Electroactive tunable metamaterial for dynamic subwavelength focusing

Abstract

Electroactive tunable metamaterial structures, devices, and methods are disclosed. In some aspects, an acoustic metamaterial structure for acoustic imaging includes a plurality of piezoactive membrane cells, each including a first passivation layer; an electrode layer in contact with the first passivation layer; a piezoelectric layer in contact with the electrode layer; a bulk membrane layer in contact with the piezoelectric layer; and a second passivation layer in contact with the bulk membrane layer, wherein the acoustic metamaterial structure is configured to steer and / or focus propagation of an acoustic signal based on electrical control signals to the piezoactive membrane cells to affect an acoustic propagation property.

Description

PCT Patent ApplicationDocket: 009062.8552.WOOOELECTROACTIVE TUNABLE METAMATERIAL FOR DYNAMIC SUBWAVELENGTH FOCUSINGSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under Grant Nos. 2031557 and 1022166 awarded by Defense Advanced Research Projects Agency. The government has certain rights in the technology.CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent document claims priority to and benefits of U.S. Provisional Patent Application No. 63 / 693,138, titled “ELEC TRO ACTIVE TUNABLE METAMATERIAL FOR DYNAMIC SUB WAVELENGTH FOCUSING” and filed on September 10, 2024. The entire contents of the aforementioned patent application are incorporated by reference as part of the disclosure of this patent document.TECHNICAL FIELD

[0003] The patent document relates to material structures, devices, systems, and processes for high-resolution ultrasound imaging and methods of manufacturing the material structures.BACKGROUND

[0004] Metamaterials are artificially engineered materials designed to have properties not found in naturally occurring materials. Typically, metamaterials’ unique characteristics arise from their structure rather than their composition.SUMMARY

[0005] Disclosed are engineered material structures, devices, systems, and methods for electroactive tunable metamaterials. Implementations of the disclosed material technology can include acoustic signal propagation and / or transduction, particularly for ultrasound imaging applications.

[0006] In some aspects, an acoustic metamaterial device for manipulating acoustic signal transmission includes a piezoelectric active membrane comprising a plurality of piezoactive cells, wherein each piezoactive cell includes a side wall having a material layer structure including a-1-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO first passivation layer on an interior side of the side wall, a first electrode layer in contact with the first passivation layer, a piezoelectric layer in contact with the first electrode layer and configured to convert acoustic energy to an electrical signal, a bulk material in contact with the piezoelectric layer configured to propagate an acoustic signal carrying the acoustic energy, and a second passivation layer in contact with the bulk material; a top wall coupled to the side wall and having an aperture spanning into an interior chamber of the piezoactive cell operable to be filled with a fluid medium that allows acoustic signals to propagate within between regions of the interior side of the side wall; and a bottom wall coupled to the side wall on an opposing side of the top wall.

[0007] In some aspects, a method for controlling acoustic signal propagation includes receiving, at acoustic metamaterial structure, an acoustic signal generated by one or more transducers of an acoustic transducer device, wherein the acoustic metamaterial structure is interfaced between the acoustic transducer device and an external medium comprising an internal target; and steering and / or focusing propagation of the acoustic signal within the acoustic metamaterial structure by applying a set of electrical control signals to piezoactive cells of the acoustic metamaterial structure to affect an acoustic propagation property including a negative refraction.

[0008] The subject matter described in this patent document can be implemented in specific ways that provide one or more of the following features.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1A shows a diagram of an example embodiment of an electroactive tunable metamaterial structure in accordance with the present technology.

[0010] FIGS. IB shows a diagram depicting an example arrangement of a material layer structure of an example embodiment of a piezoactive membrane cell, in accordance with the present technology.

[0011] FIGS. 1C shows a diagram depicting another example arrangement of a material layer structure of an example embodiment of a piezoactive membrane cell, in accordance with the present technology.

[0012] FIGS. 2A-2D show diagrams depicting an example embodiment of a method for fabricating an electroactive tunable metamaterial structure in accordance with the present technology.-2-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO

[0013] FIG. 3 shows a diagram depicting an example electroactive tunable metamaterial operating as an acoustic lens with negative refraction to enhance focusing and beam steering.

[0014] FIG. 4A shows a diagram depicting an example electroactive tunable metamaterial having a natural, non-tuned passive index.

[0015] FIG. 4B shows a diagram depicting an example electroactive tunable metamaterial being controlled by uniform membrane modulation.

[0016] FIG. 4C shows a diagram depicting an example electroactive tunable metamaterial being controlled by asymmetric membrane modulation.

[0017] FIG. 5 shows a diagram depicting an acoustic transducer device interfaced with an example embodiment of the electroactive tunable metamaterial structure, in accordance with the present technology.DETAILED DESCRIPTION

[0018] Ultrasound is one of the most versatile non-invasive tools that can be used in imaging, such as anatomical structures and studying fluid flow, and in clinical treatment or interventions, such as high intensity ultrasound imaging (HIFU) to destroy stones in the kidney. One critical challenge for ultrasound-based biomedical applications is beam forming to control focusing and improve resolution. Presently, phased arrays offer the best way to control ultrasound focusing as well as to correct distortions relying on an array of acoustic transducers to generate tailored wave fronts that can focus ultrasound dynamically and therefore perform scanning patterns and read reflected signals for imaging. However, one of the resolution limitations of phased arrays is the diffraction limit associated with the wavelength.

[0019] The diffraction limit in ultrasound applications is a limitation on the ability to distinguish two closely spaced objects, which is limited by the wavelength of the ultrasound waves utilized. The diffraction limit arises because acoustic waves, like light waves, spread out (diffract) when passing through an aperture, causing the image of a point source to appear as a blurred spot, e.g., referred to as an Airy disk. This effect prevents the clear resolution of objects smaller than approximately half the wavelength of the ultrasound, a limit that can be overcome by techniques like super-resolution ultrasound imaging.

[0020] The disclosed technology addresses these limitations through development of negativeindex acoustic metamaterials to achieve dynamic sub -wavelength focusing beyond the diffraction-3-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO limit. The disclosed metamaterial technology provides structures capable of negative refraction and the sub -wavelength focusing of acoustic signals. In some embodiments, the disclosed metamaterial technology uses piezoelectric active controlled micro acoustic components. Implementations of the disclosed material technology can include acoustic signal propagation and / or transduction, particularly for ultrasound imaging applications.

[0021] Example embodiments of the disclosed electroactive tunable metamaterials are configured such that they break the wavelength-diffraction limit of ultrasound focusing at relevant biomedical frequencies of substantially 2 MHz. Sub-wavelength ultrasound imaging and focusing has been demonstrated experimentally, but only at sub-100 kHz frequencies. Example implementations of the disclosed electroactive tunable metamaterials show focusing and beam steering capabilities, e.g., including producing a focal spot with lateral sub -wavelength size beyond the near-field at 2 MHz with electroactive control. These capabilities are a significant improvement for resolving acoustic signals (e.g., ultrasound) for shallow structures, such as blood vessels or joints, for example. In addition, example embodiments of the disclosed electroactive tunable metamaterials are configured as piezoelectric membranes to control the location of the focal point electronically, providing the capability of sub-wavelength dynamic focusing, which is critical for any real imaging technology and is envisioned to bring enormous benefits to biomedical ultrasound imaging.

