Photonic spin Hall effect in bilayer graphene moiré superlattices

Abstract

The formation of a superstructure -with a related Moiré pattern -plays a crucial role in the extraordinary optical and electronic properties of twisted bilayer graphene, including the recently observed unconventional superconductivity. Here we put forward a novel, interdisciplinary approach to determine the Moiré angle in twisted bilayer graphene based on the photonic spin Hall effect. We show that the photonic spin Hall effect exhibits clear fingerprints of the underlying Moiré pattern, and the associated light beam shifts are well beyond current experimental sensitivities in the nearinfrared and visible ranges. By discovering the dependence of the frequency position of the maximal photonic spin Hall effect shift on the Moiré angle, we argue that the latter could be unequivocally accessed via all-optical far-field measurements. We also disclose that, when combined with the Goos-Hänchen effect, the spin Hall effect of light enables the complete determination of the electronic conductivity of the bilayer. Altogether our findings demonstrate that sub-wavelength spin-orbit interactions of light provide a unprecedented toolset for investigating optoelectronic properties of multilayer two-dimensional van der Waals materials.At macroscopic scales geometric optics provides an adequate description of several photonic phenomena. However, in the sub-wavelength regime light's spatial and polarization degrees of freedom are not independent quantities, resulting in deviations from the traditional ray optics picture due to optical spin-orbit interactions 1-10 . The scattered electromagnetic field due to a linearly polarized finite width beam impinging on an interface develops a non-trivial spin texture arising from the shift of photons with contrary helicity to opposite edges of the beam cross section, a phenomenon known as the spin Hall effect of light (SHEL) 4,7-9 . The SHEL is ubiquitous to any interface and represents a remarkable failure of geometric optics at the nanoscale. Recent experimental demonstrations have shown that the SHEL is uniquely suited for applications in precision metrology, including nanoprobing 11 , thin films characterization 12,13 , mapping of absorption mechanisms in bulk semiconductors 14,15 , and multilayer graphene identification in the absence of interlayer coupling 16 . More recently, it has been proposed that the SHEL can be used to probe the quantum Hall effect in monolayer graphene 17,18 , topological phase transitions in the expanded graphene family 19 , and hyperbolic behavior in anisotropic 2D atomic crystals 20 .In recent years, another class of 2D materials has drawn a lot attention, namely van der Waals structures assembled together layer-by-layer with controllable sequence and orientation 21 . These structures exhibit unusual physical properties that cannot be found in either monolayers or in bulk. Twisted Bilayer Graphene (TBG) is one important example of these multilayer materials, where the twist produces a Moiré pattern and induces a static periodic potential from the coupling bet...

Giant photonic spin Hall effect near the Dirac points

“…Thus the emergence of photonic topological metacrystal render a important foundation to investigate photonic SHE at the unique optical interface. Unlike graphene [26][27][28] and silene [29], 3D photonic topological metacrystal are nonplanar and and possess intrinsic spin-orbit coupling that results in unique band structure. These structures, especially for Dirac points and Weyl points, exhibit unusual physical properties that cannot be found in either monolayers or in bulk materials.…”

Consequences of chirality on the response of materials

“…The effect is not exclusive to chiral materials but can also be observed for achiral materials provided that the geometry of the optical experiment itself is chiral. 55 Interesting from an application perspective is that the effect occurs in systems where helicity can be engineered such as metamaterials 56,57 and bilayer graphene [58][59][60] and may allow for new and unconventional optical elements for manipulating the polarization of light. 61…”

“…Interesting from an application perspective is that the effect occurs in systems where helicity can be engineered such as metamaterials 56,57 and bilayer graphene 58–60 and may allow for new and unconventional optical elements for manipulating the polarization of light. 61…”

Electric Field Induced Twisted Bilayer Graphene Infrared Plasmon Spectrum

“…In the superlattice of TBG, due to the strong electronic correlation of the system, the optical properties of the superlattice also have novel characteristics, such as the enhancement of fluorescence based on the corner [ 10 ], the enhancement of photochemical reaction activity [ 11 ], optoelectronics and light voltage regulation [ 12 ], optical spin Hall effect, etc. The emergence of plasmon characteristics based on the superlattice rotation angle and the study of chirality are especially more excellent [ 13 , 14 , 15 ]. Therefore, in TBG, the coupling and interaction of electrons between layers mainly depends on the distortions between the individual atomic lattices in each layer.…”