Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

TL;DR

This paper investigates how ultra-thin silicon membranes significantly alter phonon dispersion relations, notably reducing velocities of fundamental flexural modes, impacting noise control and opto-mechanical applications.

Contribution

It provides experimental measurements of phonon dispersion in silicon membranes as thin as 8 nm, revealing substantial velocity reductions not observed in bulk materials.

Findings

Phase and group velocities reduced by over tenfold

Dispersion relations significantly modified in nanostructures

Implications for noise control in MEMS/NEMS and opto-mechanical devices

Abstract

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano and micro-electromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.