Pseudo-surface acoustic waves in hypersonic surface phononic crystals

TL;DR

This paper develops a theoretical framework to analyze pseudo-surface acoustic waves in hypersonic surface phononic crystals, revealing how structural parameters influence wave localization, frequency gaps, and energy scattering at nanometer scales.

Contribution

It introduces a surface-likeness coefficient to identify pseudo-surface modes and studies their spectral properties in hypersonic phononic structures, addressing coupling challenges.

Findings

Pseudo-surface acoustic waves are identified using the surface-likeness coefficient.

Frequency gaps in the pseudo-SAW spectrum are characterized.

Structural parameters affect wave localization and energy scattering.

Abstract

We present a theoretical framework allowing to properly address the nature of surface-like eigenmodes in a hypersonic surface phononic crystal, a composite structure made of periodic metal stripes of nanometer size and periodicity of 1 micron, deposited over a semi-infinite silicon substrate. In surface-based phononic crystals there is no distinction between the eigenmodes of the periodically nanostructured overlayer and the surface acoustic modes of the semi-infinite substrate, the solution of the elastic equation being a pseudo-surface acoustic wave partially localized on the nanostructures and radiating energy into the bulk. This problem is particularly severe in the hypersonic frequency range, where semi-infinite substrate's surface acoustic modes strongly couple to the periodic overlayer, thus preventing any perturbative approach. We solve the problem introducing a surface-likeness coefficient as a tool allowing to find pseudo-surface acoustic waves and to calculate their line shapes. Having accessed the pseudo-surface modes of the composite structure, the same theoretical frame allows reporting on the gap opening in the now well-defined pseudo-SAW frequency spectrum. We show how the filling fraction, mass loading and geometric factors affect both the frequency gap, and how the mechanical energy is scattered out of the surface waveguiding modes.