A hyperbolic plasmonic surface supports highly directional propagating polaritons with extremely large density of states. Such plasmon polaritons have been realized in artificially structured metasurfaces. However, the upper bound of the achievable plasmon wave vector is limited by the structure size, which calls for a natural hyperbolic surface without any structuring. Here, we experimentally demonstrate a natural hyperbolic plasmonic surface based on thin films of WTe2 in the light wavelength range of 16 to 23 microns by far infrared absorption spectroscopy. The topological transition from the elliptic to the hyperbolic regime is further manifested by mapping the iso-frequency contours of the plasmon. Moreover, the anisotropy character and plasmon frequency exhibit prominent temperature dependence. Our study demonstrates the first natural platform to host 2D hyperbolic plasmons, which opens exotic avenues for the manipulation of plasmon propagation, light-matter interaction and light emission in planar photonics.
The collective excitations of electrons in the bulk or at the surface, namely plasmons, play an important role in the properties of materials, and have generated the field of plasmonics. We report the observation of a highly unusual plasmon mode on the surface of Van der Waals heterostructures of graphene monolayer on 2D transition metal dichalcogenide substrate. Since the exponentially decaying fields of surface plasmon wave propagating along interface is highly sensitive to the ambient refractive index variations, such heterostructures are useful for ultra-sensitive bio-sensing.
Purcell enhancement of photogalvanic currents in a van der Waals plasmonic self-cavity
Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质结构与动力学研究所）；Center for Free-Electron Laser Science（自由电子激光科学中心）；Department of Physics, Columbia University（哥伦比亚大学物理系）
In this minireview, we explore the main features and the prospect of plasmonics with two-dimensional semiconductors. Plasmonic modes in each class of van der Waals semiconductors have their own peculiarities, along with potential technological capabilities. Plasmons of transition-metal dichalcogenides share features typical of graphene, due to their honeycomb structure, but with damping processes dominated by intraband rather than interband transitions, unlike graphene. Spin-orbit coupling strongly affects the plasmonic spectrum of buckled honeycomb lattices (silicene and germanene), while the…
The multi-scale computational method of combining first-principles calculation and finite element electromagnetic simulations is used to study plasmon-enhanced interlayer charge transfer (ICT) exciton of 2D lateral and van der Waals MoS2/WS2 heterostructure with 2H phase. The weak ICT excitons are observed in the 2H lateral and van der Waals MoS$_2$/WS$_2$ heterostructures. Theoretical results reveal the physical principle of plexciton resulting from strong coupling between plasmonic and ICT exciton. The weak CT exciton can be strongly enhanced by metal plasmon, can provide a new way to observ…
Plasmon polaritons in van der Waals (vdW) materials hold promise for next-generation photonics. The ability to deterministically imprint spatial patterns of high carrier density in cavities and circuitry with nanoscale features underlies future progress in nonlinear nanophotonics and strong light-matter interactions. Here, we demonstrate a general strategy to atomically imprint low-loss graphene plasmonic structures using oxidation-activated charge transfer (OCT). We cover graphene with a monolayer of WSe$_2$, which is subsequently oxidized into high work-function WOx to activate charge transf…
We analytically describe the plasmonic edge modes for an interface that involves the twisted bilayer graphene (TBG) or other similar Moire van der Waals heterostructure. For this purpose, we employ a spatially homogeneous, isotropic and frequency-dependent tensor conductivity which in principle accounts for electronic and electrostatic interlayer couplings. We predict that the edge mode dispersion relation explicitly depends on the chiral response even in the nonretarded limit, in contrast to the collective bulk plasmonic excitations in the TBG. We obtain a universal function for the dispersio…
A plasmonic coupled-cavity system, which consists of a quarter-wave coupler cavity, a resonant Fabry-Perot detector nanocavity, and an off-resonant reflector cavity, is used to enhance the localization of surface plasmons in a plasmonic detector. The coupler cavity is designed based on transmission line theory and wavelength scaling rules in the optical regime, while the reflector cavity is derived from off-resonant resonator structures to attenuate transmission of plasmonic waves. We observed strong coupling of the cavities in simulation results, with an 86% improvement of surface plasmon loc…
Integrating and manipulating the nano-optoelectronic properties of Van der Waals heterostructures can enable unprecedented platforms for photodetection and sensing. The main challenge of infrared photodetectors is to funnel the light into a small nanoscale active area and efficiently convert it into an electrical signal. Here, we overcome all of those challenges in one device, by efficient coupling of a plasmonic antenna to hyperbolic phonon-polaritons in hexagonal-BN to highly concentrate mid-infrared light into a graphene pn-junction. We balance the interplay of the absorption, electrical an…
