Hybridized Hyperbolic Surface Phonon Polaritons at α-MoO3 and Polar Dielectric Interfaces

Multiple Hyperbolic Dispersion Branches and Broadband Canalization in a Phonon-Polaritonic Heterostructure

Hyperbolic polaritons in anisotropic crystals hold great promise for guiding the flow of light at deep-subwavelength scales. However, conventional hyperbolic dispersion with a single pair of symmetric branches inherently confines polaritons to propagate only within specific spatial directions. Here we demonstrate a multibranch in-plane hyperbolic dispersion in a phonon-polaritonic heterostructure composed of α-phase molybdenum trioxide (α-MoO3) and 4H-silicon carbide (4H-SiC). Leveraging the in-plane hyperbolicity of α-MoO3 and the interlayer coupling with 4H-SiC, the polaritons manifest distinct dispersive responses along the mutually orthogonal crystal directions of α-MoO3, enabling asymmetric multidirectional polariton propagation. Furthermore, the dispersion contours of polaritons along the [100] crystal direction of α-MoO3 evolves into flat bands as the frequency decreases, yielding broadband polariton canalization in the low-frequency region. These findings deepen our understanding of the evolution of polariton dispersions in α-MoO3/4H-SiC heterostructures and highlight the potential of this phonon-polaritonic heterostructure as a versatile platform for nanolight manipulation.

Canalization-based super-resolution imaging using an individual van der Waals thin layer

Canalization is an optical phenomenon that enables unidirectional light propagation without predefined waveguiding designs. Recently demonstrated using phonon polaritons in twisted van der Waals (vdW) layers of α-MoO3, it offers unprecedented possibilities for controlling light-matter interactions at the nanoscale. However, practical applications have been hindered by the complex sample fabrication of twisted stacks. In this work, we introduce a previously unexplored canalization phenomenon in a single-thin vdW layer (α-MoO3) interfaced with a substrate exhibiting a given negative permittivity. This enables a proof-of-concept application of polariton canalization: super-resolution nanoimaging (~λ0/220). Canalization-based imaging transcends conventional projection constraints, allowing the super-resolution images to be obtained at any desired location in the image plane. This versatility stems from the synergetic manipulation of three key parameters: incident frequency, rotation angle of the thin vdW layer, and thickness. Our results provide insights into the properties of canalization and constitute a seminal step toward multifaceted photonic applications, including imaging, data transmission, and ultracompact photonic integration.

Plasmonic resonances of metallic moiré superlattices in the infrared range

The recent surge of interest in moiré photonics arises from the possibility of exploring many groundbreaking physical phenomena in photonics. These phenomena include photonic topological states and magic-angle lasing, which offer an attractive platform for manipulating the flow and confinement of light from remarkably simple device geometries. In this work, we fabricate a series of metallic moiré superlattices supporting moiré plasmon polaritons and explore the moiré-potential induced plasmonic resonances. We demonstrate that two-dimensional moiré plasmonic superlattices exhibit transmittance and polarization-dependent responses because of the localized plasmonic resonances in the infrared range, whose modes have a near-flat dispersion band. Our findings hold the potential for the understanding of localized plasmonic resonances within moiré superlattices.

Modulation of surface phonon polaritons in MoO3 via dynamic doping of SiC substrate

Polar biaxial crystals with extreme anisotropy hold promise for the spatial control and the manipulation of polaritons, as they can undergo topological transitions. However, taking advantage of these unique properties for nanophotonic devices requires to find mechanisms to modulate dynamically the material response. Here, we present a study on the propagation of surface phonon polaritons (SPhPs) in a photonic architecture based on a thin layer of α-MoO3 deposited on a semiconducting 4H-SiC substrate, whose carrier density can be tuned through photoinduction. By employing this system, we establish a comprehensive polaritonic platform where the propagation of the hybridized SPhPs can be manipulated dynamically due to their coupling with the electron plasma. Specifically, we demonstrate that increasing the doping of the 4H-SiC substrate allows for modulating the on/off switch behavior of SPhP propagation or its controlled canalization. Furthermore, this modulation leads to a notable increase in the Purcell factor, primarily attributed to the doping-induced flat dispersion curve creating ultra-slow light. These findings have significant implications for the development of nanophotonic and quantum technologies, as they enable the utilization of polaritonic materials exclusively.

