Caustic graphene plasmons with Kelvin angle

TL;DR

This paper demonstrates that graphene plasmons exhibit a transition from Kelvin to Mach angle caustic wave patterns as the velocity of a moving charged particle increases, linking classical wave theory with plasmonic phenomena.

Contribution

It introduces caustic wave theory to graphene plasmonics and reveals a velocity-dependent transition in wave caustic angles during electron energy-loss spectroscopy.

Findings

At low velocities, graphene plasmons form Kelvin angle caustics.

At high velocities, caustics disappear and approach Mach angle.

The study connects classical wave caustics with plasmonic excitations.

Abstract

A century-long argument made by Lord Kelvin that all swimming objects have an effective Mach number of 3, corresponding to the Kelvin angle of 19.5 degree for ship waves, has been recently challenged with the conclusion that the Kelvin angle should gradually transit to the Mach angle as the ship velocity increases. Here we show that a similar phenomenon can happen for graphene plasmons. By analyzing the caustic wave pattern of graphene plasmons stimulated by a swift charged particle moving uniformly above graphene, we show that at low velocities of the charged particle, the caustics of graphene plasmons form the Kelvin angle. At large velocities of the particle, the caustics disappear and the effective semi-angle of the wave pattern approaches the Mach angle. Our study introduces caustic wave theory to the field of graphene plasmonics, and reveals a novel physical picture of graphene plasmon excitation during electron energy-loss spectroscopy measurement.