Optical Excitations and Field Enhancement in Short Graphene Nanoribbons

TL;DR

This paper theoretically investigates optical excitations in short graphene nanoribbons, revealing strong, tunable collective oscillations and field enhancements relevant for nanoantenna and nanoplasmonic applications.

Contribution

It provides a detailed atomistic analysis of optical spectra and field enhancement in finite-length graphene nanoribbons, highlighting their tunability and collective excitation behavior.

Findings

Dominant low-energy excitations with strong intensity

Stationary collective oscillations of carrier density

Robust field enhancement across size variations

Abstract

The optical excitations of elongated graphene nanoflakes of finite length are investigated theoretically through quantum chemistry semi-empirical approaches. The spectra and the resulting dipole fields are analyzed, accounting in full atomistic details for quantum confinement effects, which are crucial in the nanoscale regime. We find that the optical spectra of these nanostructures are dominated at low energy by excitations with strong intensity, comprised of characteristic coherent combinations of a few single-particle transitions with comparable weight. They give rise to stationary collective oscillations of the photoexcited carrier density extending throughout the flake, and to a strong dipole and field enhancement. This behavior is robust with respect to width and length variations, thus ensuring tunability in a large frequency range. The implications for nanoantennas and other nanoplasmonic applications are discussed for realistic geometries.