[0022] Implementations of the disclosed technology can use negative-index non-resonant index acoustic metamaterials with piezoelectric components to dynamically focus ultrasound beyond the wavelength limit to improve the ultrasound resolution. Currently, there are no existing metamaterials capable for use at relevant biomedical ultrasound frequencies, even less with dynamic capabilities, due to fabrication limitations and to resonance operation losses. Thus, the disclosed technology is envisioned to provide a new generation of non-resonant acoustic metamaterials (NRAM) that can be configured as an acoustic membrane with engineered cavities (in the tens or hundreds of microns, e.g., such as substantially 100 pm cavity diameter or size), e.g., made of silicon-based materials. In some embodiments, the cavities can have a size in a range of 10 pm to 500 pm. For example, silicon-based semiconductor materials for the passivation and / or bulk material layers offer high stability in aqueous and organic solutions and high- mechanical properties enabling high frequency operation and low-intrinsic losses. In various implementations, for example, the NRAM structure can have a range of frequency operations from-4-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO0.5 to 10 MHz based on the design. For example, stiffer and smaller membrane cells can allow to fo in frequency. By incorporating piezoelectric materials, the disclosed electroactive tunable metamaterials can be implemented such that the mechanical properties of the metamaterial can be electronically controlled, e.g., both locally and globally, allowing the electronic control of the focal point for dynamic focusing which is critical for scanning and imaging.

[0023] Example Embodiments

[0024] In some embodiments, for example, an acoustic metamaterial device for acoustic signal transmission includes a piezoelectric active membrane comprising a matrix of piezoactive cells, where each piezoactive cell includes a side wall having a material layer structure including a first passivation layer on an interior side of the side wall, a first electrode layer in contact with the first passivation layer, a piezoelectric layer in contact with the first electrode layer and configured to convert acoustic energy to an electrical signal, a bulk material in contact with the piezoelectric layer configured to propagate an acoustic signal carrying the acoustic energy, and a second passivation layer in contact with the bulk material; a top wall coupled to the side wall and having an aperture spanning into an interior chamber of the piezoactive cell operable to be filled with a fluid medium (e.g., water) that allows acoustic signals to propagate within between regions of the interior side of the side wall; and a bottom wall coupled to the side wall on an opposing side of the top wall. The matrix of the piezoelectric active membrane includes an array of adjacent piezoactive cells. In various embodiments of the piezoelectric active membrane, the piezoactive cells can be configured such that at least a portion of the side wall of a given cell is shared with an adjacent piezoactive cell in the membrane.

[0025] In some embodiments, the disclosed electroactive tunable metamaterials can be configured as two dimensional (2D) piezoactive membrane structures and as three dimensional (3D) piezoactive membrane structures. For example, the 3D piezoactive membranes can be produces by arranging a plurality of 2D piezoactive membranes in a perpendicular arrangement (e.g., stacked arrangement).

[0026] FIG. 1A shows a diagram of an example embodiment of an electroactive tunable metamaterial structure 100 in accordance with the present technology. The exemplary electroactive tunable metamaterial structure 100 shown in FIG. 1A depicts an example of a 2D piezoactive membrane that includes a plurality of piezoactive membrane cells 110 arranged in a planar matrix. The piezoactive membrane cells 110 of the electroactive tunable metamaterial-5-009062.8552. WOOOM83672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO structure 100 are each structured to include a first passivation layer 111 , an electrode layer 113 in contact with the first passivation layer 111, a piezoelectric layer 115 in contact with the electrode layer 113, a bulk material layer 117 in contact with the piezoelectric layer 115, and a second passivation layer 119 in contact with the bulk material layer 117, as shown in the cross-sectional view of the diagram of FIG. 1A. The diagram of FIG. 1A includes a select region (box) 199 depicting a portion of the material layer structure of a piezoactive membrane cell 110, which is also shown in FIG. IB.

[0027] The piezoactive membrane cells 110 are configured adjacent to each other in the matrix of the electroactive tunable metamaterial structure 100 (e.g., 2D piezoactive membrane). Each of the piezoactive membrane cells 110 includes the material layer structure (e.g., the arrangement shown in box 199 and FIG. IB) providing a surrounding wall around a chamber region 125, where acoustic signals (e.g., sound waves, such as ultrasonic waveforms) are able to propagate within the piezoactive membrane cells 110. Each of the piezoactive membrane cells 110 includes a top wall 127 and a bottom wall 129. In some embodiments, for example, the top wall 127 is formed of the material comprising the first passivation layer 111 and / or second passivation layer 119. In some embodiments, for example, the bottom wall 129 is formed of one or more materials, which can include a layer or plurality of layers comprising the bulk material layer 117 and / or the first and / or second passivation layer(s) 111, 119. As shown in the example of FIG. 1A, the bottom wall 129 is configured of an interior layer comprising the bulk material layer 117 and an outer layer comprising the first and / or second passivation layer(s) 111, 119. Each of the piezoactive membrane cells 110 includes an aperture or opening 121 that spans into the chamber region 125. The aperture 121 is configured to allow a fluid medium, e.g., water, to enter into and fill the chamber region 125 for implementations of the electroactive tunable metamaterial structure 100 as a piezoactive membrane that can be interfaced with an acoustic transducer device for a variety of applications. The chamber regions 125 of the piezoactive membrane cells 110 can be pre-filled with the fluid medium (e.g., water or aqueous solution), so that for implementations of the electroactive tunable metamaterial structure 100, the fluid medium within the chamber regions 125 of the piezoactive membrane cells 110 are trapped within. In some embodiments, for example, the chamber region 125 can have a size (e.g., width or diameter) in a range of 10 pm to 500 pm, and in some embodiments, 10 pm to 100 pm. In some embodiments, for example, the aperture 121 can have an opening size in a range of 5% to 60% of the size range of the chamber region 125-6-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO(e.g., 500 nm to 50 pm). For example, capillary issues can be corrected using surfactants in the liquid, as the fluid medium (e.g., liquid) inside the chamber region 125 is not in contact with a body of a subject during an acoustic imaging or intervention application.

[0028] In implementations of the electroactive tunable metamaterial structure 100, for example, the fluid medium that is captured within the chamber region 125 facilitates acoustic signal propagation between piezoactive membrane cells 110. The apertures 121 of each of the piezoactive membrane cells 110 provides a negative modulus for the electroactive tunable metamaterial structure 100, enabling negative-angle refraction properties for an acoustic transducer device implementing the electroactive tunable metamaterial structure 100. Moreover, the material layer structure provides the electroactive control for actuating acoustic beam steering and focus of the acoustic signals while propagating through the electroactive tunable metamaterial structure 100. The metamaterial can be controlled by an electronic controller that controls the individually-addressable piezoactive membrane cells 110 to manipulate the amplitude and / or phase to steer and focus the acoustic signal (e.g., ultrasound beam). For example, in some embodiments, the electronic controller can be a phase array controller. In some embodiments, for example, the electroactive tunable metamaterial structure 100 can include an outer sheathing or cover. The example outer sheathing or cover can be configured such that the manipulated acoustic signal propagating from the electroactive tunable metamaterial structure 100 is unimpeded and unaffected by the outer sheathing or cover (e.g., using material with acoustic impedance matching layers interfaced with the transducer array and / or with the inspection zone of the target medium, e.g., where the acoustic impedance matching is with the internal fluid medium in the electroactive tunable metamaterial structure 100).

