θ-Tunable Photoluminescence from Interlayer Excitons in Twisted Bilayer Graphene

Using resonant 2-photon excitation of interlayer electrons in twisted bilayer graphene (tBLG), we resolve photoluminescence (PL) that tunes spectrally with stacking angle, θ. This weak signal is 4- 5$\times$ 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 θ = 8$^o$ to 17$^o$. 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 θ-tuneable control over carrier thermalization, extraction and emission in optical graphene-based devices.