When two sheets of a 2D material such as graphene are stacked at a twisted angle, the 2p orbitals rehybridize, giving angle-tunable absorption resonances. By comparing the ultrafast intra-band and multi-photon transient absorption spectra, our results agree best with a recent theoretical model that predict that overlapping interlayer 2p orbitals interfere to produce symmetrized bound exciton states that are decoupled from lower lying continuum states. We further map out the excited state manifold of exciton fine states using 2-photon photoluminescence microscopy. Our spectral analysis suggests the observed photoluminescence emission is a bright exciton state that is thermally populated by a lower-lying, long lived dark-exciton state. For this dark state, both our ultrafast transient absorption and two-photon photoluminescence studies support a large binding energy of the order of ~0.6 eV. If resonantly excited, we believe twisted bilayer graphene is the first 2D metallic material that can form stable, strongly bound excitons. Our results support that twisted bilayer graphene and similar materials are novel example of 2D hybrid material, where resonantly excited excitons are strongly-bound and quasi-stable from inherently weak coupling to the continuum metallic states. The formation mechanism of this dark state is very general, and holds exciting promise for other twisted stacked 2D materials.

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