[0029] In some embodiments of electroactive tunable metamaterial structure 100, for example, the first passivation layer 111 and the second passivation layer 119 can be configured of the same material; whereas in other embodiments, for example, the first passivation layer 111 and the second passivation layer 119 can be configured of different materials. In some embodiments, the first passivation layer 111 and / or the second passivation layer 119 can be configured of a ceramic material that is electrically insulative and substantially chemically inert. For example, the first passivation layer 111 and / or the second passivation layer 119 can be or can include a silicon- based material (e.g., such as silicon carbide (SiC), silicon carbon nitride (SiCN), or a silicon nitride (SixNy, such as SiaN^). In some examples, the first passivation layer 111 and / or the second-7-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO passivation layer 1 19 can be or can include a polymer material (e.g., such as parylene or polyimide (PI)). In some embodiments, for example, the first passivation layer 111 can have a thickness in a range of 10 nm to 500 nm. The first passivation layer 111 can prevent oxidation and / or degradation (e.g., corrosion and / or electrochemical effects) produced by the fluid water on the piezoelectric layer, e.g., given the applied voltage on the piezoactive membrane cell(s) 110. In some embodiments, for example, the second passivation layer 119 can have a thickness in a range of 10 nm to 500 nm range. For example, having the first passivation layer 111 and the second passivation layer 119 on both sides of the piezoactive membrane cell 110 helps to prevent or substantially reduce buckling or unwanted bending of the cells and overall membrane towards one side or the other.

[0030] In some embodiments of electroactive tunable metamaterial structure 100, for example, the electrode layer 113 is configured of an electrically conductive material. In some examples, the electrode layer 113 includes, but is not limited to, a metal, a conductive polymer, carbon-based nanomaterials, metal oxide ceramics, and composites. For example, the electrode layer 113 can include gold, platinum, silver, aluminum, copper, stainless steel, titanium, tungsten, or a combination thereof of these or other metals, and / or or alloys thereof. For example, the electrode layer 113 can include polyaniline (PANI), polypyrrole (PPY), and / or poly(3,4- ethylenedi oxy thiophene) (PEDOT). For example, the electrode layer 113 can include carbon nanotubes (CNTs), graphene, and / or materials of activated carbon. In some embodiments, for example, the electrode layer 113 can have a thickness in a range of 100 nm to 500 nm.

[0031] In some embodiments of electroactive tunable metamaterial structure 100, for example, the piezoelectric layer 115 is configured of lead zirconate titanate (PZT) or aluminum scandium nitride (AlScN). For instance, PZT has a high piezoelectric coefficient, so it can give a high mechanical push given a certain electrical input. AlScN is a piezoelectric material capable of achieving high frequencies (e.g., greater than 1 MHz) and is compatible with CMOS processing; while the piezoelectric coefficient of AlScN is not as high as PZT, it can reach higher frequencies compared to PZT. The piezoelectric layer 115 is configured to transduce an acoustic signal that is incident (applied) on it into an electrical signal. In some embodiments, for example, the piezoelectric layer 115 can have a thickness in a range of 500 nm to 5 pm.

[0032] In some embodiments of electroactive tunable metamaterial structure 100, for example, the bulk material layer 117 is configured of an inorganic silicon-based material, for example, doped-8-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO silicon or silicon oxide, e g., such as crystalline silicon dioxide (e.g., a-quartz) or amorphous silicon dioxide (e.g., glass). In implementations of electroactive tunable metamaterial structure 100, for example, the bulk material layer 117 is configured to propagate an acoustic signal to the piezoelectric layer 115. In some embodiments, for example, the bulk material layer 117 is configured as a second electrode, e.g., in embodiments where the bulk material layer 117 is configured of an electrically insulative material, such as doped silicon. In some (optional) embodiments where the bulk material layer 117 is configured as an electrically non-conductive material (e.g., glass) or poorly conductive material, the electroactive tunable metamaterial structure 100 can include an additional (second) electrode layer coupled between the bulk material layer 117 and the second passivation layer 119, where the (optional) second electrode layer can be made of the same or different electrically conducive material as the electrode layer 113. In some embodiments, for example, the bulk material layer 117 can have a thickness in a range of 5 pm to 100 pm.

[0033] In some embodiments, the electroactive tunable metamaterial structure 100 includes electrical contacts on at least one side of the piezoactive membrane cell 110, at which the transduced electrical signal is addressable to an interfacing device. For example, each of the piezoactive membrane cells 110 of an exemplary 2D piezoactive membrane can include electrical conduits coupled to an end region of the electrode layer 113 to route the respective electrical contact to a portion or region of the piezoactive membrane that is able to be coupled to corresponding electrical contact of an interfacing device, e g., thereby facilitating the interfacing of interior piezoactive membrane cells 110 in the matrix of the electroactive tunable metamaterial structure 100. For example, the interfacing device can include a voltage source, where a first electrode in electrical connection to the electrode layer 113 and a second electrode in electrical connection with the bulk material layer 117 (or the (optional) second electrode layer) are configured to provide an applied voltage to an individual piezoactive membrane cell 110. The voltage may be an oscillating voltage, for example. The diagram of FIG. IB depicts the example material layer structure of a piezoactive membrane cell 110 interfaced with a voltage source, where the bulk material layer 117 uses a conductive material operable to serve as the second electrode. The diagram of FIG. 1C depicts the example material layer structure of a piezoactive membrane cell 110 interfaced with a voltage source, where the piezoactive membrane cell 110 includes a second electrode layer 133 operable to serve as the second electrode.-9-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO

[0034] Example piezoactive membrane structures were developed by design modeling the structures to possess properties. As an example, COMSOL simulations of crystalline silicon plates were initiated, e.g., finding that at a thickness of about 3 pm and width of about 75 pm (e.g., 75x75 pm2area), their first-mode resonance frequency in water is 2.9 MHz. From this value, a negativeindex in the ~1 to 2 MHz range can be achieved. The design included introducing side apertures into the example crystalline silicon plates, and a frequency-domain analysis was conducted to obtain the acoustic branches and tune the design to obtain a negative branch (e.g.,< 0,^ >), with an aim for speed of sound in water (e.g., c —1500 m / s and n —1) at ranges of 0.5 MHz to 2 MHz. The design modeling included exemplary thermal conductivity and viscosity medium testing to account for heat and viscous losses. The design modeling also included modeling acoustic (liquid) - structure (silicon) physics to couple acoustic waves in the medium with the vibration motion of the exemplary Si-based plates. The design modeling also included studying the behavior of ultrasound using point sources and plane waves.

