Chiral plasmon in gapped Dirac systems

Abstract

We study the electromagnetic response and surface electromagnetic modes in a generic gapped Dirac material under pumping with circularly polarized light. The valley imbalance due to pumping leads to a net Berry curvature, giving rise to a finite transverse conductivity. We discuss the appearance of nonreciprocal chiral edge modes, their hybridization and waveguiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays. DOI: 10.1103/PhysRevB.93.041413 Introduction. The Berry curvature is a topological property of the Bloch energy band and acts as an effective magnetic field in momentum space [1][2][3]. Hence, topological materials may exhibit anomalous Hall-like transverse currents in the presence of an applied electric field, in the absence of a magnetic field. Examples includes topological insulators [4] with propagating surface states that are protected against backscattering from disorder and impurities and transition metal dichalcogenides where the two valleys carry opposite Berry curvature giving rise to bulk topological charge neutral valley currents [5,6]. These bulk topological currents were also experimentally investigated in other Dirac materials, such as a gapped graphene and bilayer graphene system [7,8]. The electromagnetic response of these gapped Dirac systems, particularly that due to its surface electromagnetic modes (i.e., plasmons), are relative unexplored.In gapped graphene or monolayer transition metal dichalcogenides, electrons in the two valleys have opposite Berry curvature, ensured by time-reversal symmetry (TRS) of their chiral Hamiltonians [5]. Hence, far field light scattering properties of these atomically thin systems does not differentiate between circularly polarized light, i.e., zero circular dichroism in the classical sense. Optical pumping with circularly polarized light naturally breaks TRS, and a net planar chirality ensues. However, under typical experimental conditions, the transverse conductivity due to Berry curvature is less than the quantized conductivity e 2 / , and the associated optical dichroism effect is not prominent. These effects, however, can potentially be amplified through enhanced light-matter interaction with plasmons [9][10][11][12][13].In this Rapid Communication, we discuss the emergence of chiral electromagnetic plasmonic modes and their associated optical dichroism effect. We consider a gapped Dirac system under continuous pumping with circularly polarized light. We discuss the appearance of edge chiral plasmons and how they can allow launching of one-way propagating edge plasmons in a semi-infinite geometry. We also consider the hybridization of these chiral edge modes in a nanoribbon geometry and the possibility of nonreciprocal waveguiding. Their far-field optical properties reveal resonant absorption accompanied by sizable polarization rotation.

Towards properties on demand in quantum materials

“…Recently, the aforementioned strategies for creating topologically protected electronic edge/surface states have been extended to other types of excitations. For example, chiral plasmons-one-way-propagating collective oscillations of the itinerant electron sea-have been proposed to occur at the edges of anomalous Hall metals and other multivalley conducting media where both time-reversal and inversion symmetries are broken 27,150 . Plasmon-polariton chirality is a direct consequence of non-vanishing Berry curvature in a medium that can be induced on demand by driving, for example, valley degenerate semiconductors with circularly polarized light.…”

Real-space mapping of tailored sheet and edge plasmons in graphene nanoresonators

“…Further, we believe that the developed understanding of grapheneplasmonic near-field contrasts is broadly applicable to other 2D materials. Among others, near-field microscopy could be applied for exploring polariton edge modes in thin films of van-der-Waals crystals, plasmons in nanopatterned topological insulators, non-reciprocal 1D plasmons 32,33 or plasmons and phonons in mid-infrared and terahertz detectors based on 2D materials and heterostructures.…”

Hybrid surface waves in two-dimensional Rashba-Dresselhaus materials

“…In this setting, Maxwell's equations allow solutions in the form of two orthogonal modes representing the TE state with nonzero (H x , E y , H z ) and the transverse magnetic (TM) state with (E x , H y , E z ), respectively. The general solution of the source-free Maxwell equations with boundary conditions e z × (E + − E − ) = 0 and e z × (H + − H − ) = 4π c σE (here, E stands for the components of the electric field parallel to the plane of a quantum well) can be searched in the form of a linear superposition of the TE and TM surface modes 14,18,19,38 ,…”

“…In this geometry, a surface electromagnetic mode is formed, because the tangential component of the electric field at the conducting interface generates surface current density which leads to a discontinuity in the tangential magnetic field [15][16][17] . Examples of such a conducting interface are two-dimensional electron gas (2DEG) in quantum-well structures or electrons in graphene characterized by the linear dispersion relation 14,18,19 . In this paper we focus on a theoretical analysis of the optical response and propagation properties of hybrid surface electromagnetic waves in thin film semiconductors with a spin-orbit interaction (SOI) of the Rashba and Dresselhaus type.…”