# Excitonic effects in third harmonic generation: the case of carbon nanotubes and nanoribbons

## TL;DR

This paper investigates how excitonic effects influence third harmonic generation in carbon nanotubes and nanoribbons, revealing that many-body interactions significantly alter the nonlinear optical response by reducing peak intensities and redistributing spectral weight.

## Contribution

It introduces a new ab-initio methodology to analyze excitonic effects on nonlinear susceptibilities in low-dimensional nanostructures.

## Key findings

- Excitonic effects significantly modify third-harmonic generation spectra.
- Many-body effects reduce main peak intensity in nonlinear response.
- Spectral weight is redistributed among excitonic resonances.

## Abstract

Linear and nonlinear optical properties of low dimensional nanostructures have attracted a large interest in the scientific community as tools to probe the strong confinement of the electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [Science 308, 838 (2005)] and graphene nanoribbons [Nat. Comm. 5, 4253 (2014)] is dominated by bounded electron-hole pairs, the excitons. The role of excitons in linear response has been widely studied, but still little is known on their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013)] based on well-established ab-initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent particle spectrum and redistribute the spectral weight among several excitonic resonances.