Real-Space Imaging of the Tailored Plasmons in Twisted Bilayer Graphene

Abstract

We report a systematic plasmonic study of twisted bilayer graphene (TBLG)two graphene layers stacked with a twist angle. Through real-space nanoimaging of TBLG single crystals with a wide distribution of twist angles, we find that TBLG supports confined infrared plasmons that are sensitively dependent on the twist angle. At small twist angles, TBLG has a plasmon wavelength comparable to that of single-layer graphene (SLG). At larger twist angles, the plasmon wavelength of TBLG increases significantly with apparently lower damping. Further analysis and modeling indicate that the observed twist-angle-dependence of TBLG plasmons in the Dirac linear regime is mainly due to the Fermi-velocity renormalization, a direct consequence of interlayer electronic coupling. Our work unveils the tailored plasmonic characteristics of TBLG and deepens our understanding of the intriguing nano-optical physics in novel van der Waals (vdW) coupled two-dimensional (2D) materials. Main textGraphene Dirac plasmons [1-6], which are collective oscillations of Dirac fermions in graphene, have been widely investigated in recent years by using both the electron energy loss spectroscopy [7-9] and optical imaging/spectroscopy [10][11][12][13][14][15][16][17][18][19][20][21] techniques. These quasiparticles demonstrate many superior characteristics including high confinement, long lifetime, strong field enhancement, broad spectral range, electrical tunability and a broad spectral range from terahertz to infrared .

Functional Mid‐Infrared Polaritonics in van der Waals Crystals

“…When the twist angle is large (θ ≳ 3°), the period of the moiré superlattice is small enough to consider the twisted bilayer graphene (TBG) as a uniform material, having continuous band structure which depends on the twist angle (Figure b) and can be electrostatically tuned . Accordingly, the plasmonic dispersion in TBG also depends on θ. Hu et al probed the infrared graphene plasmons in single‐layer graphene (SLG) and TBG with different twist angles using the s‐SNOM at excitation wavelength λ 0 ≈ 11 µm (Figure c). According to the measurements, the plasmon dispersion in TBG is close to that of SLG at smaller angles, while shifting to larger plasmonic wavelengths (and hence, lower damping) as the angle increases .…”

“…Accordingly, the plasmonic dispersion in TBG also depends on θ. Hu et al probed the infrared graphene plasmons in single‐layer graphene (SLG) and TBG with different twist angles using the s‐SNOM at excitation wavelength λ 0 ≈ 11 µm (Figure c). According to the measurements, the plasmon dispersion in TBG is close to that of SLG at smaller angles, while shifting to larger plasmonic wavelengths (and hence, lower damping) as the angle increases . From the analytical model for conductivity in both cases, the estimated SLG plasmon wavelength was λ P SLG ≈ 279 nm, while the plasmon wavelength in TBG was λ P TBG ≈ 393, 387, 340, and 278 nm for θ ≈ 27°, 12°, 5°, and 3°, respectively.…”

“…a) Schematics of the moiré superlattice in TBG with twist angle θ, with period indicated by the red arrow. Adapted with permission . Copyright 2017, American Physical Society.…”

Scaling and Reflection Behaviors of Polaritons in Low‐Dimensional Materials

“…i) The scaling behavior of plasmons in TBLG. Both the Fermi velocity v F and the GP wavelength λ p decrease with decreasing twist angle θ. Reproduced with permission . Copyright 2017, American Physical Society.…”

“…Furthermore, the plasmon wavelength of TBLG sensitively changes with the twist angle θ as well. In TBLG, the plasmon wavelength is directly related to the carrier density as well as the twist angle θ via …”