Non-dispersive one-way signal amplification in sonic metamaterials

TL;DR

This paper introduces a novel sonic metamaterial design that achieves broadband, one-way, non-dispersive amplification of sound waves across an entire frequency band using spacetime modulation, enabling lossless and distortion-free wave propagation.

Contribution

It presents a new mechanism for one-way sound amplification in metamaterials via spacetime modulation, extending parametric amplification to broadband and non-dispersive regimes.

Findings

Nearly uniform amplification across the lowest-frequency band.

Amplification occurs only in the forward-propagating band.

Wave packets are amplified without distortion or loss.

Abstract

Parametric amplification -- injecting energy into waves via periodic modulation of system parameters -- is typically restricted to specific multiples of the modulation frequency. However, broadband parametric amplification can be achieved in active metamaterials which allow local parameters to be modulated both in space and in time. Inspired by the concept of luminal metamaterials in optics, we describe a mechanism for one-way amplification of sound waves across an entire frequency band using spacetime-periodic modulation of local stiffnesses in the form of a traveling wave. When the speed of the modulation wave approaches that of the speed of sound in the metamaterial -- a regime called the sonic limit -- nearly all modes in the forward-propagating acoustic band are amplified, whereas no amplification occurs in the reverse-propagating band. To eliminate divergences that are inherent to the sonic limit in continuum materials, we use an exact Floquet-Bloch approach to compute the dynamic excitation bands of discrete periodic systems. We find wide ranges of parameters for which the amplification is nearly uniform across the lowest-frequency band, enabling amplification of wavepackets while preserving their speed, shape, and spectral content. Our mechanism provides a route to designing acoustic metamaterials which can propagate wave pulses without losses or distortion across a wide range of frequencies.