The ability to confine light into tiny spatial dimensions is important for applications such as microscopy, sensing and nanoscale lasers. While plasmons offer an appealing avenue to confine light, Landau damping in metals imposes a trade-off between optical field confinement and losses. We show that a graphene-insulator-metal heterostructure can overcome that trade-off, and demonstrate plasmon confinement down to the ultimate limit of the lengthscale of one atom. This is achieved by far-field excitation of plasmon modes squeezed into an atomically thin hexagonal boron nitride dielectric h-BN s…
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. At larger twist angles, the plasmon wavelength of TBLG increases significantly with apparently lower damping. Further analysis and modeling indicate that the observed twist-ang…
Van der Waals heterostructures employing graphene and hexagonal boron nitride (hBN) crystals have emerged as a promising platform for plasmonics thanks to the tunability of their collective modes with carrier density and record values for plasmonics figures of merit. In this Article we investigate theoretically the role of moiré-pattern superlattices in nearly aligned graphene on hBN by using continuum-model Hamiltonians derived from ab initio calculations. We calculate the system's energy loss function for a variety of chemical potential values that are accessible in gated devices. Our calcul…
Using a unified macroscopic QED formalism, we derive an integral equation for the van der Waals energy of a two-level atomic system near a carbon nanotube. The equation is valid for both strong and weak atom-vacuum-field coupling. By solving it numerically, we demonstrate the inapplicability of weak-coupling-based van der Waals interaction models in a close vicinity of the nanotube surface.
The spatial dependence of the van der Waals (vdW) energy between graphene and hexagonal boron-nitride (h-BN) is investigated using atomistic simulations. The van der Waals energy between graphene and h-BN shows a hexagonal superlattice structure identical to the observed Moiré pattern in the local density of states (LDOS) which depends on the lattice mismatch and misorientation angle between graphene and h-BN. Our results provide atomistic features of the weak van der Waals interaction between graphene and BN which are in agreement with experiment and provide an analytical expression for the s…
Graphene constitutes one of the key elements in many functional van der Waals heterostructures. However, it has negligible optical visibility due to its monolayer nature. Here we study the visibility of graphene in various van der Waals heterostructures and include the effects of the source spectrum, oblique incidence and the spectral sensitivity of the detector to obtain a realistic model. A visibility experiment is performed at different wavelengths, resulting in a very good agreement with our calculations. This allows us to reliably predict the conditions for better visibility of graphene i…
Van der Waals heterostructures of graphene and hexagonal boron nitride feature a moiré superlattice for graphene's Dirac electrons. Here, we review the effects generated by this superlattice, including a specific miniband structure featuring gaps and secondary Dirac points, and a fractal spectrum of magnetic minibands known as Hofstadter's butterfly.
We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of \Mo* and hexagonal boron nitride (hBN). The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.
Van der Waals materials are a new platform to study two-dimensional systems, including magnetic order. Since the number of spins is relatively small, measuring the magnetization is challenging. Here we report spin Hall magnetoresistance (SMR) up to room temperature caused by the magnetic surface texture of exfoliated flakes of magnetic van der Waals materials. For the antiferromagnet FePS3 the SMR amounts to 0.1 % for an applied magnetic field of 7 T at 5 K which implies a substantial canting of the magnetic moments relative to the colinear antiferromagnetic order. The canting is substantial e…
Even if individual two-dimensional materials own various interesting and unexpected properties, the stacking of such layers leads to van der Waals solids which unite the characteristics of two dimensions with novel features originating from the interlayer interactions. In this topical review, we cover fabrication and characterization of van der Waals heterosructures with a focus on heterobilayers made of monolayers of semiconducting transition metal dichalcogenides. Experimental and theoretical techniques to investigate those heterobilayers are introduced. Most recent findings focusing on diff…
We report experimental measurements for ultrathin (< 15 nm) van der Waals heterostructures exhibiting external quantum efficiencies exceeding 50%, and show that these structures can achieve experimental absorbance > 90%. By coupling electromagnetic simulations and experimental measurements, we show that pn WSe2/MoS2 heterojunctions with vertical carrier collection can have internal photocarrier collection efficiencies exceeding 70%.
