Confined modes at the edge arbitrarily inclined with respect to optical axes of nonmagnetic anisotropic 2D materials are considered. By developing the exact Wiener-Hopf and approximated Fetter methods we studied edge modes dispersions, field and charge density distributions. The 2D layer is described by the Lorentz-type conductivities in one or both directions, which is realistic for natural anisotropic 2D materials and resonant hyperbolic metasurfaces. We demonstrate that, due to anisotropy, the edge mode exists only at wave vectors exceeding the nonzero threshold value if the edge is tilted with respect to the direction of the resonant conductivity. The dominating contribution to field and charge density spatial profiles is provided by evanescent 2D waves, which are confined both in space near the 2D layer and along the layer near its edge. The degree of field confinement along the layer is determined by wave vector or frequency mismatch between the edge mode and continuum of freely propagating 2D modes. Our analysis is suitable for various types of polaritons (plasmon-, phonon-, exciton-polaritons etc.) at large enough wave vectors. Thanks to superior field confinement in all directions perpendicular to the edge these modes look promising for modern plasmonics and sensorics.
Motivated by the recent emergence of a new class of anisotropic 2D materials, we examine their electromagnetic modes and demonstrate that a broad class of the materials can host highly directional hyperbolic plasmons. Their propagation direction can be manipulated on-the-spot by gate doping, enabling hyperbolic beams reflection, refraction and bending. The realization of these natural 2D hyperbolic media opens up a new avenue in dynamic control of hyperbolic plasmons not possible in the 3D version.
In this paper we reveal the physics behind the formation of tri- and tetra-hyperbolic phases in anisotropic metamaterials without magnetoelectric coupling and describe the anti-crossing splitting phenomenon in the hyperbolic dispersion which arises due to the hybridization of the plasmonic and magnetic Bloch high-k polaritons. This considerably deepens the understanding of the high-k polaritons and the topology of the optical iso-frequency surfaces in k-space and will find applications in optical nano-resolution imaging and emission rate and directivity control. To accomplish this, we develop …
In plasmonics, and particularly in plasmonic photochemistry, the effect of hot-electron generation is an exciting phenomenon driving new fundamental and applied research. However, obtaining a microscopic description of the hot-electron states represents a challenging problem, limiting our capability to design efficient nanoantennas exploiting these excited carriers. This paper addresses this limitation and studies the spatial distributions of the photophysical dynamic parameters controlling the local surface photochemistry on a plasmonic nanocrystal. We found that the generation of energetic e…
In the fast growing two-dimensional (2D) materials family, anisotropic 2D materials, with their intrinsic in-plane anisotropy, exhibit a great potential in optoelectronics. One such typical material is black phosphorus (BP), with a layer-dependent and highly tunable band gap. Such intrinsic anisotropy adds a new degree of freedom to the excitation, detection and control of light. Particularly, hyperbolic plasmons with hyperbolic q-space dispersion are predicted to exist in BP films, where highly directional propagating polaritons with divergent densities of states are hosted. Combined with a t…
In this work, we study the near-field radiative heat transfer between two suspended sheets of anisotropic 2D materials. It is found that the radiative heat transfer can be enhanced with orders-of-magnitude over the blackbody limit for nanoscale separation. The enhancement is attributed to the excitation of anisotropic and hyperbolic plasmonic modes. Meanwhile, a large thermal modulation effect, depending on the twisted angle of principal axes between the upper and bottom sheets of anisotropic 2D materials, is revealed. The near-field radiative heat transfer for different concentrations of elec…
Bimetallic plasmonic nanoparticles enable tuning of the optical response and chemical stability by variation of the composition. The present numerical simulation study compares Ag-Au alloy, Ag@Au core-shell, and Au@Ag core-shell bimetallic plasmonic nanoparticles of both spherical and anisotropic (nanotriangle and nanorods) shapes. By studying both spherical and anisotropic (with LSPR in the near-infrared region) shapes, cases with and without interband transitions of Au can be decoupled. Explicit comparisons are facilitated by numerical models supported by careful validation and examination o…