[0035] Example piezoactive membrane structures can be fabricated in both 2D and 3D. For example, silicon-on-insulator wafers can be used to provide a substrate, e.g., with a top silicon layer of a particular thickness or thickness range (e.g., 75 pm thick for some embodiments). Photolithography can be used to pattern an array of cross bars with a width of 5 pm, followed by reactive ion etching (RTE), e g., deep-reactive ion etching for some implementations, to etch into the silicon to a depth of about the top silicon layer (e.g., 75 pm for some embodiments), thereby forming silicon walls (plates) with a particular thickness based on the pattern (e.g., 5 pm thickness for some embodiments, defined by the width of the cross bar). For example, the photoresist used in photolithography can have a thickness in a range of 1 pm to 10 pm. Dry-stamping can be used (e.g., using a polymer stamp) to align and transfer a silicon layer on top of the silicon walls. Wafer bonding can be used to bind the two layers. The top layer includes apertures to serve as side apertures to produce negative modulus and also fill the cavities with water. The example fabricated metamaterial structure forms a 2D piezoactive membrane structure operable as a 2D waveguide with an array of parallel and transversal silicon plates adding side channels on the side. The example metamaterial structure can be used as a device to be submerged into degassed water to control acoustic signal focus and steering for an acoustic transducer device to perform acoustic measurements.-10-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO

[0036] The fabrication technique can further produce a 3D piezoactive membrane structures by stacking multiple 2D piezoactive membrane structures together. To produce the exemplary 3D metamaterial structure, for example, a parylene layer can be spin coated onto a 2D piezoactive membrane structure, e.g., to give mechanical strength. The 2D piezoactive membrane structure can be adhered (e g., via adhesive, such as glue, epoxy, etc.) on a substrate (e.g., PDMS / glass stamp) as a carrying chip. One or more other 2D piezoactive membrane structures can be stacked upon a previous 2D piezoactive membrane structure, e.g., with the aid of microscopy and / or a micromanipulator. This can be repeated until a desired amount of 2D piezoactive membrane structures are used to build the 3D piezoactive membrane structure.

[0037] FIGS. 2A-2D show diagrams depicting an example embodiment of a method for fabricating an electroactive tunable metamaterial structure in accordance with the present technology. The method can be implemented to produce various example embodiments of the electroactive tunable metamaterial structure 100, e.g., including 2D piezoactive membrane structures and 3D piezoactive membrane structures.

[0038] FIG. 2A shows a diagram of a process 210 of the fabrication method to pattern an array of structures, e.g., of photoresist using photolithography, on a substrate, e.g., silicon substrate, such as a silicon wafer. In some implementations of the process 210, the pattern can include an array of rectangular or square cross-bar / wall structures, like that illustrated in the diagram of FIG. 2A; whereas in some implementations, for example, the pattern can include other patterns, such as a circular pattern, triangular pattern, ovular pattern, hexagonal pattern, or any other shape pattern, or a combination of any different patterns. The relative dimensions between portions of the side wall (or side walls) of the piezoactive membrane cells is determined as part of the formation of the cross-bar / wall structures, and the relative dimensions is a parameter used in operating the electroactive tunable metamaterial structure for the manipulations (steering and / or focusing) of acoustic signals.

[0039] FIG. 2B shows a diagram of a process 220 of the fabrication method to perform deep anisotropic etching, e.g., deep-reactive ion etching, into the substrate (e.g., silicon), such as etch to a depth of about a top layer of the substrate (e.g., silicon top layer), thereby forming walls based on the pattern of the array of structures. For example, the formed walls based on the pattern can provide the bulk material layer 117 of the piezoactive membrane cells 110.-11-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO

[0040] FIG. 2C shows a diagram of a process 230 of the fabrication method to deposit a piezoelectric material layer and an electrically-conductive material layer (e.g., via masked deposition) alongside the formed walls, e.g., providing the piezoelectric layer 115 and the electrode layer 113, respectively; and depositing a passivation layer along a side of the electrically- conductive material layer via deposition to form the remaining portion of the side walls and a top wall over the plurality of cells, where the formation of the top wall leaves an opening over the previously-etched regions for each of the cells. In this matter, the process 230 creates a top wall with open apertures to produce a 2D piezoactive membrane structure.

[0041] For example, in some implementations of the process 230, the piezoelectric material layer can be formed by sputtering of AlScN, evaporation, and subsequent patterning of electrical contacts, and finally deposition of SiN as passivation layer, e.g., by plasma enhanced chemical vapor deposition. For example, the fabrication process to add the piezoelectric films can start with a sputtering deposition of AlScN with a sacrificial layer deposited directionally at a tilted angle to allow the AlScN deposition only on one side of the membranes. AlScN has a high resistance, so it does not produce a short circuit between different components. Then, metal contacts like molybdenum (Mo) can be deposited by directional metal evaporation also at angle for shadow evaporation, allowing the deposition only on one side of the membranes. The passivating SiN will be deposited by isotropic plasma chemical vapor deposition. The silicon bulk can be heavily doped and serve as electrical ground. The contacts for each cell can be contacted with the top layer (as illustrated in FIG. 1A) that can be modified with electrical contacts to the membranes and connection pins for electrical control.

[0042] FIG. 2D shows a process 240 to create 3D piezoactive membrane structures by stacking multiple 2D piezoactive membrane structures, e.g., via dry transfer using polymer-based stamps for adhesion of stacked 2D piezoactive membrane structures. In some implementations of the process 240, a micromanipulator can be used to perform the stacking. In some embodiments of the fabrication method (not shown), for example, after the process 230, a process to form an electrical contact associated with each cell (coupled to the electrical layer) can include pattern etching a portion of the top wall to reach the underlying electrically-conductive material layer and depositing an electrically conductive material through the etched portion, which can include forming the electrical contact over the top wall with a diameter or length larger than the diameter or length of the etched portion. Alternatively, for example, the process to form the electrical-12-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO contact can include producing the electrical contact structure prior to forming the top wall as part of the process 230.

[0043] Example Implementations

[0044] Focusing and refraction'. To demonstrate the capability of the example 2D piezoactive membrane structure (e.g., NRAM) to produce a negative angle of refraction and control focusing at example acoustic signal frequencies of 0.5 MHz, 1 MHz, and 2 MHz, a point source acoustic signal generator (an example piezoelectric 1-3 transducer, e.g., composites with silver epoxy paste as contacts) can be used. Example piezoelectric films were diced to sizes of ~ 1 mm to achieve sizes similar to the wavelength ( ). The point source acoustic signal generator device can be supported on a glass substrate. Notably, for example, for experimental implementations using 3D piezoactive membrane structure (NRAM,) a concave transducer or a phased array can be used to produce a focused field before the NRAM, which can be reproduced as a focal point behind the NRAM. Example experimental implementations tested the focusing and refraction using the example NRAMs. The acoustic field can be mapped with a needle hydrophone mounted on an XYZ stage with a step resolution of 20 microns. The example implementation of the example 2D piezoactive membrane structure showed that the beam reverses direction along the water- metamaterial interface, as shown in FIG. 3, leading to a convergent beam and a focal point, thereby operating as an acoustic lens with negative refraction behavior. In some embodiments, a FLIR A35 thermal camera can be used to make a spatial map of potential heating losses.

[0045] For example, in the implementations for 0.5 MHz, 1 MHz, and 2 MHz, the metamaterial does not operate at resonance and has broadband operation. Also, for example, the metamaterial does not suffer significant energy losses, as such losses are below -10 dB.

[0046] Negative-index materials is the amplification of sub -wavelength components can lead to improved resolution by achieving sub -wavelength focusing. The disclosed technology develops negative-index metamaterials with sub-X. focusing of the metamaterials. Optimizing the NRAM thickness the sub-Z. components can be sufficiently amplified to reach penetration depths beyond X to achieve sub-k focusing beyond the near-field. For example, the exemplary 2D and 3D piezoactive membrane structure can be used for focusing ultrasound to ~X / 3 (e.g., 250 pm) to ~X / 8 (e.g., 100 pm) at 2 MHz.

