Acoustic flat lensing using an indefinite medium

TL;DR

This paper demonstrates acoustic flat lensing by tuning a phononic array to exhibit indefinite medium behavior, confirmed through theoretical analysis and time-domain experiments with airborne acoustic waves, enabling precise 3D wave control.

Contribution

The study introduces a novel method to achieve acoustic flat lensing using an indefinite medium in a phononic array, validated by both theoretical and experimental results.

Findings

Confirmation of indefinite medium behavior via flat band and isofrequency contours.

Successful formation of coherent directive beams and focal spots.

Experimental validation with airborne acoustic waves at 10.6 kHz.

Abstract

Acoustic flat lensing is achieved here by tuning a phononic array to have indefinite medium behaviour in a narrow frequency spectral region along the acoustic branch. This is confirmed by the occurrence of a flat band along an unusual path in the Brillouin zone and by interpreting the intersection point of isofrequency contours on the corresponding isofrequency surface; coherent directive beams are formed whose reflection from the array surfaces create lensing. Theoretical predictions are corroborated by time-domain experiments, airborne acoustic waves generated by a source with a frequency centered about $10.6$ kHz, placed at three different distances from one side of a finite phononic crystal slab, constructed from polymeric spheres, yield distinctive focal spots on the other side. These experiments evaluate the pressure field using optical feedback interferometry and demonstrate precise control of the three-dimensional wave trajectory through a sonic crystal.