Brown, Michael David; (2018) Optical and single element transducers for the generation of arbitrary acoustic fields. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Precise control over the temporal and spatial properties of acoustic fields in 2 or 3-D is essential for nearly all modern, biomedical applications of ultrasound. At present, piezoelectric arrays dominate, however, despite their ubiquity they have a number of drawbacks that compromise the fidelity with which the output field can be manipulated, particularly at high frequencies and in three dimensions. The development of new novel alternatives for manipulating acoustic fields in 3-D is therefore essential. This thesis presents several new techniques through which this can be achieved using both the optical generation of ultrasound and single element piezoelectric transducers. First, the use of multiple Q-switch laser sources in combination with binary amplitude holograms is investigated for the generation of single and multi-focal acoustic fields. The conditions required for the generation of a focus are established numerically and the method is validated experimentally. Next, two approaches are developed for the generation of arbitrary spatial distributions of pressure using a single optical pulse. The first employs multi-layer optical absorbers: structures composed of several absorbing layers each individually patterned such that the field constructively interferes at a set of target points. The second uses tailored optically absorbing surface profiles: arbitrary surface shapes, fabricated through 3-D printing, designed to geometrically focus over a continuous pattern. Finally, the last chapter of the thesis investigates the use of multi-frequency kinoforms for mapping the field of single element piezoelectric transducers onto multiple complex target distributions. The properties of these kinoforms are explored in depth numerically and experimentally it is shown that multiple complex distributions can be generated in a target plane using this approach.

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