[0047] In some example experimental implementations, the same metamaterials used for subwavelength focusing was used, but using an acoustic source with dimension S-100-250 pm (<k).-13-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOOIn this case, 1-3 composite piezoelectric structures can be produced down to 100-250 pm using silver epoxy paste as contacts mounted on a glass slide with electrical contacts. The acoustic signals used in the example sub-X focusing implementations can have frequencies at 0.5, 1.0 and 2.0 MHz.

[0048] The measurements used an XYZ stage water tank with a ONDA hydrophone with a 20 micron step resolution to move the source from the metamaterial to vary the focal distance ‘d’. For d < X, the sub-X focusing built a mirror image of the sub-X source S~100-250 pm. For example, for some implementations, the negative-index and thickness of the metamaterial can be modified to enhance amplification of the sub-X components for sub-X focusing beyond near-field aiming at depths of -5X (4 mm). For example, applications of this capability can include ultrasound imaging of shallow structures such as blood vessels and bone joints in which the sub-X capability enhances ultrasound resolution, e.g., including achieving a focal point with lateral size of -100 pm to 250 pm at 2 MHz with losses below -10 dB at depths of d~l mm to 4 mm.

[0049] Some example implementations of the disclosed electroactive tunable metamaterial technology include dynamic tuning of acoustic index and active sub -wavelength focusing using exemplary 2D piezoactive membrane structures.

[0050] One critical requirement to use metamaterials for medical imaging is the ability to control actively its acoustic index and focal point. This allows for scanning the focal point through a certain area of a sample for imaging, as well as controlling the acoustic index to adapt to tissue variations and optimize focusing. The previous example implementations for negative refraction and focusing and for sub-X focusing demonstrate the capability of example piezoactive membrane structures for sub-wavelength focusing, and the following example implementations demonstrate active electromechanical tuning of the metamaterial by integrating piezoelectric components into the exemplary piezoactive membrane structures. By adding piezoelectric materials, mechanical properties of the metamaterial can be electrically modulated and therefore dynamically optimize beam focusing.

[0051] For example, piezoelectric materials, such as AlScN, were integrated into the exemplary metamaterial architecture to achieve an active tunable metamaterial allowing control in real time of the metamaterial properties and its focal point. AlScN can get excited in the >100 MHz range, for example, ensuring its operation at our target frequency of 2 MHz. AlScN films can be deposited by sputtering on silicon and can be driven by metallic contacts. AlScN resonators-14-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO can also be passivated by silicon nitride films allowing its operation in liquid environments. The role of the AlScN in the exemplary metamaterials can be to tune the mechanical properties of the metamaterial membranes — not serving as a source of the ultrasound energy.

[0052] For example, piezoactive cell behavior can be measured under electrical control using a Laser Doppler Vibrometer to monitor the membrane motion. For example, in static mode, the change in stiffness of the cell can be characterized by applying a DC signal; and with an AC signal, the control of the phase of oscillations can control metamaterial properties.

[0053] Example implementations for dynamic tuning and focusing included uniform and non- uniform modulation of the metamaterial properties and focusing under a DC and AC voltage. For example, the voltage was controlled on the piezoelectric component in two ways: DC and AC voltage, and uniform vs non-uniform voltage across the metamaterial was studied. For DC voltage, the applied volage can create tensile / compressive stress (e.g., depending on poling conditions) on the piezoelectric fdm, changing the effective bending modulus and therefore changing the effective impedance and acoustic index. On top of the DC, an AC signal can be applied to modulate amplitude and phase of the membranes vibration to control the phase and amplitude of the ultrasound propagation. In addition, the spatial distribution of the applied voltages (AC or DC) can be controlled.

[0054] FIG. 4A shows a diagram depicting an example electroactive tunable metamaterial having a natural, non-tuned passive index (nO), e.g., via no electrical actuation (V=0). However, applying a uniform DC voltage to every membrane, the overall acoustic index can be changed to nS and therefore modulate the focal length to control the positioning of the focal point.

[0055] FIG. 4B shows a diagram depicting an example electroactive tunable metamaterial being controlled by applying a DC voltage, demonstrating uniform membrane modulation by applying a uniform voltage (V 0) on the piezoelectric material of the example metamaterial, thereby changing the stiffness and acoustic index. Yet, furthermore, a certain voltage distribution can be applied to locally control the acoustic index of the metamaterial.

[0056] FIG. 4C shows a diagram depicting an example electroactive tunable metamaterial being controlled by applying different voltages at the top and bottom regions of the metamaterial. This unequal, asymmetric modulation (Vtop t Vbot) allows vertical displacement of the focal point. For example, the asymmetric modulation can produce two different indices at top (nl) and bottom (n2) such that nl > n2, allowing to shift the focal point vertically and in principle raster the focal-15-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO point for scanning and imaging. Using a phased array controller, for example, different phases (e.g., via AC signal) can be applied at specific locations to fully control index and phase locally through the metamaterial. In some example cases, the example implementations showed the tunable negative metamaterial having a negative index and tunability of n=-0.8 to -1.2 taking as reference nwater=l.

[0057] FIG. 5 shows a diagram depicting an acoustic transducer device 500 interfaced with an example embodiment of the electroactive tunable metamaterial structure 100, in accordance with the present technology. The acoustic transducer device 500 (e.g., ultrasound imaging probe device) includes an acoustic transducer array 510, which the example electroactive tunable metamaterial structure 100 is coupled to one or more transducers of the acoustic transducer array 510. The acoustic transducer device 500 can include an electronic controller (not shown) for the example electroactive tunable metamaterial structure 100 to control (e.g., provide digital command signals for programming) the individually-addressable piezoactive membrane cells 110 (e.g., ‘acoustic pixels’) of the example electroactive tunable metamaterial structure 100 to manipulate the amplitude and / or phase to steer and focus an acoustic signal 590 (e.g., ultrasound beam) to be propagated toward a target 599 within a medium 598 (e.g., such as an anatomic structure in a subject’s body). For example, in some embodiments, the electronic controller can be a phase array controller that can control each piezoactive membrane cell 110 of the electroactive tunable metamaterial structure 100 and thereby manipulate the propagation of the acoustic signals 590 from the acoustic transducer device 500 toward the target 599 in the medium 598 with controlled focus and steering. The medium 598 can include a volume of a subject’s body and any interfacing gel or fluid between the body (e.g., skin) and the electroactive tunable metamaterial structure 100 (coupled to the acoustic transducer device 500). As depicted by the diagram of FIG. 5, the example electroactive tunable metamaterial structure 100 includes a spacing 150 between rows of adjacent or neighboring piezoactive membrane cells 110, which can facilitate electrical connections between the individually-addressable piezoactive membrane cells 110 and the electronic controller. Also, as depicted by the diagram in FIG. 5, in some embodiments, for example, the example electroactive tunable metamaterial structure 100 can include an outer sheathing or cover 170, which can include a material with one or more acoustic impedance matching layers interfaced with the transmission region of the electroactive tunable metamaterial structure 100 to contact the medium 598, e.g., where the acoustic impedance matching is with the internal fluid medium in the-16-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO electroactive tunable metamaterial structure 100 and the medium 598. In some embodiments, for example, the outer sheathing or cover 170 can include a material (with one or more acoustic impedance matching layers) interfaced between the reception region of the electroactive tunable metamaterial structure 100 and the acoustic transducer array 510 (not shown).

