title: Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons.
abstract: 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. By applying a novel combination of multiphoton transient absorption (TA) microscopy and TEM, we resolve the electronic structure and ensuing relaxation by probing resonant excitations of 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. Two-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.
authors.affiliation: Department of Physics, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Hiral
authors.initials: H
authors.lastname: Patel
authors.affiliation: Department of Chemistry & Chemical Biology, Cornell University , Ithaca, New York 14850, United States.|Kavli Institute at Cornell for Nanoscale Science , Ithaca, New York 14853, United States.
authors.forename: Robin W
authors.initials: RW
authors.lastname: Havener
authors.affiliation: Department of Chemistry & Chemical Biology, Cornell University , Ithaca, New York 14850, United States.|Kavli Institute at Cornell for Nanoscale Science , Ithaca, New York 14853, United States.
authors.forename: Lola
authors.initials: L
authors.lastname: Brown
authors.affiliation: Department of Physics, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Yufeng
authors.initials: Y
authors.lastname: Liang
authors.affiliation: Department of Physics, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Li
authors.initials: L
authors.lastname: Yang
authors.affiliation: Department of Chemistry & Chemical Biology, Cornell University , Ithaca, New York 14850, United States.|Kavli Institute at Cornell for Nanoscale Science , Ithaca, New York 14853, United States.
authors.forename: Jiwoong
authors.initials: J
authors.lastname: Park
authors.affiliation: Department of Physics, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Matt W
authors.initials: MW
authors.lastname: Graham
keywords: Graphene; excitons; ghost Fano resonance; ultrafast microscopy