Two-dimensional (2D) van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. The efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through magnetic proximity effect (MPE). Here, we report the investigation of magnetic proximity effect in 2D CrBr3/graphene van der Waals heterostructures, which is probed by Zeeman spin Hall effect thro…
We develop a theory for interlayer tunneling in van der Waals heterostructures driven under a strong electromagnetic field, using graphene/{\it h}-BN/graphene as a paradigmatic example. Our theory predicts that strong anti-resonances appear at bias voltage values equal to an integer multiple of the light frequency. These features are found to originate from photon-assisted resonant tunneling transitions between Floquet sidebands of different graphene layers, and are unique to two-band systems due to the interplay of both intraband and interband tunneling transitions. Our results point to the p…
Indirect excitons (IXs) in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by a high binding energy making them stable at room temperature and giving the opportunity for exploring fundamental phenomena in excitonic systems and developing excitonic devices operational at high temperatures. We present the observation of IXs at room temperature in van der Waals TMD heterostructures based on monolayers of MoS$_2$ separated by atomically thin hexagonal boron nitride. The IXs realized in the TMD heterostructure have lifetimes orders of magnitude longer than lif…
Vertical stacking of monolayers via van der Waals assembly is an emerging field that opens promising routes toward engineering physical properties of two-dimensional (2D) materials. Industrial exploitation of these engineering heterostructures as robust functional materials still requires bounding their measured properties so to enhance theoretical tractability and assist in experimental designs. Specifically, the short-range attractive van der Waals forces are responsible for the adhesion of chemically inert components and are recognized to play a dominant role in the functionality of these s…
We demonstrated n-type and p-type carrier injections into a transition metal dichalcogenide (TMD) WSe2 using van der Waals (vdW) contacts of two-dimensional (2D) materials: graphite for an n-type contact and NbSe2 for a p-type contact. Instead of conventional methods such as the evaporation of metals on TMD, 2D metals were transferred onto WSe2 in order to form van der Waals contacts. With these contacts, we demonstrated a small Schottky barrier height for both carrier polarities. Our finding reveals the potential of a high-performance vdW metal/semiconductor contact for use in electronics app…
We consider the role of deformations in graphene heterostructures with hexagonal crystals (including strain, wrinkles and dislocations) on the geometrical properties of moire patterns characteristic for a pair of two incommensurate misaligned isostructural crystals. By, employing a phenomenological theory to describe generic moire perturbation in van der Waals heterostructures of graphene and hexagonal crystals we investigate the electronic properties of such heterostructure.
We investigate the force between plasmonic nanoparticle and highly excited two-level system (molecule). Usually van der Waals force between nanoscale electrically neutral systems is monotonic and attractive at moderate and larger distances and repulsive at small distances. In our system, the van der Waals force acting on molecule has optical nature. At moderate distances it is attractive as usual but its strength highly increases in a narrow distance ranges ("lacunas"). We show that quantum fluctuations of (quasi)continuum of multipole plasmons of high, nearly infinite degree altogether form e…
We studied the van der Waals interactions of two finite, solid, cylindrical rods at arbitrary angle and position with respect to each other. An analytic interpolative formula for the interaction potential energy is constructed, based on various asymptotic cases. The potential can be readily used for numerical and analytic description of multi-wall carbon nanotubes, metallic nanorods, rod-shaped colloids, or any other similar objects with significant van der Waals interactions.