[0058] In some aspects, for example, a piezoactive membrane cell in accordance with the disclosed technology can be used for ultrasound imaging. In some aspects, an acoustic metamaterial structure including a plurality of the piezoactive membranes cell arranged in parallel for ultrasound imaging is disclosed. In some aspects, a method of manufacturing the acoustic metamaterial structure is disclosed.

[0059] For example, in some implementations, the disclosed acoustic metamaterial structure acts as an acoustic lens with negative acoustic index operating near 1 MHz, allowing for focusing ultrasound with high-resolution beyond the diffraction limit and with electronic control on the position of the focal point having sizes less than the wavelength. Piezoelectric components, for example, piezoelectric films, such as aluminum scandium nitride (AlScN) thin films, are integrated to the membranes with independent electrical contacts to each membrane for achieving tunability of the mechanical properties of the membranes as well as the phase of their oscillations, providing significant capabilities to control the propagation of ultrasound through the metamaterial.

[0060] The membranes include cells with side-holes to induce a negative phase shift in the ultrasound propagation and cause an effective negative density and negative stiffness, respectively, resulting in negative phase velocities and negative angles of refraction. The frequency operation depends strongly on the resonance frequency of the acoustic components. A higher resonance frequency corresponds to a higher frequency of operation of the metamaterial. As frequency is inversely proportional to the size of the acoustic components, scaling down the acoustic components is critical, although challenging, to achieve higher frequency of operation. In this disclosure, the membranes and side-holes may be on the order of 100 microns in size, which allows resonance frequencies close to 1 MHz in liquid.

[0061] When a voltage is applied to the piezoelectric film, then the piezoelectric film is strained. This results in a change in stiffness of the membranes and therefore changing the overall acoustic properties (for example, frequency, acoustic impedance, effective density, effective mass, acoustic index, etc.) of the membranes. Since the strain is directly proportional to the applied voltage, the properties can be tuned electrically. This, in turn, changes the acoustic index of the-17-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO metamaterial, tuning the focal point for scanning during imaging procedures and resolution for ultrasound imaging, as well as controlling the phase of ultrasound locally through the metamaterial. By applying an oscillating voltage, the phase of the ultrasound propagation can be manipulated.

[0062] The disclosed technology provides an acoustic dynamic lens or meta-lens that is advantageous in many aspects. First, the acoustic metamaterial structure can operate in water. Second, the acoustic metamaterial structure can operate at a high frequency, such as 1 MHz and higher, causing it to be suitable for biomedical ultrasound imaging. Third, the active electromechanical control allows the acoustic metamaterial structure to tune the acoustic lens properties and therefore the focusing location and capabilities for ultrasound imaging applications.

[0063] In some example embodiments in accordance with the present technology, a piezoactive membrane is disclosed. The piezoactive membrane includes a first passivation layer, a first electrode layer in contact with the first passivation layer, a piezoelectric layer in contact with the first electrode layer, a membrane bulk (providing or including a second electrode) in contact with the piezoelectric layer, and a second passivation layer in contact with the membrane bulk. The first passivation layer and / or the second passivation layer can be polymer based (such as parylene or polyimide (PI)), or silicon based (such as Silicon Carbide (SiC), Silicon carbon nitride (SiCN), or Silicon Nitride (SiNx)). The first electrode layer comprises gold (Au). The piezoelectric layer comprises lead zirconate titanate (PZT) or AlScN. PZT has a high piezoelectric coefficient, so it can give a high mechanical push given a certain electrical input. AlScN is a novel piezoelectric film that achieved higher frequencies and is compatible with CMOS processing. Although its piezoelectric coefficient is not as high as PZT, it can reach higher frequencies. The membrane bulk may include a second electrode and may be made of inorganic silicon-based materials, for example, doped silicon, with lateral size of 100 microns. The first electrode and the second electrode are configured to be applied to a voltage. The voltage may be an oscillating voltage. In some example embodiments, the positions of the piezoelectric layer and the membrane bulk may be interchangeable. In some example embodiments, the membrane bulk may be sandwiched between two piezoelectric layers.Examples

[0064] In some embodiments in accordance with the present technology (example Al), an acoustic metamaterial device for manipulating acoustic signal transmission includes a-18-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO piezoelectric active membrane comprising a plurality of piezoactive cells, wherein each piezoactive cell includes a side wall having a material layer structure including a first passivation layer on an interior side of the side wall, a first electrode layer in contact with the first passivation layer, a piezoelectric layer in contact with the first electrode layer and configured to convert acoustic energy to an electrical signal, a bulk material in contact with the piezoelectric layer configured to propagate an acoustic signal carrying the acoustic energy, and a second passivation layer in contact with the bulk material; a top wall coupled to the side wall and having an aperture spanning into an interior chamber of the piezoactive cell operable to be filled with a fluid medium that allows acoustic signals to propagate within between regions of the interior side of the side wall; and a bottom wall coupled to the side wall on an opposing side of the top wall.

[0065] Example A2 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the piezoelectric active membrane is configured as two dimensional (2D) array of adjacent piezoactive cells.

[0066] Example A3 includes the acoustic metamaterial device of example A2 or any of examples A1-A29, wherein the acoustic metamaterial device is configured as a three dimensional (3D) array of piezoelectric active membranes comprising two or more 2D arrays of adjacent piezoactive cells in a stacked configuration.

[0067] Example A4 includes the acoustic metamaterial device of example A3 or any of examples A1-A29, wherein, for the 3D array of piezoelectric active membranes, each of the piezoelectric active membranes includes a space between rows of adjacent or neighboring piezoactive membrane cells.

[0068] Example A5 includes the acoustic metamaterial device of example Al or any of examples A1-A29, further comprising an outer cover comprising a material with one or more acoustic impedance matching layers interfaced with an acoustic transmission region of the piezoelectric active membrane.

[0069] Example A6 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the acoustic metamaterial device is configured to control a direction of the acoustic signals while propagating through the acoustic metamaterial device to steer and / or focus the acoustic signals’ propagation toward a target in a medium.

[0070] Example A7 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the apertures of each of the piezoactive membrane cells provide a-19-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO negative modulus to enable negative-angle refraction properties for controlling a direction or directions of the acoustic signals.

[0071] Example A8 includes the acoustic metamaterial device of example Al or any of examples A1-A29, further comprising an electronic controller in electrical communication with each of the piezoactive membrane cells to apply an electrical control signal to any of the piezoactive membrane cells of the acoustic metamaterial device to manipulate an amplitude and / or phase to steer and / or focus the acoustic signals.

[0072] Example A9 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the chamber region has a width size or a diameter size in a range of 10 pm to 500 pm.

[0073] Example A10 includes the acoustic metamaterial device of example A9 or any of examples Al -A29, wherein the aperture has an opening size in a range of 5% to 60% of the width size or the diameter size of the chamber region.

