The high-conductivity, low cost and ease of fabrication have made graphene the poster-child for next-generation electronics. Unfortunately as a gapless semimetal, excited electrons in graphene recombine far too fast for electrons to be efficiently harvested as a photocurrent. Instead, we consider the interlayer electronic relaxation process when the 2p electronic states are forced to rehybridize when stacked in a twisted bilayer graphene(tBLG) configuration. This stacking produces a prominent absorption peak originally thought to be a van Hove singularity. As an incisive space-time probe of tBLG interlayer electron population, we apply one and two-photon confocal scanning transient absorption microscopy. Our resulting ultrafast maps of electronic population directly image both bright and dark stable, strongly-bound excitons states that rule-out the van Hove singularity model. Instead, we find the constrained interlayer 2p orbitals produce two closely spaced Fano resonances that destructively interfere to produce a dark, strongly bound exciton state that is decoupled from lower-lying continuum states. This observation of strongly bound, stable excitons is a first for a 2D or 3D metallic system.

Associated Paper: H. Patel, R. Havener, L. Brown, Y. Liang, L. Yang, J. Park, M.W. Graham (2015), Nano Letters, 15, 5932-7