications in single-cell analysis, three-dimensional cell assembly, high-throughput screening, and microscale and nanoscale device assembly.

Equilibrium gigahertz acoustics reveals long-range confinement in liquids

Understanding how the mechanical properties of liquids confined within nanometer-scale gaps differ from bulk behavior is central to biophysics, lubrication, catalysis, electrochemistry, and surface science. Yet the characterization of ultrathin confined liquids remains challenging, as many existing approaches rely on destructive or intrusive contact-based techniques, mostly measuring the liquid flow in the low frequency regime. Here, we present a non-invasive, all-optical technique based on ultrafast laser ultrasonics that probes confined liquids at equilibrium in the gigahertz frequency range. The method measures the phase and amplitude of time-domain Brillouin scattering signals transmitted through liquid layers whose thickness is varied step by step with subnanometer effective sampling. Supported by numerical modeling of acoustic propagation and optical detection, these signals allow us to extract the thickness-dependent acoustic velocity and attenuation of confined liquids. We show that nanometric confinement modifies the GHz acoustic response of glycerol, the liquid crystal 8CB, and a butyl-based ionic liquid over unexpectedly long spatial scales. These effects extend from a few nanometers to several tens of nanometers and reveal bound interfacial layers, acoustic stiffening, and enhanced solid-like behavior under confinement. Our results open a route to probing liquid confinement in a scarcely explored regime: dynamically measured at gigahertz frequencies, yet sufficiently weakly perturbative to preserve the equilibrium confined state.

Synergistic design of an SU-8-based CMUT array for low-frequency ultrasonic detection under liquid loading

Capacitive micromachined ultrasonic transducers (CMUTs) operating under liquid loading conditions suffer from significant added-mass effects and radiation damping, which degrade low-frequency vibration response and acoustic output performance. To address this issue, a synergistically optimized SU-8-based CMUT structure is proposed by combining a slotted membrane with an inverted trapezoidal stepped cavity. The slotted membrane reduces the effective bending stiffness to enhance mechanical compliance, while the stepped cavity improves electric-field distribution and displacement space. A coupled electromechanical-acoustic model is established in COMSOL Multiphysics to investigate the effects of membrane geometry, cavity configuration, and array topology. Simulation results show that, compared with a conventional flat-cavity CMUT, the proposed structure under water loading achieves a 29.8% reduction in resonant frequency, together with increases of 225.8% in membrane displacement and 40.3% in acoustic pressure. Furthermore, a 30 × 30 offset open hollow-square array is designed to improve directivity performance while reducing the number of active elements. The optimized array achieves a main-lobe width of 17.15° and a sidelobe level of −2.95 dB using only 81.8% of the array elements. These results demonstrate that the proposed membrane–cavity–array codesign provides a physically interpretable strategy for improving low-frequency CMUT performance under liquid loading.