[0074] Example Al l includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein at least some of the piezoactive cells are configured such that at least a portion of the side wall of a given piezoactive cell is shared with an adjacent piezoactive cell in the piezoelectric active membrane.

[0075] Example Al 2 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the first passivation layer and the second passivation layer are of a same material or materials, or wherein the first passivation layer and the second passivation layer are of a different material or materials.

[0076] Example A13 includes the acoustic metamaterial device of example A12 or any of examples A1-A29, wherein one or both of the first passivation layer and the second passivation layer includes a silicon-based material or a polymer material.

[0077] Example A14 includes the acoustic metamaterial device of example A13 or any of examples A1-A29, wherein the silicon-based material includes at least one of silicon carbide (SiC), silicon carbon nitride (SiCN), or silicon nitride (SisN^.

[0078] Example A15 includes the acoustic metamaterial device of example A13 or any of examples A1-A29, wherein the polymer material includes at least one of parylene or polyimide.-20-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO

[0079] Example Al 6 includes the acoustic metamaterial device of example Al 2 or any of examples A1-A29, wherein one or both of the first passivation layer and the second passivation layer is configured to have a thickness in a range of 10 nm to 500 nm.

[0080] Example Al 7 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the electrode layer includes at least one of a metal, a conductive polymer, a carbon-based nanomaterial, a metal oxide ceramic, or a composite material.

[0081] Example Al 8 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the electrode layer is configured to have a thickness in a range of 100 nm to 500 nm.

[0082] Example A19 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the piezoelectric layer includes at least one of lead zirconate titanate (PZT) or aluminum scandium nitride (AlScN).

[0083] Example A20 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the piezoelectric layer is configured to have a thickness in a range of 500 nm to 5 pm.

[0084] Example A21 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the bulk material layer includes a doped silicon or silicon oxide.

[0085] Example A22 includes the acoustic metamaterial device of example A21 or any of examples A1-A29, wherein the bulk material layer includes the doped silicon and is configured to operate as a second electrode of the acoustic metamaterial device.

[0086] Example A23 includes the acoustic metamaterial device of example A21 or any of examples A1-A29, wherein the bulk material layer includes the silicon oxide, which is electrically nonconductive or poorly conductive, and wherein the side wall of the piezoactive cell further comprises a second electrode layer coupled between the bulk material layer and the second passivation layer.

[0087] Example A24 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the bulk material layer is configured to have a thickness in a range of 5 pm to 100 pm.

[0088] Example A25 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein each of the piezoactive cells comprises a plurality of electrical contacts-21-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO configured on at least one side of the piezoactive cell and coupled to an end region of the electrode layer.

[0089] Example A26 includes the acoustic metamaterial device of example A25 or any of examples A1-A29, wherein the piezoelectric active membrane further comprises a plurality of electrical conduits coupled to the plurality of electrical contacts, respectively.

[0090] Example A27 includes the acoustic metamaterial device of example Al or any of examples A1-A29, at least some of the piezoactive cells of the piezoelectric active membrane are configured in a square shape, a cylindrical shape, a rectangular shape, a triangular shape, a hexagonal shape, or a shape with at least one curved side.

[0091] Example A28 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the fluid medium includes water.

[0092] Example A29 includes the acoustic metamaterial device of example Al or any of examples A1-A29, wherein the acoustic metamaterial device is included in an ultrasound imaging system comprising an ultrasound transducer probe device, the ultrasound transducer probe device including a plurality of acoustic transducer elements, and the acoustic transducer elements coupled to the acoustic metamaterial device.

[0093] In some embodiments in accordance with the present technology (example A30), a method for controlling acoustic signal propagation includes receiving, at acoustic metamaterial structure, an acoustic signal generated by one or more transducers of an acoustic transducer device, wherein the acoustic metamaterial structure is interfaced between the acoustic transducer device and an external medium comprising an internal target; and steering and / or focusing propagation of the acoustic signal within the acoustic metamaterial structure by applying a set of electrical control signals to piezoactive cells of the acoustic metamaterial structure to affect an acoustic propagation property including a negative refraction.

[0094] Example A31 includes the method of example A30, wherein the applying the set of electrical control signals includes applying different voltages at a first region and a second region of the acoustic metamaterial structure with an unequal, asymmetric modulation (Vist-region V2nd- region).

[0095] Example A32 includes the method of example A31, wherein the applied different voltages with the unequal, asymmetric modulation causes a change of focal point of the acoustic signal in the medium.-22-009062.8552. WOOOU83672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO

[0096] Example A33 includes the method of any of examples A30-A32, wherein the acoustic metamaterial structure includes the acoustic metamaterial device of any of examples A1-A29.

[0097] In some embodiments in accordance with the present technology (example A34), an acoustic metamaterial structure for ultrasound imaging, comprising a plurality of piezoactive membranes, each comprising a first passivation layer; a first electrode layer in contact with the first passivation layer and operable as a first electrode; a piezoelectric layer in contact with the first electrode layer; a membrane bulk in contact with the piezoelectric layer configured to propagate an acoustic signal and operable as a second electrode; and a second passivation layer in contact with the membrane bulk.

[0098] In some embodiments in accordance with the present technology (example Bl), an acoustic metamaterial structure for ultrasound imaging comprises a plurality of piezoactive membranes, where each piezoactive membrane comprises a first passivation layer; a first electrode layer in contact with the first passivation layer; a piezoelectric layer in contact with the first electrode layer; a membrane bulk in contact with the piezoelectric layer, comprising a second electrode; and a second passivation layer in contact with the membrane bulk.

[0099] Example B2 includes the acoustic metamaterial structure of example Bl or any of examples Bl -B10, wherein both the first passivation layer and the second passivation layer are polymer based or silicon based.

[0100] Example B3 includes the acoustic metamaterial structure of example B2 or any of examples Bl -B10, wherein the first passivation layer comprises Silicon Nitride (SiNx).

[0101] Example B4 includes the acoustic metamaterial structure of example Bl or any of examples Bl -B10, wherein the first electrode layer comprises gold (Au).

[0102] Example B5 includes the acoustic metamaterial structure of example Bl or any of examples B1-B10, wherein the piezoelectric layer comprises lead zirconate titanate (PZT) or aluminum scandium nitride (AlScN).

[0103] Example B6 includes the acoustic metamaterial structure of example Bl or any of examples Bl -B10, wherein the membrane bulk comprises doped silicon.

[0104] Example B7 includes the acoustic metamaterial structure of example Bl or any of examples B 1 -B 10, wherein the first electrode and the second electrode are configured to be applied to a voltage.-23-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO

[0105] Example B8 includes the acoustic metamaterial structure of example Bl or any of examples Bl -B10, wherein the membrane bulk has a lateral size of 100 microns.

[0106] Example B9 includes the acoustic metamaterial structure of example B8 or any of examples Bl -B10, wherein the voltage is an oscillating voltage.

[0107] Example B10 includes the acoustic metamaterial structure of example Bl or any of examples Bl -B9, wherein the plurality of piezoactive membranes is arranged in parallel.Conclusion

[0108] While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

[0109] Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.

