Enabling propagation of anisotropic polaritons along forbidden directions via a topological transition
University of Oviedo（奥维耶多大学）；Center of Research on Nanomaterials and Nanotechnology, CINN (CSIC-Universidad de Oviedo)（纳米材料与纳米技术研究中心，CINN（西班牙国家研究委员会-奥维耶多大学））；Moscow Institute of Physics and Technology（莫斯科物理技术学院）；Institute of Science and Technology Austria（奥地利科学技术研究所）；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯基克，巴斯克科学基金会）
Jiahua Duan, Gonzalo Álvarez-Pérez, Kirill V. Voronin, Iván Prieto, Javier Taboada-Gutiérrez, Valentyn S. Volkov, Javier Martín-Sánchez, Alexey Y. Nikitin, Pablo Alonso-González
Recent discoveries of polaritons in van der Waals (vdW) crystals with directional in-plane propagation, ultra-low losses, and broad spectral tunability have opened the door for unprecedented manipulation of the flow of light at the nanoscale. However, despite their extraordinary potential for nano-optics, these unique polaritons also present an important limitation: their directional propagation is intrinsically determined by the crystal structure of the host material, which imposes forbidden directions of propagation and hinders its control. Here, we theoretically predict and experimentally demonstrate that directional polaritons (in-plane hyperbolic phonon polaritons) in a vdW biaxial slab (alpha-phase molybdenum trioxide) can be steered along previously forbidden directions by inducing an optical topological transition, which naturally emerges when placing the slab on a substrate with a given negative permittivity (4H-SiC). Importantly, due to the low-loss nature of this topological transition, we are able to visualize in real space exotic intermediate polaritonic states between mutually orthogonal hyperbolic regimes, which permit to unveil the unique topological origin of the transition. This work provides new insights into the emergence of low-loss optical topological transitions in vdW crystals, offering a novel route to efficiently steer the flow of energy at the nanoscale.
The morphology of a columnar thin film (CTF) of silver renders it an effectively biaxially anisotropic continuum. CTFs of silver deposited on one-dimensional gratings of photoresist showed strong blazing action and asymmetrically coupled optical radiation to surface plasmon-polariton (SPP) waves propagating only along one direction supported by either the CTF/photoresist or the CTF/air interfaces. Homogenization of the CTFs using the Bruggeman formalism revealed them to display hyperbolic dispersion, and the dispersion of SPP waves was adequately described thereby.
We present a generalized 4 $\times$ 4 matrix formalism for the description of light propagation in birefringent stratified media. In contrast to previous work, our algorithm is capable of treating arbitrarily anisotropic or isotropic, absorbing or non-absorbing materials and is free of discontinous solutions. We calculate the reflection and transmission coefficients and derive equations for the electric field distribution for any number of layers. The algorithm is easily comprehensible and can be straight forwardly implemented in a computer program. To demonstrate the capabilities of the appro…
Ultrafast Control of Lifetime in High Q Anisotropic Plasmon Polaritons
Photonics Initiative, Advanced Science Research Center, City University of New York（纽约市立大学先进科学研究中心光子学倡议）；Center for Nano Science and Technology, Fondazione Istituto Italiano di Tecnologia（意大利技术研究院纳米科学与技术中心）；Physics Program, Graduate Center, City University of New York（纽约市立大学研究生院物理学项目）
Direct observation of anisotropic surface phonon polaritons on α-quartz
Chuo University（中央大学）；Hokkaido University（北海道大学）；National Institute of Informatics（国立情报学研究所）；Kanagawa Institute of Industrial Science and Technology（神奈川县工业技术研究机构）
Launching of Visible-Range Hyperbolic Polaritons by Gold Nanoantennas in a natural van der Waals crystal
Donostia International Physics Center (DIPC)（多诺西亚国际物理中心）；Department of Physics, University of Oviedo（奥维耶多大学物理系）；Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU)（巴斯克地区大学）；Center of Research on Nanomaterials and Nanotechnology, CINN (CSIC–Universidad de Oviedo)（纳米材料与纳米技术中心，CINN（西班牙国家研究委员会-奥维耶多大学））；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯科奎，巴斯克科学基金会）
Perhaps the most significant progress to the field of infrared optics and nanophotonics has been made through the real space realisation of polaritons in two-dimensional materials that provide maximum light confinement functionalities. The recent breakthrough discovery of in-plane hyperbolicity in the natural van der Waals material has revealed a most exciting optical property which enable an in-plane anisotropic dispersion. Yet, the most intriguing feature of in-plane anisotropic dispersion is the manipulation of polaritons at the nano scale. This development has opened a new window of opport…
The canonical boundary-value problem for surface-plasmon-polariton (SPP) waves guided by the planar interface of a dielectric material and a plasmonic material was solved for cases wherein either partnering material could be a uniaxial material with optic axis lying in the interface plane.Numerical studies revealed that two different SPP waves, with different phase speeds, propagation lengths, and penetration depths, can propagate in a given direction in the interface plane; in contrast, the planar interface of isotropic partnering materials supports only one SPP wave for each propagation dire…
We demonstrate all-optical linear polarization control of exciton-polariton condensates in anisotropic elliptical optical traps. The cavity inherent TE-TM splitting lifts the ground state spin degeneracy with emerging fine structure modes polarized linear parallel and perpendicular to the trap major axis with the condensate populating the latter. Our findings show a new type of polarization control with exciting perspectives in both spinoptronics and studies on extended systems of interacting nonlinear optical elements with anisotropic coupling strength and adjustable fine structure.
