End-to-end design of multi-functional acoustic holograms via heterogeneous physics constraints.

Internal solitary wave (ISW) is a kind of nonlinear internal wave commonly observed in the shallow water, which has the characteristics of large amplitude, short period and strong current velocity. With the distribution of the temperature and the salinity in the water column perturbed by ISWs, the sound speed profile becomes range-dependent, and thus affecting the characteristics of the underwater acoustic propagation. The ISWs usually propagate at a speed of the order of 1 m/s , and moving internal waves cause the energy in each acoustic mode to fluctuate dramatically. In this paper, the modal intensity is defined as the squared modulus of the modal coefficient, and is used to measure the sound energy in each mode. Based on the coupled mode theory, the expression of the acoustic modal intensity during the propagation of internal waves is derived in this paper, and the modal intensity is taken as the linear superposition of the oscillating term and the trend term. Most of previous researches were limited to the study of the time-varying characteristics of the acoustic modal intensity during the propagation of internal waves in the time domain or frequency domain. In this paper, the mechanism of modal intensity fluctuations is studied simultaneously in the time domain and the frequency domain with the aid of the short-time Fourier transform. Both the theoretical derivation and the numerical simulation show that the internal solitary wave gives rise to the energy transfer among acoustic modes, i.e., the mode coupling. The dynamic propagation of internal waves further leads to the modal interference, which behaves as an oscillating term in the modal intensity, and causes the modal intensity to fluctuate rapidly with time. The amplitude of the trend term changes with time due to the mode stripping (the difference in attenuation coefficients between different modes), which in turn adds a time-varying offset to the oscillations induced by the modal interference. The trend of the modal intensity and the time-varying characteristics of the amplitude of each frequency component in the oscillating term are closely associated with the modal attenuation. Meanwhile, the depth-integrated intensity is chosen as the measure of the total received acoustic intensity, and the influences of modal intensity fluctuations on the acoustic energy at the receivers during the propagation of internal waves are studied. It is demonstrated that the modal intensity with high energy which oscillates most dramatically will dominate the temporal variation of the received acoustic energy.

Acoustic holograms are 3-D-printed structures that can passively modulate the transmitted wavefront from a single-element source and generate arbitrary-shape beams in multi-layered media. This approach has been of great interest in the brain therapy field, where the skull-induced beam aberration leads to undesired off-targeting effects while the complex brain anatomy may limit optimal target coverage. Acoustic holograms have been shown to successfully address these limitations by providing a more simple and cost-efficient alternative as compared to phased arrays. This presentation will cover assessment of acoustic holograms in both animal and human applications, including hologram design technical details. First, an overview of potentially useful acoustic patterns generated transcranially for brain applications will be presented. Second, the basics of hologram design are described, with different approaches of wavefront processing and heights distribution generation depending on single-element transducer geometry. Third, an overview of the preclinical blood–brain barrier (BBB) opening and neuromodulation outcomes in mice will be presented, followed by BBB opening outcomes in non-human primates. Finally, in-silico feasibility of clinical BBB opening and neuromodulation will be reported. Hologram-assisted focused ultrasound (FUS) is a promising and powerful technology which provides new avenues into a novel and simple approach for cost-efficient and rapid FUS application in the brain.

The propagation of internal electromagnetic waves in an inviscid chiral fluid in the presence of the external constraint of transverse magnetic field is investigated. These waves are shown to be generated due to the stabilizing nature of the distribution of charge density. It is shown that the effect of the external constraint of magnetic field in a chiral fluid is analogous to the effect of viscosity in ordinary fluids. The wave equation, derived from the conservation of mass and momentum together with Maxwell’s equations and suitable auxiliary equations for chiral materials, reveals

Investigation of the generation and propagation of low frequency internal waves: A case study for the east coast of India

In this paper, more than 1000 SAR (ENVISAT, ERS-2) and optical (MODIS, HJ-1A/1B) images are used to analyze the distribution, sources and propagation of internal waves in South China Sea. The distribution of internal waves shows that, internal waves mainly occur in three regions of South China Sea: 1) Northern South China Sea, 2) Western South China Sea, 3) Southern South China Sea. Internal waves are observed all year in the South China Sea, most during summer, least in winter. The sources and propagation of internal waves are obtained from the distribution of internal waves. 1) in Northern South China Sea, most internal waves origin in Luzon Strait and propagate westward across Dongsha Island, then some internal waves turn into northwestward, others continue to propagate westward.2) in Western South China Sea, internal waves generate at the shelf, and propagate to the shore or off the shore.3) in Southern South China Sea, internal waves generate at the shelf. Some internal waves propagate to the Kalimantan shore, some propagate southwestward.