[0110] Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.-24-009062.8552. WOOO\183672915.1

Claims

PCT Patent ApplicationDocket: 009062.8552.WOOOCLAIMS1. An acoustic metamaterial device for manipulating acoustic signal transmission, comprising: a piezoelectric active membrane comprising a plurality of piezoactive cells, wherein each piezoactive cell includes: a side wall having a material layer structure including a first passivation layer on an interior side of the side wall, a first electrode layer in contact with the first passivation layer, a piezoelectric layer in contact with the first electrode layer and configured to convert acoustic energy to an electrical signal, a bulk material in contact with the piezoelectric layer configured to propagate an acoustic signal carrying the acoustic energy, and a second passivation layer in contact with the bulk material; a top wall coupled to the side wall and having an aperture spanning into an interior chamber of the piezoactive cell operable to be filled with a fluid medium that allows acoustic signals to propagate within between regions of the interior side of the side wall; and a bottom wall coupled to the side wall on an opposing side of the top wall.

2. The acoustic metamaterial device of claim 1, wherein the piezoelectric active membrane is configured as two-dimensional (2D) array of adjacent piezoactive cells.

3. The acoustic metamaterial device of claim 2, wherein the acoustic metamaterial device is configured as a three-dimensional (3D) array of piezoelectric active membranes comprising two or more 2D arrays of adjacent piezoactive cells in a stacked configuration.

4. The acoustic metamaterial device of claim 3, wherein, for the 3D array of piezoelectric active membranes, each of the piezoelectric active membranes includes a space between rows of adjacent or neighboring piezoactive membrane cells.

5. The acoustic metamaterial device of claim 1, further comprising: an outer cover comprising a material with one or more acoustic impedance matching layers interfaced with an acoustic transmission region of the piezoelectric active membrane.

6. The acoustic metamaterial device of claim 1, wherein the acoustic metamaterial device is configured to control a direction of the acoustic signals while propagating through the acoustic-25-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO metamaterial device to steer and / or focus the acoustic signals’ propagation toward a target in a medium.

7. The acoustic metamaterial device of claim 1, wherein the apertures of each of the piezoactive membrane cells provide a negative modulus to enable negative-angle refraction properties for controlling a direction or directions of the acoustic signals.

8. The acoustic metamaterial device of claim 1, further comprising: an electronic controller in electrical communication with each of the piezoactive membrane cells to apply an electrical control signal to any of the piezoactive membrane cells of the acoustic metamaterial device to manipulate an amplitude and / or phase to steer and / or focus the acoustic signals.

9. The acoustic metamaterial device of claim 1, wherein the chamber region has a width size or a diameter size in a range of 10 pm to 500 pm.

10. The acoustic metamaterial device of claim 9, wherein the aperture has an opening size in a range of 5% to 60% of the width size or the diameter size of the chamber region.11 . The acoustic metamaterial device of claim 1, wherein at least some of the piezoactive cells are configured such that at least a portion of the side wall of a given piezoactive cell is shared with an adjacent piezoactive cell in the piezoelectric active membrane.

12. The acoustic metamaterial device of claim 1, wherein the first passivation layer and the second passivation layer are of a same material or materials, or wherein the first passivation layer and the second passivation layer are of a different material or materials.

13. The acoustic metamaterial device of claim 12, wherein one or both of the first passivation layer and the second passivation layer includes a silicon-based material or a polymer material.

14. The acoustic metamaterial device of claim 13, wherein the silicon-based material includes at least one of silicon carbide (SiC), silicon carbon nitride (SiCN), or silicon nitride (Si3N4).-26-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO15. The acoustic metamaterial device of claim 13, wherein the polymer material includes at least one of parylene or polyimide.

16. The acoustic metamaterial device of claim 12, wherein one or both of the first passivation layer and the second passivation layer is configured to have a thickness in a range of 10 nm to 500 nm.

17. The acoustic metamaterial device of claim 1, wherein the electrode layer includes at least one of a metal, a conductive polymer, a carbon-based nanomaterial, a metal oxide ceramic, or a composite material.

18. The acoustic metamaterial device of claim 1, wherein the electrode layer is configured to have a thickness in a range of 100 nm to 500 nm.

19. The acoustic metamaterial device of claim 1, wherein the piezoelectric layer includes at least one of lead zirconate titanate (PZT) or aluminum scandium nitride (AlScN).

20. The acoustic metamaterial device of claim 1, wherein the piezoelectric layer is configured to have a thickness in a range of 500 nm to 5 pm.

21. The acoustic metamaterial device of claim 1, wherein the bulk material layer includes a doped silicon or silicon oxide.

22. The acoustic metamaterial device of claim 21, wherein the bulk material layer includes the doped silicon and is configured to operate as a second electrode of the acoustic metamaterial device.

23. The acoustic metamaterial device of claim 21, wherein the bulk material layer includes the silicon oxide, which is electrically nonconductive or poorly conductive, and wherein the side wall of the piezoactive cell further comprises a second electrode layer coupled between the bulk material layer and the second passivation layer.

24. The acoustic metamaterial device of claim 1, wherein the bulk material layer is configured to have a thickness in a range of 5 pm to 100 pm.-27-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO25. The acoustic metamaterial device of claim 1, wherein each of the piezoactive cells comprises a plurality of electrical contacts configured on at least one side of the piezoactive cell and coupled to an end region of the electrode layer.

26. The acoustic metamaterial device of claim 25, wherein the piezoelectric active membrane further comprises a plurality of electrical conduits coupled to the plurality of electrical contacts, respectively.

27. The acoustic metamaterial device of claim 1, at least some of the piezoactive cells of the piezoelectric active membrane is configured in a square shape, a cylindrical shape, a rectangular shape, a triangular shape, a hexagonal shape, or a shape with at least one curved side.

28. The acoustic metamaterial device of claim 1, wherein the fluid medium includes water.

29. The acoustic metamaterial device of claim 1, wherein the acoustic metamaterial device is included in an ultrasound imaging system comprising an ultrasound transducer probe device, the ultrasound transducer probe device including a plurality of acoustic transducer elements, and the acoustic transducer elements coupled to the acoustic metamaterial device.

30. A method for controlling acoustic signal propagation, comprising: receiving, at acoustic metamaterial structure, an acoustic signal generated by one or more transducers of an acoustic transducer device, wherein the acoustic metamaterial structure is interfaced between the acoustic transducer device and an external medium comprising an internal target; and steering and / or focusing propagation of the acoustic signal within the acoustic metamaterial structure by applying a set of electrical control signals to piezoactive cells of the acoustic metamaterial structure to affect an acoustic propagation property including a negative refraction.

31. The method of claim 30, wherein the applying the set of electrical control signals includes applying different voltages at a first region and a second region of the acoustic metamaterial structure with an unequal, asymmetric modulation (Vist-region V2nd-region).-28-009062.8552. WOOO\183672915.1PCT Patent ApplicationDocket: 009062.8552.WOOO32. The method of claim 31 , wherein the applied different voltages with the unequal, asymmetric modulation causes a change of focal point of the acoustic signal in the medium.

33. An acoustic metamaterial structure for ultrasound imaging, comprising a plurality of piezoactive membranes, each comprising: a first passivation layer; a first electrode layer in contact with the first passivation layer and operable as a first electrode; a piezoelectric layer in contact with the first electrode layer; a membrane bulk in contact with the piezoelectric layer configured to propagate an acoustic signal and operable as a second electrode; and a second passivation layer in contact with the membrane bulk.-29-009062.8552. WOOO\183672915.1

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