Enhancement of high-order harmonic generation in graphene by mid-infrared and terahertz fields

TL;DR

This paper theoretically demonstrates that combining mid-infrared and terahertz fields significantly enhances high-order harmonic generation in graphene, with potential for improved nonlinear optical applications.

Contribution

It reveals that the enhancement arises from coherent coupling between MIR and THz-induced transitions, beyond simple population effects.

Findings

10-fold increase in fifth harmonic intensity

25-fold increase in seventh harmonic intensity

Enhancement driven by coherent coupling mechanisms

Abstract

We theoretically investigate high-order harmonic generation (HHG) in graphene under mid-infrared (MIR) and terahertz (THz) fields based on a quantum master equation. Numerical simulations show that MIR-induced HHG in graphene can be enhanced by a factor of 10 for fifth harmonic and a factor of 25 for seventh harmonic under a THz field with a peak strength of 0.5 MV/cm by optimizing the relative angle between the MIR and THz fields. To identify the origin of this enhancement, we compare the fully dynamical calculations with a simple thermodynamic model and a nonequilibrium population model. The analysis shows that the enhancement of the high-order harmonics mainly results from a coherent coupling between MIR- and THz-induced transitions that goes beyond a simple THz-induced population contribution.