When two sheets of graphene stack in a twisted bilayer graphene (tBLG)configuration, the resulting constrained overlap between interplanar 2porbitals produce angle-tunable electronic absorption resonances. Using a novelcombination of multiphoton transient absorption (TA) microscopy and TEM, weresolve the resonant electronic structure, and ensuing electronic relaxationinside single tBLG domains. Strikingly, we find that the transient electronicpopulation in resonantly excited tBLG domains is enhanced many fold, forming amajor electronic relaxation bottleneck. 2-photon TA microscopy shows thisbottleneck effect originates from a strongly bound, dark exciton state lying0.37 eV below the 1-photon absorption resonance. This stable coexistence ofstrongly bound excitons alongside free-electron continuum states has not beenpreviously observed in a metallic, 2D material.
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