Numerical studies of internal solitary wave generation and evolution by gravity collapse

Non-hydrostatic model for internal wave generations and propagations using immersed boundary method

The physical implementation of immersive boundary conditions (IBCs) allows acoustic or elastic waves to propagate seamlessly between a physical domain, such as a wave propagation laboratory, and a numerical simulation virtually enclosing the physical domain. IBCs correctly account for all wavefield interactions between both domains, including higher-order long-range scattering. In this contribution, IBCs are physically implemented in a two-dimensional (2-D) acoustic waveguide. The boundary surrounding the waveguide is densely populated with hundreds of loudspeakers that apply the necessary boundary conditions. The required signals to be injected at the boundary are predicted in real-time by (1) measuring the pressure field and its gradient on two acoustically transparent auxiliary surfaces of microphones inside the waveguide and (2) extrapolating the wavefields to the boundary by evaluating a Kirchhoff-Helmholtz integral using a low-latency, FPGA-enabled data acquisition, computation and control system. Here, we demonstrate the first real-time, 2-D physical immersive wave propagation experiments. We present the setup, as well as a suite of experiments that demonstrate the ability of IBCs to actively suppress broadband incident fields at the boundary of the waveguide and to correctly reproduce all orders of wavefield scattering between the physical experiment and the numerical simulation.The physical implementation of immersive boundary conditions (IBCs) allows acoustic or elastic waves to propagate seamlessly between a physical domain, such as a wave propagation laboratory, and a numerical simulation virtually enclosing the physical domain. IBCs correctly account for all wavefield interactions between both domains, including higher-order long-range scattering. In this contribution, IBCs are physically implemented in a two-dimensional (2-D) acoustic waveguide. The boundary surrounding the waveguide is densely populated with hundreds of loudspeakers that apply the necessary boundary conditions. The required signals to be injected at the boundary are predicted in real-time by (1) measuring the pressure field and its gradient on two acoustically transparent auxiliary surfaces of microphones inside the waveguide and (2) extrapolating the wavefields to the boundary by evaluating a Kirchhoff-Helmholtz integral using a low-latency, FPGA-enabled data acquisition, computation and control system. H...

We numerically investigate the propagation of surface and internal waves which are caused by a gravity current in a container. A level set approach is used to capture interfaces without explicit tracking. To model the complex boundary of the container, a virtual boundary method is applied to the level set formulation. In this study, it is shown that multifluid flows in a deformed container can be simulated through the combination of the above two methods.

In this paper, we are interested in studying the propagation of the over diagnostic ultrasound waves through complex biological vascular networks such as the tumor tissue. Evidence shows that the over diagnostic wave propagates through complex media with power law of non-integer order \(t^{-\nu }, \, 1< \nu <2\). Evidence shows also that the vascular morphology of the tumor is non-smooth and is a complex media that means it is a fractal media. The wave propagates through this fractal media which exhibits with extremely long jumps whose length is distributed according to the Levy long tail \(\sim |x|^{-1-\alpha }\), \(0<\alpha <2\). Therefore, the space–time-fractional forced wave equation with attenuation, or the so-called multi-term wave equation, mathematically models this medicine problem. This equation mathematically models many other physical, biological, chemical, and environmental problems. We get the approximate solution of this model to study the time evolution of the propagated wave by adopting the backward Grunwald–Letnikov scheme joining with the common finite difference method. We investigate numerically the effect of the time fractional on the propagation of the wave as well as the effect of the space-fractional order for the three cases as: \(0<\alpha <1\), \(1<\alpha < 2\), and \(\alpha =1\). The stability condition of each approximate solution is also discussed separately. Finally, we prove that the space-fractional order \( \alpha \) has no effect on the stability condition.

