Acoustic beamwidth compressor using gradient-index phononic crystals

We report a novel approach to effectively couple acoustic energy into a two-dimensional phononic-crystal waveguide by an acoustic beamwidth compressor using the concept of a gradient-index phononic crystal (GRIN PC). The GRIN PC-based beamwidth compressor is composed of a square array of solid scatterers embedded in epoxy. By gradually modulating the density and elastic modulus of the scatterers along the direction transverse to the phononic propagation, the beamwidth compressor can efficiently compress the wide acoustic beam to the scale of the phononic-crystal waveguide. This acoustic beamwidth compressor is investigated through a finite-difference time-domain method. A beam-size conversion ratio of 6.5 : 1 and a transmission efficiency of up to 90% is obtained over the working frequency range of the phononic-crystal waveguide. Potential applications for this device include acoustic biosensors and signal processors.