Nonlinear Response of Bilayer Graphene at Terahertz Frequencies

TL;DR

This paper develops a density-matrix formalism to study the nonlinear terahertz response of bilayer graphene, revealing strong harmonic generation influenced by interband and intraband dynamics at low temperatures.

Contribution

It introduces a novel theoretical approach using a density-matrix formalism within the length gauge for bilayer graphene's nonlinear response at terahertz frequencies.

Findings

Third harmonic amplitude reaches 30% of the fundamental at 1 THz with 1.5 kV/cm.

Third harmonic amplitude increases with incident frequency, reaching 53% at 2 THz.

Strong harmonic generation results from interplay between interband and intraband dynamics.

Abstract

A density-matrix formalism within the length gauge is developed for the purpose of calculating the nonlinear response of intrinsic bilayer graphene at terahertz frequencies. Employing a tight-binding model, we find that interplay between the interband and intraband dynamics leads to strong harmonic generation at moderate field amplitudes. Specifically, we find that at low temperature (10 K), the reflected field of undoped suspended bilayer graphene exhibits a third harmonic amplitude that is 30% of the fundamental in the reflected field for an incident 1 THz single-cycle pulse with a field amplitude of 1.5 kV/cm. More interestingly, we find that as the central frequency of the incident radiation is increased, the third harmonic amplitude also increases; reaching a maximum of 53% for an incident frequency of 2 THz and amplitude of 2.5 kV/cm.