We present a new design technique for frequency-diverse holograms used in millimeter- and submillimeter-wave imaging. A neural-network backed imaging method utilizes quasirandom hologram structure, which disperses the field in the region of interest across the WR-3.4 (220-330 GHz) band. The global optimum for the hologram structure is generally not known. A new, semiempirical optimization method utilizes the experimental data used in localization tasks. The data is a collection of the of the local field at the object plane measured with a probe ($S _{\mathbf{21}}$), and the corresponding localization performance derived from the reflection spectra of a corner cube ($S _{\mathbf{11}}$). The frequency diversity of the local field at the object is correlated with the localization error. The proposed design technique uses the obtained empirical correlation data to inform a new hologram design.

We present the theory, design, fabrication and evaluation of off-plane computer-generated waveguide holograms (OP-CGWHs). The hologram structure is composed of an array of rectangular elements each containing a waveguide grating coupler. The function of the rectangular elements is twofold: outcoupling the guided optical wave, and introducing phase shift. The phase shift of each rectangular element can be determined by controlling the dislocation of the grating grooves along the guided wave propagation direction. In addition, the phase pattern of the OP-CGWH can be designed using many existing algorithms for computer-generated holograms (CGH), such as the iterative Fourier transform algorithm (IFTA). A design example is presented for a waveguide array generator, which outcouples a Gaussian-like incident guided wave and simultaneously produces an array of spots in free space. Such an OP-CGWH device, based on an AlGaAs-GaAs waveguide, was fabricated using electron-beam lithography and reactive ion beam etching. Experimental results are presented that demonstrate the effectiveness of the proposed idea. The uniformity error and the power efficiency of the fabricated OP-CGWH array generator were measured to be approximately 8% and 30%, respectively.

Accurate reconstruction of pressure fields using phase-only acoustic holograms is critical for applications requiring high spatial precision, such as targeted ultrasound therapies. In this study, we investigate the effect of excitation frequency on reconstruction accuracy by performing a controlled sweep from 0.75 to 4.0 MHz, while keeping all other parameters such as aperture size, simulation grid, target patterns, and optimization settings constant. To evaluate performance, we employ five quantitative metrics: Mean Squared Error (MSE), Peak Signal-to-Noise Ratio (PSNR), Cross-Correlation, Uniformity, and Efficiency. The results show that reconstruction fidelity improves as frequency increases, particularly in the low-to-mid range, where finer spatial features become resolvable due to the shorter wavelengths. However, beyond a certain point, the gains begin to taper, and in some cases, high frequencies introduce subtle artifacts such as edge ringing or increased variance. Moreover, higher frequencies are associated with increased acoustic attenuation and imposing stricter fabrication demands on holographic elements. These findings suggest that frequency selection in acoustic holography must be application-specific, as both low and high frequencies offer distinct advantages depending on the target characteristics and system constraints.

Acoustic vortex beams have great potential for contactless particle manipulation and torque-based biomedical applications. However, focusing acoustic waves through highly aberrating layers such as the human skull at ultrasonic frequencies results in strong phase aberrations, which prevent the generation of sharp acoustic images. In the case of a wavefront containing phase dislocations, skull aberrations can inhibit the focusing of acoustic vortex beams inside the cranial cavity. In this paper, we demonstrate that phase-conjugated acoustic holograms can encode time-reversed fields, allowing compensation of the aberrations of the skull and, simultaneously, the generation of a focused vortex inside an ex vivo human skull. The method is applied to single-element geometrically focused sources and results in a very simple and compact ultrasonic system. This work will pave the way to designing low-cost particle-trapping systems, clot manipulation, and the exertion of acoustic-radiation forces and torques in the brain for biomedical applications.

This chapter considers the simplest example of free harmonic internal waves of small amplitude propagating in an undisturbed horizontally homogeneous ocean. A detailed analysis of the basic boundary value problem for amplitude functions of these internal waves for a finite depth is given in Section 3.1. Examples of solutions of the basic boundary value problem for typical vertically undisturbed density distributions in the ocean are presented in Section 3.2. If the vertical scale of internal waves is small, the ocean may be considered as infinitely deep. In this case internal waves propagate not only horizontally, but also in the vertical direction. This case is examined in Section 3.3. A theory of the propagation of linear internal wave packets is worked out in Section 3.4 based on geometric optics’ approximations. The influence of a fine structure of the ocean’s density field on the propagation of free infinitesimal internal waves is studied in Section 3.5. The influence of the Earth’s rotation (taking into account all components of the Coriolis force) on the propagation of free internal waves is considered in Section 3.6.