  When two sheets of graphene stack in a twisted bilayer graphene (tBLG)
configuration, the resulting constrained overlap between interplanar 2p
orbitals produce angle-tunable electronic absorption resonances. Using a novel
combination of multiphoton transient absorption (TA) microscopy and TEM, we
resolve the resonant electronic structure, and ensuing electronic relaxation
inside single tBLG domains. Strikingly, we find that the transient electronic
population in resonantly excited tBLG domains is enhanced many fold, forming a
major electronic relaxation bottleneck. 2-photon TA microscopy shows this
bottleneck effect originates from a strongly bound, dark exciton state lying
0.37 eV below the 1-photon absorption resonance. This stable coexistence of
strongly bound excitons alongside free-electron continuum states has not been
previously observed in a metallic, 2D material.