V-shaped nanoantennas are among the popular choices for the unit element of a metasurface, a nanostructured surface used for its ability to mold and control the wavefront of light. In general, the motivation for choosing the V-antenna as the unit element comes from its bimodal nature, where the introduction of the second mode offers extra control over the scattered wavefronts. Here, through near-field scanning optical microscopy, we study a 1D metastructure comprised of V-antennas in the context of generating asymmetric surface plasmon polariton (SPP) wavefronts. The key point is that the use …
Light-matter coupling in van der Waal's materials holds significant promise in realizing Bosonic condensation and superfluidity. The underlying semiconductor's crystal asymmetry, if any, can be utilized to form anisotropic half-light half-matter quasiparticles. We demonstrate generation of such highly anisotropic exciton-polaritons at the interface of a biaxial layered semiconductor, stacked on top of a distributed Bragg reflector. The spatially confined photonic mode in this geometry couples with polarized excitons and their Rydberg states, creating a system of highly anisotropic polariton ma…
Exciton polaritons in semiconductor microcavities exhibit many fundamental physical effects, with some of them amenable to being controlled by external fields. The polariton transport is affected by the polaritonic spin-orbit interaction, which is caused by the splitting of transverse-electric and transverse-magnetic (TE-TM) modes. This is the basis for a polaritonic Hall effect, called optical spin Hall effect (OSHE), which is related to the formation of spin/polarization textures in momentum space, determining anisotropic ballistic transport, as well as related textures in real space. Owing …
In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a lot of research interest due to their capability of shaping nanoscale field distributions and controlling nanophotonic energy flows. Here we report a nano-optical imaging study of waveguide exciton polaritons (EPs) in tin sulfide (SnS) in the near-infrared (IR) region using the scattering-type scanning near-field optical microscopy (s-SNOM). With s-SNOM, we mapped in real space the propagative EPs in SnS, which show sensitive dependence on the excitation energy and sample thickness. Moreover, we found …
Compression dramatically changes the transport and localization properties of graphene. This is intimately related to the change of symmetry of the Dirac cone when the particle hopping is different along different directions of the lattice. In particular, for a critical compression, a semi-Dirac cone is formed with massless and massive dispersions along perpendicular directions. Here we show direct evidence of the highly anisotropic transport of polaritons in a honeycomb lattice of coupled micropillars implementing a semi-Dirac cone. If we optically induce a vacancy-like defect in the lattice,…
As one of the most fundamental optical phenomena, refraction between isotropic media is characterized by light bending towards the normal to the boundary when passing from a low- to a high-refractive-index medium. However, in anisotropic media, refraction is a much more exotic phenomenon. The most general case of refraction between two anisotropic media remains unexplored, particularly in natural media and at the nanoscale. Here, we visualize and comprehensively study refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media, and, importantly, we do it with the us…
Polaritons in polar biaxial crystals with extreme anisotropy offer a promising route to manipulate nanoscale light-matter interactions. The dynamical modulation of their dispersion is great significance for future integrated nano-optics but remains challenging. Here, we report a momentum-directed strategy, a coupling between the modes with extra momentum supported by the interface and in-plane hyperbolic polaritons, to tailor topological transitions of anisotropic polaritons in biaxial crystals. We experimentally demonstrate such tailored polaritons at the interface of heterostructures between…
Spatiotemporal Visualization of Long-Range Anisotropic Plasmon Polaritons in Hyperbolic MoOCl2
The University of Chicago（芝加哥大学）；James Franck Institute, The University of Chicago（詹姆斯·弗兰克研究所，芝加哥大学）；Department of Chemistry, The University of Chicago（化学系，芝加哥大学）；Department of Physics, The University of Chicago（物理系，芝加哥大学）
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 …
Organic exciton-photon polariton lasers have recently been shown to be remarkable candidates for the realization of efficient sources of coherent light operating at room temperature. While their thresholds are now comparable with conventional organic photon lasers, tuning of molecular conformation and orientation as a means to further enhance their performance remains largely unexplored. Here, we first report a two-fold reduction in the threshold of a polariton laser based on a high-Q microcavity filled with an active layer of poly(9,9-dioctylfluorene) (PFO) when 15% β-phase conformation is in…