Monolithically Structured van der Waals Materials for Volume-Polariton Refraction and Focusing

Polaritons, hybrid light and matter waves, offer a platform for subwavelength on-chip light manipulation. Recent works on planar refraction and focusing of polaritons all rely on heterogeneous components with different refractive indices. A fundamental question, thus, arises whether it is possible to configure two-dimensional monolithic polariton lenses based on a single medium. Here, we design and fabricate a type of monolithic polariton lens by directly sculpting an individual hyperbolic van der Waals crystal. The in-plane polariton focusing through sculptured step-terraces is triggered by geometry-induced symmetry breaking of momentum matching in polariton refractions. We show that the monolithic polariton lenses can be robustly tuned by the rise of van der Waals terraces and their curvatures, achieving a subwavelength focusing resolution down to 10% of the free-space light wavelength. Fusing with transformation optics, monolithic polariton lenses with gradient effective refractive indices, such as Luneburg lenses and Maxwell's fisheye lenses, are expected by sculpting polaritonic structures with gradually varied depths. Our results bear potential in planar subwavelength lenses, integrated optical circuits, and photonic chips.

Casimir interaction driven by hyperbolic polaritons

Casimir interaction is an intriguing phenomenon that is induced by electromagnetic quantum fluctuations, which dominates the interaction between microstructures at small separations and is essential for micro- and nano-electromechanical systems (MEMS and NEMS). However, Casimir interaction driven by hyperbolic polaritons remains an unexplored frontier. In this work, we investigate the Casimir interaction between natural hyperbolic material hexagonal boron nitride from the perspective of force distribution with different optical axis orientations for the first time. The attractive Casimir force is remarkably enhanced due to the excitation of volume-confined hyperbolic polaritons (HPs). Furthermore, distinct repulsive contributions to the force are observed due to surface-confined HPs that only exist when the optical axis is in-plane. The HPs are associated with a striking thickness dependence of spectral force properties, suggesting that the discrete volume-confined HPs lead to the attractive-repulsive transition of Casimir force. This work sheds light on the relation between HPs and the vacuum fluctuation-induced force, which could offer new opportunities for the development of the MEMS and NEMS.

Manifestation of anharmonicities in terms of Fano scattering and phonon lifetime of scissors modes in α-MoO3

α-MoO3 exhibits promising potential in the field of infrared detection and thermoelectricity owing to its exceptional characteristics of ultra-low-loss phonon polaritons (PhPs). It is of utmost importance to comprehend the phonon interaction exhibited by α-MoO3 in order to facilitate the advancement of phonon-centric devices. The intriguing applications of α-MoO3 for phonon-centric technology are found to be strongly dependent on scissors Raman modes. In this study, we have investigated the temperature-dependent asymmetric Raman line-shape characteristics of two scissors modes, Ag(1) and B1g(1), in the orthorhombic phase of bulk α-MoO3 within a temperature range spanning from 138 K to 498 K at 633 nm excitation wavelength. The Fano-Raman line-shape function was employed to analyze the asymmetry in terms of electron-phonon coupling strength, which varies from 0.050 to 0.313 and -0.017 to -0.192 for Ag(1) and B1g(1) modes, respectively, with temperature. This asymmetric behavior of Ag(1) and B1g(1) scissors modes are attributed to interference between the electronic energy continuum and discrete TO and LO phonon states, respectively. Therefore, the line-shape asymmetry in two scissors modes with increasing temperature stemming from the Fano resonance is also consistent with a 488 nm excitation wavelength. Additionally, anharmonicity caused by temperature results in redshift, and linewidth broadening of these two scissors modes through cubic-phonon decay has been observed. Moreover, the ultrashort lifetime of these optical phonons diminishes from ∼1.37 ps to ∼0.53 ps with increasing temperature due to the dominance of cubic-phonon decay over quartic-phonon decay. Our findings strongly emphasize the significance of investigating anharmonic interactions with Fano resonance to acquire an extensive comprehension of the vibrational characteristics of α-MoO3 for novel functionalities.

Probing Hyperbolic Shear Polaritons in β-Ga2O3 Nanostructures Using STEM-EELS

Phonon polaritons, quasiparticles arising from strong coupling between electromagnetic waves and optical phonons, have potential for applications in subdiffraction imaging, sensing, thermal conduction enhancement, and spectroscopy signal enhancement. A new class of phonon polaritons in low-symmetry monoclinic crystals, hyperbolic shear polaritons (HShPs), have been verified recently in β-Ga2O3 by free electron laser (FEL) measurements. However, detailed behaviors of HShPs in β-Ga2O3 nanostructures still remain unknown. Here, by using monochromatic electron energy loss spectroscopy in conjunction with scanning transmission electron microscopy, the experimental observation of multiple HShPs in β-Ga2O3 in the mid-infrared (MIR) and far-infrared (FIR) ranges is reported. HShPs in various β-Ga2O3 nanorods and a β-Ga2O3 nanodisk are excited. The frequency-dependent rotation and shear effect of HShPs reflect on the distribution of EELS signals. The propagation and reflection of HShPs in nanostructures are clarified by simulations of electric field distribution. These findings suggest that, with its tunable broad spectral HShPs, β-Ga2O3 is an excellent candidate for nanophotonic applications.

