# Second harmonic generation in graphene dressed by a strong terahertz field

## TL;DR

This paper reports the observation of enhanced second-harmonic generation in monolayer graphene under an ultra-strong terahertz pulse, revealing a nonperturbative light-matter interaction regime with significant nonlinear optical response.

## Contribution

It introduces a novel experimental observation of enhanced second-harmonic generation in graphene driven by strong terahertz fields and develops an analytic theory to explain this effect.

## Key findings

- Enhanced second-harmonic signal observed in graphene
- Strong asymmetric carrier distribution causes nonlinear response
- Analytic model accurately predicts intensity and polarization

## Abstract

We observe enhanced second-harmonic generation in monolayer graphene in the presence of an ultra-strong terahertz field pulse with a peak amplitude of 250 kV/cm. This is a strongly nonperturbative regime of light-matter interaction in which particles get accelerated to energies exceeding the initial Fermi energy of 0.2 eV over a timescale of a few femtoseconds. The second-harmonic current is generated as electrons drift through the region of momenta corresponding to interband transition resonance at an optical frequency. The resulting strongly asymmetric distortion of carrier distribution in momentum space gives rise to an enhanced electric-dipole nonlinear response at the second harmonic. We develop an approximate analytic theory of this effect which accurately predicts observed intensity and polarization of the second-harmonic signal.