Tunable Excitons in Biased Bilayer Graphene

TL;DR

This paper demonstrates that biased bilayer graphene exhibits tunable excitonic optical responses, with exciton binding energies adjustable via external bias, revealing unique strong excitonic behaviors due to its effective one-dimensional density of states and pseudospin effects.

Contribution

It reveals the dominant role of bound excitons in the optical response of biased bilayer graphene and how their properties can be tuned by external bias, differing from traditional two-dimensional systems.

Findings

Excitons dominate the optical absorbance spectrum.

Exciton binding energy can be tuned from zero to tens of meV.

Unique excitonic behaviors arise from the system's effective one-dimensional density of states.

Abstract

Recent measurements have shown that a continuously tunable bandgap of up to 250 meV can be generated in biased bilayer graphene [Y. Zhang et al., Nature 459, 820 (2009)], opening up pathway for possible graphene-based nanoelectronic and nanophotonic devices operating at room temperature. Here, we show that the optical response of this system is dominated by bound excitons. The main feature of the optical absorbance spectrum is determined by a single symmetric peak arising from excitons, a profile that is markedly different from that of an interband transition picture. Under laboratory conditions, the binding energy of the excitons may be tuned with the external bias going from zero to several tens of meV's. These novel strong excitonic behaviors result from a peculiar, effective ``one-dimensional'' joint density of states and a continuously-tunable bandgap in biased bilayer graphene. Moreover, we show that the electronic structure (level degeneracy, optical selection rules, etc.) of the bound excitons in a biased bilayer graphene is markedly different from that of a two-dimensional hydrogen atom because of the pseudospin physics.