# Optical selection rules of zigzag graphene nanoribbons

## TL;DR

This paper develops an analytical tight-binding model to understand the optical selection rules in zigzag graphene nanoribbons, revealing how wave function parity influences optical transitions and their relation to nanotube spectra.

## Contribution

It provides the first analytical expressions for wave functions and optical transition rules in zigzag graphene nanoribbons, highlighting differences and correlations with armchair nanotubes.

## Key findings

- Optical selection rules depend on wave function parity, with ΔJ odd for valence-conduction transitions.
- The absorption spectra of ribbons and nanotubes are correlated through van Hove singularities.
- Matrix elements show smooth behavior at Dirac points and edge state intersections.

## Abstract

We present an analytical tight-binding theory of the optical properties of graphene nanoribbons with zigzag edges. Applying the transfer matrix technique to the nearest-neighbor tight-binding Hamiltonian, we derive analytical expressions for electron wave functions and optical transition matrix elements for incident light polarized along the structure axis. It follows from the obtained results that optical selection rules result from the wave function parity factor $(-1)^J$, where $J$ is the band number. These selection rules are that $\Delta J$ is odd for transitions between valence and conduction subbands and that $\Delta J$ is even for transitions between only valence (conduction) subbands. Although these selection rules are different from those in armchair carbon nanotubes, there is a hidden correlation between absorption spectra of the two structures that should allow one to use them interchangeably in some applications. The correlation originates from the fact that van Hove singularities in the tubes are centered between those in the ribbons if the ribbon width is about a half of the tube circumference. The analysis of the matrix elements dependence on the electron wave vector for narrow ribbons shows a smooth non-singular behavior at the Dirac points and the points where the bulk states meet the edge states.