We investigate unusual surface plasmons polariton (SPP) propagation and light-matter interactions in ultrathin black phosphorus (BP) films, a 2D material that exhibits exotic electrical and physical properties due to its extremely anisotropic crystal structure. Recently, it has been speculated that the ultra-confined surface plasmons supported by BP may present various topologies of wave propagation bands, ranging from anisotropic elliptic to hyperbolic, across the mid- and near-infrared regions of the electromagnetic spectrum. By carefully analyzing the natural nonlocal anisotropic optical co…
Plasmon coupling and hybridization in complex nanostructures constitutes a fertile playground for controlling light at the nanoscale. Here, we present a semi-analytical model to describe the emergence of hybrid plasmon modes guided along 2D nano-slits. In particular, we find two new coupled plasmonic resonances arising from symmetric and antisymmetric hybridizations of the edge plasmons of the constituent half-sheets. These give rise to an antibonding and a bonding mode, lying above and below the energy of the bare edge plasmon. Our treatment is notably generic, being able to account for slits…
Anisotropic nanostructured porous Pd films are fabricated using oblique angle deposition in vacuum on a glass substrate. They display a dichroic response, due to localised surface plasmon resonances (LSPR) within the nanoparticles forming the film, dependent on the incident light polarisation. Ultrasensitive hydrogen sensing is reached by using these films in conjunction with a differential optical technique derived from the reflectance anisotropy spectroscopy. The evolution of the samples optical responses is monitored during the formation of Pd hydride in both the dilute alpha-phase and the …
We study collective modes in anisotropic double layer systems. To this end, we derive a zero-$T$ dynamic polarization function for a conductor with a paraboloidal dispersion. Following this, we demonstrate the dependence of the plasmonic modes on the relative orientation, doping, and system anisotropy.
We propose multiple approaches for controlling Wien's displacement law in the near-field leading to narrowband, tunable, spatially-coherent high temperature thermal sources. We show that narrowband super-planckian thermal emission relevant to the wavelength ranges for near-field thermophotovoltaics can be achieved by utilizing engineered plasmonic states of metals with a high melting point. Along with conventional thin film surface plasmon polaritons of high temperature plasmonic metals, we also introduce multiple different approaches to narrowband near field thermal emission a) epsilon-near-z…
A comprehensive analysis of hybrid TM-TE polarized surface electromagnetic waves supported by different few-layer anisotropic metasurfaces is presented. A generalized 4$\times$4 T-matrix formalism for arbitrary anisotropic 2D layers is developed, from which the general relations for the surface waves dispersions and scattering coefficients are deduced. Using this formalism and the effective conductivity approach, the dispersions and iso-frequency contours (IFCs) topology of the surface waves in various hybrid uniaxial metasurfaces are studied. The existence of hyperbolic plasmon-exciton polari…
We extend the model of exciton-plasmon materials to include a ro-vibrational structure of molecules using wave-packet propagations on electronic potential energy surfaces. The new model replaces conventional two-level emitters with more complex molecules allowing to examine the influence of alignment and vibrational dynamics on strong coupling with surface plasmon-polaritons. We apply the model to a hybrid system comprising a thin layer of molecules placed on top of a periodic array of slits. Rigorous simulations are performed for two types of molecular systems described by vibrational bound-b…
The interaction of free electrons with electromagnetic excitation is the fundamental mechanism responsible for ultra-strong confinement of light that, in turn, enables biosensing, near-field microscopy, optical cloaking, sub-wavelength focusing, and super-resolution imaging. These unique phenomena and functionalities critically rely on the negative permittivity of optical elements resulting from the free electrons. As result, progress in nanophotonics and nano-optics is often related to the development of new negative permittivity (plasmonic) media at the optical frequency of interest. Here we…
A chain of metallic particles, of sufficiently small diameter and spacing, allows linearly polarized plasmonic waves to propagate along the chain. In this paper, we consider how these waves are altered by an anisotropic host (such as a nematic liquid crystal) or an applied magnetic field. In a liquid crystalline host, with principal axis (director) oriented either parallel or perpendicular to the chain, we find that the dispersion relations of both the longitudinal ($L$) and transverse ($T$) modes are significantly altered relative to those of an isotropic host. Furthermore, when the director …
