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Proceedings Paper • Open Access

Ultrafast microscopy captures the dynamics of bound excitons in twisted bilayer van der Waals materials
Author(s): Hiral Patel; Kyle T. Vogt; Lujie Huang; Jiwoong Park; Matt W. Graham

Paper Abstract

Stacking and twisting 2D van der Walls (vdW) materials can create unique electronic properties that are not accessible in a single sheet of material. When two sheets of van der Waals material such as graphene are stacked in an off-axis angle, in a twisted bilayer graphene (tBLG) configuration, electronic properties are modified from interlayer orbital hybridization effects. For instance, in tBLG we can access both massless and massive chiral quasiparticles characteristics of graphene and bilayer graphene, as well as angle tunable optical resonances that are not present in graphene or bilayer graphene. In addition, first principle simulation predicts that upon optical resonant excitation of tBLG, bound exciton formation is a possibility due to cancelation of exciton-continuum coupling from anti-symmetric superposition of degenerate resonant transitions. In order to study possible bound exciton formation, we map out the electronic structure of single grain tBLG using multi-photon transient absorption microscopy. Surprisingly, upon resonant optical excitations, tBLG shows enhanced transient response with longer carrier compared to AB stacked bilayer graphene. Further, we find that the origin of this unexpected optical response can be best explained by the presence of a lower lying bound exciton state predicted by recent theoretical simulations. This suggests that tBLG is a novel 2D hybrid material that enables the creation of both strongly-bound excitons along-side highly-conductive continuum states. Recently, the family of 2D vdW materials has grown appreciably. As such, there are countless possibilities for stacking and twisting 2D vDw materials to produce similar interlayer electronic states for next generation optoelectronics.

Paper Details

Date Published: 8 May 2017
PDF: 10 pages
Proc. SPIE 10193, Ultrafast Bandgap Photonics II, 101930V (8 May 2017); doi: 10.1117/12.2262003
Show Author Affiliations
Hiral Patel, Oregon State Univ. (United States)
Kyle T. Vogt, Oregon State Univ. (United States)
Lujie Huang, Cornell Univ. (United States)
Jiwoong Park, Cornell Univ. (United States)
Matt W. Graham, Oregon State Univ. (United States)

Published in SPIE Proceedings Vol. 10193:
Ultrafast Bandgap Photonics II
Michael K. Rafailov, Editor(s)

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