  Gradient index (GRIN) acoustic devices have spatially inhomogeneous
refractive index profile and allow flexible control of the propagation of
acoustic waves. Previous GRIN acoustic lenses are mostly inherently
two-dimensional designs that are difficult to be extended to all three
dimensions. Besides, manually designing the spatially inhomogeneous structure
is both time-consuming and error-prone. In this work, we proposed and
numerically verified an automated computer-aided design tool: GRadient Index
Pick-and-Place (GRIPP) algorithm, for generating three-dimensional GRIN
acoustic wave controlling devices with scalable and 3D printable structures.
The algorithm receives as inputs a spatial distribution of refractive index and
a pre-defined library of gradient index unit cells, and outputs a 3D model of
GRIN device that is ready to be 3D printed. The tool enables rapid design and
realization of a large variety of 3D GRIN acoustic devices, which can be useful
in areas such as speaker system design, airborne ultrasonic sensing, as well as
therapeutic ultrasound.
