Using resonant 2-photon excitation of interlayer electrons in twisted bilayer graphene (tBLG), we resolve photoluminescence (PL) that tunes spectrally with stacking angle, {\theta}. This weak signal is 4- 5× larger than the non-resonant background and is emitted from the interlayer band anti-crossing regions traditionally associated with van Hove singularity resonances. However, our observation of resonant PL emission with delayed ~1 ps electronic thermalization suggests interlayer carriers may instead form bound-excitons. Using both the 2-photon PL and intraband transient absorption spectra, we observe bright and dark state peak-splitting associated with an interlayer exciton binding energy ranging from 0.5 to 0.7 eV for {\theta} = 8o to 17o. These results support theoretical models showing interlayer excitons in tBLG are stabilized by a vanishing exciton-coupling strength to the metallic continuum states. This unexpected dual metal-exciton optical property of tBLG suggests possible {\theta}-tuneable control over carrier thermalization, extraction and emission in optical graphene-based devices.
@article{arxiv.1806.06312,
title = {{\theta}-Tunable Photoluminescence from Interlayer Excitons in Twisted Bilayer Graphene},
author = {Hiral Patel and Lujie Huang and Cheol-Joo Kim and Jiwoong Park and Matt W. Graham},
journal= {arXiv preprint arXiv:1806.06312},
year = {2019}
}