First direct measurement of optical phonons in 2D plasma crystals

TL;DR

This paper reports the first direct experimental measurement of optical phonons in a 2D plasma crystal, revealing out-of-plane modes with negative optical dispersion and confirming theoretical predictions.

Contribution

It introduces a novel particle imaging method to directly measure the dispersion relation of out-of-plane phonons in 2D plasma crystals.

Findings

Out-of-plane phonon modes exhibit negative optical dispersion.

Experimental results agree well with theoretical models.

Plasma wakes influence the phonon dispersion relation.

Abstract

Spectra of phonons with out-of-plane polarization were studied experimentally in a 2D plasma crystal. The dispersion relation was directly measured for the first time using a novel method of particle imaging. The out-of-plane mode was proven to have negative optical dispersion, comparison with theory showed good agreement. The effect of the plasma wakes on the dispersion relation is briefly discussed.