Interlayer exciton spectral dynamics and photoemission in twisted bilayer graphene

Upon resonant two-photon excitation of twisted bilayer graphene (tBLG), we observe weak light emission that is spectrally tunable with the stacking angle. This signal is best ascribed to photoluminescence from thermalized bright and dark strongly-bound interlayer exciton states. The spectral fine-structure and long-lived exciton kinetics are further obtained from individual tBLG domains through two-photon photoluminescence excitation spectra and intraband transient absorption spectra on single tBLG grains. Spectral peaks in both two-photon photoluminescence and transient spectrum of intraband exciton transition independently quantify show the interlayer exciton states have binding energies ranging from ~0.4-0.75 eV, as a function of the stacking angle from 8^o to 17^o. This suggests that photoluminescence collected from the tBLG domain is enabled by the quasi-stable interlayer excitons that coexist alongside graphene continuum states. Theoretical simulations suggest this stable coexistence results from a vanishing exciton-continuum coupling strength resulting from the rehybridization of two degenerate van-Hove singularity-like transitions predicted in the tight-binding model for tBLG.