SUMMARY

Two-dimensional membrane arrays can support an interesting acoustic wave field, which is evanescent in the direction normal to the array while surface waves can propagate in the immersion fluid immediately above the array. These waves are a result of the resonant membranes coupling to the fluid medium and propagate with a group and phase speed lower than that of the bulk wave of the surrounding fluid. This proposed research will examine and utilize these evanescent surface waves using Capacitively Micromachined Ultrasonic Transducers (CMUT) as a specific example. CMUT arrays can generate and detect membrane displacement capacitively, and are shown to support the surface waves capable of subwavelength focusing. The first objective is to gain a greater understanding of the physics of the evanescent surface waves through simulations and experiments. The simulation models the electrostatic actuation of CMUT membranes and solves for the displacement of all membranes in a finite array using a boundary element method. These simulations calculate the dispersion curve of the surface wave along with the phase and group speeds. Experiments are used to validate the model and simulations. The second aspect of the proposed research is to use simulations to design an array capable of subwavelength focusing, relative to the bulk wave’s wavelength. Subwavelength focusing setup uses a CMUT array comprised of an imaging area of densely populated membranes and several exterior membranes surrounding the imaging area. A broadband, time-reversal technique will be implemented for imaging. The transmit capability of CMUT membranes will be used to generate the surface waves while a high frequency laser vibrometer will be used to record the displacement measurements. By being able to focus with subwavelength resolution to each membrane in the imaging area, 2D imaging can be performed. The imaging array will be characterized by the known response of the ideal array in immersion in regards to focusing to each membrane in the imaging area. Any object placed on or near the array will distort the focused field, and the information contained in the distorted signals will be used to generate an image.