Backward Propagating Acoustic Waves in Single Gold Nanobeams

TL;DR

This study uses femtosecond pump-probe spectroscopy to observe backward propagating acoustic phonons in suspended gold nanobeams, revealing size-dependent modes and demonstrating nanoscale backward wave phenomena.

Contribution

It reports the first nanoscale observation of backward propagating acoustic waves in gold nanobeams, linking them to width dilatational modes.

Findings

Backward acoustic phonons observed at ~3 GHz in nanostructures

Backward waves serve as signatures of width dilatational modes

Size-dependent measurements distinguish confined acoustic modes

Abstract

Femtosecond pump-probe spectroscopy has been carried out on suspended gold nanostructures with a rectangular cross section lithographed on a silicon substrate. With a thickness fixed to 110 nm and a width ranging from 200 nm to 800 nm, size dependent measurements are used to distinguish which confined acoustic modes are detected. Furthermore, in order to avoid any ambiguity due to the measurement uncertainties on both the frequency and size, pump and probe beams are also spatially shifted to detect guided acoustic phonons. This leads us to the observation of backward propagating acoustic phonons in the gigahertz range ($\sim$3 GHz) in such nanostructures. While backward wave propagation in elastic waveguides has been predicted and already observed at the macroscale, very few studies have been done at the nanoscale. Here, we show that these backward waves can be used as the unique signature of the width dilatational acoustic mode.