We discuss two aspects of the plasmofluidic assembly of plasmonic nanostructures at the metal fluid interface. First, we experimentally show how three and four spot evanescent wave excitation can lead to unconventional assembly of plasmonic nanoparticles at the metal fluid interface. We observed that the pattern of assembly was mainly governed by the plasmon interference pattern at the metal fluid interface, and further led to interesting dynamic effects within the assembly. The interference patterns were corroborated by 3D finite difference time domain simulations. Secondly, we show how aniso…
Homogenization of plasmonic crystals: Seeking the epsilon-near-zero effect
Texas A&M University（德克萨斯农工大学）；John A. Paulson School of Engineering and Applied Sciences, Harvard University（哈佛大学约翰·A·保尔森工程与应用科学学院）；Department of Physics, Harvard University（哈佛大学物理系）；School of Mathematics, University of Minnesota（明尼苏达大学数学学院）；Department of Mathematics, and Institute for Physical Science and Technology, and Center for Scientific Computation and Mathematical Modeling, University of Maryland（马里兰大学数学系、物理科学与技术研究所及科学计算与数学建模中心）
We consider plasmonic metasurfaces constituted by an arbitrary periodic arrangement of spherical metallic nanoparticles. Each nanoparticle supports three degenerate dipolar localized surface plasmon (LSP) resonances. In the regime where the interparticle distance is much smaller than the optical or near-infrared wavelength associated with the LSPs, the latter couple through the dipole-dipole interaction and form collective plasmonic modes which extend over the whole metasurface. Within a Hamiltonian model which we solve exactly, we derive general expressions which enable us to extract analytic…
We study the effects of an applied in-plane uniaxial strain on the plasmon dispersions of monolayer, bilayer and double-layer phosphorene structures in the long-wavelength limit within the linear elasticity theory. In the low energy limit, these effects can be modeled through the change in the curvature of the anisotropic energy band along the armchair and zigzag directions. We derive analytical relations for the plasmon modes under uniaxial strain and show that the direction of the applied strain is important. Moreover, we observe that along the armchair direction, the changes of the plasmon …
Two-dimensional materials supporting deep-subwavelength plasmonic modes can also exhibit strong magneto-optical responses. Here, we theoretically investigate magnetoplasmons (MPs) in monolayer black phosphorus (BP) structures under moderate static magnetic fields. We consider three different structures, namely, a continuous BP monolayer, an edge formed by a semi-infinite sheet, and finally, a triangular wedge configuration. Each of these structures shows strongly anisotropic magneto-optical responses induced both by the external magnetic field and by the intrinsic anisotropy of the BP lattice.…
Layered and two-dimensional (2D) materials such as graphene, boron nitride, transition metal dichalcogenides(TMDCs), and black phosphorus (BP) have intriguing fundamental physical properties and bear promise of numerous important applications in electronics and optics. Of them, BP is a novel 2D material that has been theoretically predicted to acquire plasmonic behavior for frequencies below ~0.4 eV when highly doped. The electronic properties of BP are unique due to an anisotropic structure, which could strongly influence collective electronic excitations. Advantages of BP as a material for n…
Group-IV monochalcogenides are emerging as a new class of layered materials beyond graphene, transition metal dichalcogenides (TMDCs), and black phosphorus (BP). In this paper, we report experimental and theoretical investigations of the band structure and transport properties of GeSe and its heterostructures. We find that GeSe exhibits a markedly anisotropic electronic transport, with maximum conductance along the armchair direction. Density functional theory calculations reveal that the effective mass is 2.7 times larger along the zigzag direction than the armchair direction; this mass aniso…
We systematically investigate the properties of bulk, surface and edge plasmons in Weyl semimetals in presence of a magnetic field. It is found that unidirectional plasmons with different properties exist on different surfaces, which is in consistent with the nontrivial topology of the three-dimensional (3D) bulk plasmons. These novel plasmons possess momentum-location lock and may travel between surfaces. The anomalous Hall conductivity brings about abundant anisotropic plasmon dispersions, from linear to parabolic or even hyperbolic bands. With the help of a semiclassical picture for the for…
Strongly anisotropic media where the principal components of the dielectric tensor have opposite signs are called hyperbolic. Such materials length exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces the size-dependent limitations on optical confinement. The low-loss, mid-infrared, natural hyperbolic material, hexagonal boron nitride i…
The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, opening the possibility for manipulating material properties, but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edge…
