Efficient, High-purity, Robust Sound Frequency Conversion with a Linear Metasurface

TL;DR

This paper introduces a rotating linear vortex metasurface that achieves high-purity and efficient sound frequency conversion at audible frequencies, demonstrating robustness and potential for advanced acoustic applications.

Contribution

It presents the first experimental demonstration of a rotating linear vortex metasurface for high-purity, efficient sound frequency conversion with robustness and scalability.

Findings

Achieved over 95% mode purity in sound frequency conversion.

Realized conversion efficiency above 65% at 3000 Hz.

Demonstrated stability against rotational speed variations.

Abstract

The intrinsic limitation of the material nonlinearity inevitably results in the poor mode purity, conversion efficiency and real-time reconfigurability of the generated harmonic waves, both in optics and acoustics. Rotational Doppler effect provides us an intuitive paradigm to shifting the frequency in a linear system, which needs to be facilitated by a spiraling phase change upon the wave propagation. Here we numerically and experimentally present a rotating linear vortex metasurface and achieve close-to-unity mode purity (above 95%) and conversion efficiency (above 65%) in audible sound frequency as low as 3000 Hz. The topological charge of the transmitted sound is almost immune from the rotational speed and transmissivity, demonstrating the mechanical robustness and stability in adjusting the high-performance frequency conversion in situ. These features enable us to cascade multiple vortex metasurfaces to further enlarge and diversify the extent of sound frequency conversion, which are experimentally verified. Our strategy takes a step further towards the freewheeling sound manipulation at acoustic frequency domain, and may have far-researching impacts in various acoustic communications, signal processing, and contactless detection.