Two-Dimensional Phononic Crystals: Disorder Matters

TL;DR

This study investigates how disorder in two-dimensional phononic crystals affects their hypersonic and thermal properties, revealing that disorder modifies phonon dispersion and coherence without changing thermal conductivity.

Contribution

It provides experimental insights into the impact of disorder on phononic properties of 2D PnCs, including a new criterion for phonon coherence prediction.

Findings

Disorder modifies phonon dispersion and coherence in hypersonic range.

Thermal conductivity remains unaffected by disorder.

Surface roughness influences phonon coherence without altering thermal transport.

Abstract

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.