Optical-phonon resonances with saddle-point excitons in twisted-bilayer graphene

TL;DR

This paper investigates how twisting bilayer graphene affects its optical properties, revealing saddle-point excitons and their resonances with phonons, which could impact optoelectronic applications.

Contribution

It demonstrates the tunability of optical responses in twisted-bilayer graphene via twisting angle, highlighting the role of saddle-point excitons in optical and Raman scattering phenomena.

Findings

Resonances with saddle-point excitons are observed in twisted-bilayer graphene.

Separate Stokes and anti-Stokes Raman resonances are achieved.

Resonance processes are correlated and involve the same phonon.

Abstract

Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the density of states that can be tuned by changing the twisting angle $\theta$. A $\theta$-defined tBLG has been produced and characterized with optical reflectivity and resonance Raman scattering. The $\theta$-engineered optical response is shown to be consistent with persistent saddle-point excitons. Separate resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the two-dimensional saddle-point excitons, similar to what has been previously observed for one-dimensional carbon nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the two processes are correlated and that they share the same phonon.