Raman enhancement techniques are essential for fundamental studies in light-matter interactions and find widespread application in microelectronics, bio-chemical sensing, and clinical diagnosis. Two-dimensional (2D) materials and their van der Waals heterostructures (vdWHs) are emerging rapidly as potential platforms for Raman enhancement. Here, we experimentally demonstrate a new technique of Raman enhancement driven by nonradiative energy transfer (NRET) achieving a $10$-fold enhancement in the Raman intensity in a vertical vdWH comprising of a monolayer transition metal dichalcogenide (1L-T…
A version of the Green's functions theory of the Van der Waals forces which can be conveniently used in the presence of spatial dispersion is presented. The theory is based on the fluctuation-dissipation theorem and is valid for interacting bodies, separated by vacuum. Objections against theories acounting for the spatial dispersion are discussed.
We predict a discriminatory interaction between a chiral molecule and an achiral molecule which is mediated by a chiral body. To achieve this, we generalize the van der Waals interaction potential between two ground-state molecules with electric, magnetic and chiral response to non-trivial environments. The force is evaluated using second-order perturbation theory with an effective Hamiltonian. Chiral media enhance or reduce the free interaction via many-body interactions, making it possible to measure the chiral contributions to the van der Waals force with current technology. The van der Waa…
We derive a first--principles method of determining the van der Waals or Casimir pressure in a dissipative and dispersive planar multilayered system by calculating the Maxwell stress tensor in a fictitious layer of vacuum, that is eventually made to vanish, introduced in the structure. This is illustrated by calculating the van der Waals pressure in a thin film with dissipative properties embedded between two semi--infinite media.
The recent emergence of 2D van der Waals magnets down to atomic layer thickness provides an exciting platform for exploring quantum magnetism and spintronics applications. The van der Waals nature stabilizes the long-range ferromagnetic order as a result of magnetic anisotropy. Furthermore, giant tunneling magnetoresistance and electrical control of magnetism have been reported. However, the potential of 2D van der Waals magnets for magnonics, magnon-based spintronics, has not been explored yet. Here, we report the experimental observation of long-distance magnon transport in quasi-twodimensio…
The field of plasmonics seeks to find materials with an intensive plasmon (large plasmon pole weight) with low Landau, phonon and other losses (small decay width). In this paper we propose a new class of materials that show exceptionally good plasmonic properties. These materials consist of van der Waals stacked 'plasmon active' layers (atomically thin metallic layers) and 'supporting' layers (atomically thin wide band gap insulating layers). One such material that can be experimentally realized - lithium intercalated hexagonal boron-nitride is studied in detail. We show that its 2D plasmon in…
The van der Waals heterostructures are a fertile frontier for discovering emergent phenomena in condensed matter systems. They are constructed by stacking elements of a large library of two-dimensional materials, which couple together through van der Waals interactions. However, the number of possible combinations within this library is staggering, and fully exploring their potential is a daunting task. Here we introduce van der Waals metamaterials to rapidly prototype and screen their quantum counterparts. These layered metamaterials are designed to reshape the flow of ultrasound to mimic ele…
Van der Waals-Casimir dispersion interactions between two apposed graphene layers, a graphene layer and a substrate, and in a multilamellar graphene system are analyzed within the framework of the Lifshitz theory. This formulation hinges on a known form of the dielectric response function of an undoped or doped graphene sheet, assumed to be of a random phase approximation form. In the geometry of two apposed layers the separation dependence of the van der Waals-Casimir interaction for both types of graphene sheets is determined and compared with some well known limiting cases. In a multilamell…
There has been a huge increase of interests in two-dimensional van der Waals materials over the past ten years or so with the conspicuous absence of one particular class of materials: magnetic van der Waals systems. In this Viewpoint, we point it out and illustrate how we might be able to benefit from exploring these so-far neglected materials.