Active tuning of highly anisotropic phonon polaritons in van der Waals crystal slabs by gated graphene
University of Oviedo（奥维耶多大学）；Center of Research on Nanomaterials and Nanotechnology, CINN (CSIC-Universidad de Oviedo)（纳米材料与纳米技术研究中心，CINN（CSIC-奥维耶多大学））；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；Moscow Institute of Physics and Technology（莫斯科物理与技术学院）；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯基克，巴斯克科学基金会）
Anisotropic crystals have recently attracted considerable attention because of their ability to support polaritons with a variety of unique properties, such as hyperbolic dispersion, negative phase velocity, or extreme confinement. Particularly, the biaxial crystal $α$-MoO$_3$ has been demonstrated to support phonon polaritons, light coupled to lattice vibrations, with in-plane anisotropic propagation and unusually long lifetime. However, the lack of theoretical studies on electromagnetic modes in biaxial crystal slabs impedes a complete interpretation of the experimental data, as well as an e…
We present the characteristics of a simple waveguiding structure constructed by anisotropic birefringent crystal-metal-chiral medium, anisotropic-metal-chiral in short, and reveal the chiral-dependent dispersion and propagation properties of the surface plasmon polaritons (SPPs). We demonstrate its remarkable discrimination ability to the magnitude and sign of both the real and imaginary part of the chirality parameter. The anisotropy plays a key role in such performance and shows tuneable ability in enantiomeric discrimination even when the chirality parameter is complex-valued. Most importan…
Natural hyperbolic materials (HMs) in two dimensions (2D) have an extraordinarily high anisotropy and a hyperbolic dispersion relation. Some of them can even sustain hyperbolic polaritons with great directional propagation and light compression to deeply sub-wavelength scales due to their inherent anisotropy. Herein, the anisotropic optical features of 2D natural HMs are reviewed. Four hyperbolic polaritons (i.e., phonon polaritons, plasmon polaritons, exciton-polaritons, and shear polaritons) as well as their generation mechanism are discussed in detail. The natural merits of 2D HMs hold prom…
Terahertz (THz) electromagnetic radiation is key to optically access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging between optics and solid-state physics. Confinement of THz light to the nanometer length scale is desirable for local probing of such excitations in low dimensional systems, thereby inherently circumventing the large footprint and low spectral density of far-field THz optics. For that purpose, phonon polaritons (PhPs, i.e., light coupled to lattice vibrations in polar crystals) in anisotropic van der Waals (vdW)…
Metasurfaces with strongly anisotropic optical properties can support deep subwavelength-scale confined electromagnetic waves (polaritons) that promise opportunities for controlling light in photonic and optoelectronic applications. We develop a mid-infrared hyperbolic metasurface by nanostructuring a thin layer of hexagonal boron nitride supporting deep subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic dispersion. By applying an infrared nanoimaging technique, we visualize the concave (anomalous) wavefronts of a diverging polariton beam, which represent a landmark …
Topological photonics provides an important platform for the development of photonic devices with robust disorder-immune light transport and controllable helicity. Mixing photons with excitons (or polaritons) gives rise to nontrivial polaritonic bands with chiral modes, allowing the manipulation of helical lasers in strongly coupled light-matter systems. In this work, we demonstrate helical polariton lasing from topological valleys of an organic anisotropic microcrystalline cavity based on tailored local nontrivial band geometry. This polariton laser emits light of different helicity along dif…
Twistoptics in Planar Heterostructures with an Arbitrary Number of Rotated 3D Thin Layers and 2D Conductive Sheets
University of Oviedo（奥维耶多大学）；Center of Research on Nanomaterials and Nanotechnology CINN (CSIC–Universidad de Oviedo)（纳米材料与纳米技术中心CINN（西班牙国家研究委员会-奥维耶多大学））；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；Center for Biomolecular Nanotechnologies, Istituto Italiano di Tecnologia（意大利理工学院生物分子纳米技术研究中心）；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯科奎，巴斯克科学基金会）
Phonon polaritons in natural anisotropic crystals hold great promise for infrared nano-optics. However, the direct electrical control of these polaritons is difficult, preventing the development of active polaritonic devices. Here we propose the heterostructures of graphene on a biaxial crystal (α-phase molybdenum trioxide) slab and theoretically study the hybridized plasmon-phonon polaritons with dependence on the Fermi level of graphene from three aspects: dispersion relationships, iso-frequency contours, and the quantum spin Hall effects. We demonstrate the distinct wavelength tunability of…
