Binary volume acoustic holograms

Abstract

Acoustic holograms or phase conjugate acoustic lenses are a simple inexpensive method for forming complex acoustic fields. They are 3D printable phase plates that can map an incident field onto a pre-defined phase distribution such that it diffracts to form a desired field. These phase plates areanalogues of thin optical holograms with a thickness d ≅ λ the design wavelength. Another class of optical holograms are “thick” or “volume” holograms, composed of weak periodic variations in the refractive index, for which d ⪢ λ. These volume holograms are significantly more selective to the wavelength and direction of the incident field allowing for greater ability to multiplex patterns within a single hologram. In this work, we explore the generation of volume acoustic holograms using multi-material 3-D printing. First, we numerically assess the impact of sound speed contrast, absorption, and quantisation on efficiency. A greedy optimization approach is then introduced for the design of a volumetric hologram from a desired set of input/output fields. We then validate the approach experimentally using test samples printed on an Objet Connex 350. Several holograms are fabricated demonstrating both spatial and frequency multiplexing of different patterns in a single volume.