The emergence of graphene and two-dimensional van der Walls materials renewed interest to investigation of the low-frequency noise in the low-dimensional systems. The layered van der Waals materials offers unique opportunities for studying the low-frequency noise owing to the properties controlled by the thickness of these materials, and tunable carrier concentration. In this review, we describe unusual low-frequency noise phenomena in quasi-2D and quasi-1D van der Waals materials. We also demonstrate that the low-frequency noise spectroscopy is a powerful tool for investigation of the electro…
The exfoliation of two naturally occurring van der Waals minerals, graphite and molybdenite, arouse an unprecedented level of interest by the scientific community and shaped a whole new field of research: 2D materials research. Several years later, the family of van der Waals materials that can be exfoliated to isolate 2D materials keeps growing, but most of them are synthetic. Interestingly, in nature plenty of naturally occurring van der Waals minerals can be found with a wide range of chemical compositions and crystal structures whose properties are mostly unexplored so far. This Perspectiv…
Here, we propose quantum spin Hall-like phononic states in van der Waals bilayer systems with antiferromagnetic ordering. Antiferromagnetic ordering in bilayer systems with small interlayer interaction makes the total Chern number zero, where the Chern numbers for each layer are opposite to each other. In bilayer CrI3 where antiferromagnetic ordering has been experimentally demonstrated, we show that the quantum spin Hall-like states appear with spatially separated chiral edge modes.
Van der Waals interactions between flat surfaces in uniaxial anisotropic media are investigated in the nonretarded limit. The main focus is the effect of nonzero tilt between the optical axis and the surface normal on the strength of van der Waals attraction. General expressions for the van der Waals free energy are derived using the surface mode method and the transfer-matrix formalism. To facilitate numerical calculations a temperature-dependent three-band parameterization of the dielectric tensor of the liquid crystal 5CB is developed. A solid slab immersed in a liquid crystal experiences a…
Research on graphene and other two-dimensional atomic crystals is intense and likely to remain one of the hottest topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first - already remarkably complex - such heterostructures (referred to as 'van der Waals') have recently been fabricated and investigated revealing unusual properties and new phenomena. Here we review this emerging research area and attempt to identif…
We demonstrate near unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (< 15 nm) Van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of th…
The van der Waals interaction between two polarizable atoms is considered. In three dimensions the standard form with an attractive $1/|R|^6$ potential is obtained from second-order quantum perturbation theory. When the electron motion is restricted to lower dimensions (but the $1/|R|$ Coulomb potential is retained), new terms in the expansion appear and alter both the sign and the $|R|$ dependence of the interaction.
We show that for nanoparticles with giant polarizability $α$ (a prominent example being the recently studied Na$_{14}$F$_{13}$ molecular clusters) van der Waals forces are significantly enhanced at interparticle distances shorter than $R_0=(α/2πε_0)^{1/3}$. For an adequate description of this phenomenon, nonlinear effects must be taken into account. We show that, contrary to some theoretical claims, an accurate treatment of nonlinearity does not lead to any repulsive forces.
In recent years, noncollinear topological textures have long gained increasing research attentions for their high values of both fundamental researches and potential applications. The recent discovery of intrinsic orders in magnetic and polar two-dimensional van der Waals materials provides a new ideal platform for the investigation of noncollinear topological textures. Here, we review the theoretical and experimental progresses on noncollinear topological textures in two-dimensional van der Waals materials in very recent years. During these years, magnetic skyrmions of both Bloch and Néel typ…
The van der Waals interactions between two parallel graphitic nanowiggles (GNWs) are calculated using the coupled dipole method (CDM). The CDM is an efficient and accurate approach to determine such interactions explicitly by taking into account the discrete atomic structure. Our findings show that the van der Waals forces vary from attraction to repulsion as nanoribbons move along their lengths with respect to each other. This feature leads to a number of stable and unstable positions of the system during the movement process. These positions can be tuned by changing the length of GNW. Moreov…
We show that the usual sum of $R^{-6}$ contributions from elements separated by distance $R$ can give \emph{qualitatively} wrong results for the electromagnetically non-retarded van der Waals interaction between non-overlapping bodies. This occurs for anisotropic nanostructures that have a zero electronic energy gap, such as nanowires, conducting nanotubes, and nano-layered systems including metals and graphene planes. In all these cases our analytic microscopic calculations give an interaction falling off with a power of separation different from the conventional value. We discuss implication…