Polaritons, coupled excitations of photons and dipolar matter excitations, can propagate along anisotropic metasurfaces with either hyperbolic or elliptical dispersion. At the transition from hyperbolic to elliptical dispersion (corresponding to a topological transition), various intriguing phenomena are found, such as an enhancement of the photonic density of states, polariton canalization and hyperlensing. Here we investigate theoretically and experimentally the topological transition and the polaritonic coupling of deeply subwavelength elements in a uniaxial infrared-phononic metasurface, a…
Polar van der Waals (vdW) crystals that support phonon polaritons have recently attracted much attention because they can confine infrared and terahertz (THz) light to deeply subwavelength dimensions, allowing for the guiding and manipulation of light at the nanoscale. The practical applications of these crystals in devices rely strongly on deterministic engineering of their spatially localized electromagnetic field distributions, which has remained challenging. This study demonstrates that polariton interference can be enhanced and tailored by patterning the vdW crystal α-MoO3 into microstruc…
Hyperbolic crystals like α-MoO$_3$ can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Ra…
We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index (1.52). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcou…
Phonon polaritons (PhPs),light coupled to lattice vibrations,with in-plane hyperbolic dispersion exhibit ray-like propagation with large wavevectors and enhanced density of optical states along certain directions on a surface. As such, they have raised a surge of interest as they promise unprecedented possibilities for the manipulation of infrared light with planar circuitry and at the nanoscale. Here, we demonstrate, for the first time, the focusing of in-plane hyperbolic PhPs propagating along thin slabs of MoO3. To that end, we developed metallic nanoantennas of convex geometries for both t…
Van der Waals heterostructures assembled from layers of 2D materials have attracted considerable interest due to their novel optical and electrical properties. Here we report a scattering-type scanning near field optical microscopy study of hexagonal boron nitride on black phosphorous (h-BN/BP) heterostructures, demonstrating the first direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures. Strikingly, the measured in-plane optical anisotropy along armchair and zigzag crystal axes exceeds the ratio of refractive indices of BP in the x-y plane. We explain that…
Hyperbolic metasurfaces based on van der Waals (vdW) materials support propagation of extremely anisotropic polaritons towards nanoscale light compression and manipulation, and thus has great potential in the applications of planar hyperlens, nanolasing, quantum optics and ultrasensitive infrared spectroscopy. Two-dimensional hexagonal boron nitride (h-BN) as a vdW metasurface can manipulate the propagation of hyperbolic polaritons at the level of single atomic layers, possessing higher degree of field confinement and lower losses than the conventional media. However, active manipulation of hy…
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 …
Optically reconfigurable canalization of exciton-polaritons in a non-hyperbolic perovskite
Nanyang Technological University（南洋理工大学）；B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine（乌克兰国家科学院B.维尔金低温物理与工程研究所）；Institut Universitaire de France (IUF)（法兰西学院）；National University of Singapore（新加坡国立大学）
The exploitation of phonon-polaritons in nanostructured materials offers a pathway to manipulate infrared (IR) light for nanophotonic applications. Notably, hyperbolic phonons polaritons (HP2) in polar bidimensional crystals have been used to demonstrate strong electromagnetic field confinement, ultraslow group velocities, and long lifetimes (~ up to 8 ps). Here we present nanobelts of α-phase molybdenum trioxide (α-MoO3) as a low-dimensional medium supporting HP2 modes in the mid- and far-IR ranges. By real-space nanoimaging, with IR illuminations provided by synchrotron and tunable lasers, w…
Recent discoveries have shown that when two layers of van der Waals (vdW) materials are superimposed with a relative twist angle between their respective in-plane principal axes, the electronic properties of the coupled system can be dramatically altered. Here, we demonstrate that a similar concept can be extended to the optics realm, particularly to propagating polaritons, hybrid light-matter interactions. To do this, we fabricate stacks composed of two twisted slabs of a polar vdW crystal (MoO3) supporting low-loss anisotropic phonon polaritons (PhPs), and image the propagation of the latter…
