Design and Fabrication of Acoustic Metamaterials Unit Cell in Near-megahertz Range

Ultrasound is strongly attenuated, reflected and aberrated in the skull and on the interface, due to the complex structure and acoustic impedance mismatch of biological layer. Acoustic metamaterials with effective negative density and modulus, have been proved to enhance the ultrasound transmission in theoretical and experimental physics, which also can enable for bidirectional ultrasound delivery for ultrasonic imaging and neurostimulation. However, it is quite challenging to prepare and fabricate the double-negative metamaterials in biomedical application frequency range due to the limitation of fabrication and characterization methods. Therefore, we present the design of negative refractive index acoustic metamaterials that operate at near-megahertz frequencies and is intended for the eventual aim of enabling enhanced acoustic transmission through high impedance-contrast biological layers. The negative properties are composed by a linear array of unit cells Helmholtz resonators and membranes. The dispersion relation, the effective modulus and density of the proposed negative metamaterials are calculated by COMSOL Multiphysics. The enhancement of ultrasound transmission through high-impedance-contrast layer is proven by a full three-dimensional model of the metamaterials. The transmission improves from 80\% transmission through a high impedance layer alone to near 100\% transmission through the metamaterial-plus-high-impedance layer combination. As the preliminary experimental realization, the unit cell structure created using a novel nanofabrication approach, is presented and characterized between 230 kHz and 410 kHz. We study the vibration behavior of silicon nitride membrane in water to further improve the resonant frequency of silicon nitride membrane above 1 MHz. We demonstrate that the water mass loading effect is the major role for resonance frequency reduction. Meanwhile, the non-dimensionalized added virtual mass incremental (NAVMI) factors are experimentally calculated to estimate the resonance reduction. We estimate that silicon nitride membranes with widths below 50 $\upmu$m are required to build negative metamaterials operating above 1 MHz. Based on the scaling law, we developed a new microfabrication method to integrate horizontal suspended thin films with functional nanomaterials into three-dimensional architectures for highly-compact micro/nanoscale devices. This method also reduces the spacing between the horizontal membrane structure from 200 $\upmu$m to 2 $\upmu$m, which provides the possibility for high-frequency range metamaterial component.This work demonstrates the first membrane-Helmholtz resonator based metamaterials component works in near-megahertz range, which provides a new potential method to achieve ultrasonic transmission enhancement for biomedical application. Our approach also paves the way for the development of active acoustic metamaterials, acoustic metasurface and superlensing