# Gradient index phononic crystals and metamaterials

## TL;DR

This paper reviews how gradient index phononic crystals and metamaterials can be designed to control wave propagation, enabling the creation of various acoustic devices through graded structures and ray theory analysis.

## Contribution

It provides a comprehensive overview of devices based on graded phononic crystals for controlling bulk and guided acoustic waves.

## Key findings

- Effective gradient index can be achieved in finite phononic crystals.
- Wave propagation can be modeled using ray theory in graded structures.
- Various acoustic devices have been designed using these principles.

## Abstract

Phononic crystals and acoustic metamaterials are periodic structures whose effective properties can be tailored at will to achieve extreme control on wave propagation. Their refractive index is obtained from the homogenization of the infinite periodic system, but it is possible to locally change the properties of a finite crystal in such a way that it results in an effective gradient of the refractive index (GRIN). In such case the propagation of waves can be accurately described by means of ray theory, and different refractive devices can be designed in the framework of wave propagation in inhomogeneous media. In this paper we review the different devices that have been studied for the control of both bulk or guided acoustic waves based on graded phononic crystals.

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Source: https://tomesphere.com/paper/1812.09655