Strongly nonlinear Bernstein modes in graphene reveal plasmon-enhanced near-field magnetoabsorption

TL;DR

This paper reports the experimental observation of nonlinear Bernstein modes in graphene, demonstrating strong local heating and plasmonic effects that enable nonlinear magnetoplasmonics at relatively low radiation intensities.

Contribution

It provides the first experimental evidence of nonlinear Bernstein modes in graphene using near-field terahertz excitation, revealing plasmon-enhanced nonlinear behavior and polarization dependence.

Findings

Sharp resonances at Bc/2 and Bc/3 saturate at low intensities

Resonances are insensitive to circular polarization but polarization-dependent

Strong local heating due to resonant Bernstein magnetoplasmons

Abstract

Bernstein modes -- hybrid magnetoplasmon excitations arising from the coupling between cyclotron motion and collective oscillations in two-dimensional electron systems -- offer direct access to non-local electrodynamics. These modes can exhibit rich nonlinear behavior akin to strong-coupling phenomena in cavity quantum electrodynamics, but reaching nonlinear regime has remained experimentally challenging. Here we report the observation of nonlinear Bernstein modes in graphene using terahertz excitation with near-field enhancement from embedded metallic contacts. Photoresistance spectroscopy reveals sharp resonances at Bc/2 and Bc/3 that saturate at radiation intensities nearly an order of magnitude lower than the cyclotron resonance. We ascribe this to strong local heating of the electron gas due to resonant excitation of high-amplitude Bernstein magnetoplasmons, associated with a combination of the field-concentration effect of the near field and plasmonic amplification that is resonantly enhanced in the region of Bernstein gaps. Polarization-resolved measurements further confirm the near-field origin: Bernstein resonances are insensitive to circular helicity but strongly depend on the angle of linear polarization, in sharp contrast to the cyclotron resonance response. Our results establish graphene as a platform for nonlinear magnetoplasmonics, opening opportunities for strong-field manipulation of collective electron dynamics, out-of-equilibrium electron transport, and solid-state analogues of cavity quantum electrodynamics.