Temporal dynamics of surface phonon polaritons in polar dielectric nanoparticles with nonlocality

Surface phonon polaritons (SPhPs) supported by polar dielectrics have been a promising platform for nanophotonics in mid-infrared spectral range. In this work, the temporal dynamic behavior of polar dielectric nanoparticles without (or with) spatial dispersion/nonlocality driven by the ultrashort Gaussian pulses is carried out. We demonstrate that three possible scenarios for the temporal evolutions of the dipole moment including ultrafast oscillations with the decay, exponential decay, and keeping a Gaussian shape exist, when the pulse duration of the incident field is much shorter than, similar to, and much longer than the localized SPhP lifetime. Once the nonlocal effect is considered, the oscillation period becomes large slightly, and the exponential decay turns fast. Furthermore, nonlocality-induced novel temporal behavior is found such as the decay with long-period oscillations when the center frequency of the incident pulse lies at the frequency of adjacent longitudinal resonant modes. The positive and negative time-shifts of the dielectric response reveal that the excitation of the dipole moment will be delayed or advanced. These temporal evolutions can pave the way towards potential applications in the modulation of ultrafast signals for the mid-infrared optoelectronic nanodevices.

Near-Unity Light-Matter Interaction in Mid-Infrared van der Waals Metasurfaces

Accessing mid-infrared radiation is of great importance for a range of applications, including thermal imaging, sensing, and radiative cooling. Here, we study light interaction with hexagonal boron nitride (hBN) nanocavities and reveal strong and tunable resonances across its hyperbolic transition. In addition to conventional phonon-polariton excitations, we demonstrate that the high refractive index of hexagonal boron nitride outside the Reststrahlen band allows enhanced light-matter interactions in deep subwavelength (<λ/15) nanostructures across a broad 7-8 μm range. Emergence and interplay of Fabry-Perot and Mie-like resonances are examined experimentally and theoretically. Near-unity absorption and high quality (Q ≥ 80) resonance interaction in the vicinity of the hBN transverse optical phonon is further observed. Our study provides avenues to design highly efficient and ultracompact structures for controlling mid-infrared radiation and accessing strong light-matter interactions with hBN.

Polariton Microfluidics for Nonreciprocal Dragging and Reconfigurable Shaping of Polaritons

Dielectric environment engineering is an efficient and general approach to manipulating polaritons. Liquids serving as the surrounding media of polaritons have been used to shift polariton dispersions and tailor polariton wavefronts. However, those liquid-based methods have so far been limited to their static states, not fully unleashing the promise offered by the mobility of liquids. Here, we propose a microfluidic strategy for polariton manipulation by merging polaritonics with microfluidics. The diffusion of fluids causes gradient refractive indices over microchannels, which breaks the symmetry of polariton dispersions and realizes the microfluidic analogue to nonreciprocal polariton dragging. Based on polariton microfluidics, we also designed a set of on-chip polaritonic elements to actively shape polaritons, including planar lenses, off-axis lenses, Janus lenses, bends, and splitters. Our strategy expands the toolkit for the manipulation of polaritons at the subwavelength scale and possesses potential in the fields of polariton biochemistry and molecular sensing.

Isotopic effects on in-plane hyperbolic phonon polaritons in MoO3

Hyperbolic phonon polaritons (HPhPs), hybrids of light and lattice vibrations in polar dielectric crystals, empower nanophotonic applications by enabling the confinement and manipulation of light at the nanoscale. Molybdenum trioxide (α-MoO3) is a naturally hyperbolic material, meaning that its dielectric function deterministically controls the directional propagation of in-plane HPhPs within its reststrahlen bands. Strategies such as substrate engineering, nano- and heterostructuring, and isotopic enrichment are being developed to alter the intrinsic die ectric functions of natural hyperbolic materials and to control the confinement and propagation of HPhPs. Since isotopic disorder can limit phonon-based processes such as HPhPs, here we synthesize isotopically enriched 92MoO3 (92Mo: 99.93 %) and 100MoO3 (100Mo: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural α-MoO3, which is composed of seven stable Mo isotopes. Real-space, near-field maps measured with the photothermal induced resonance (PTIR) technique enable comparisons of inplane HPhPs in α-MoO3 and isotopically enriched analogues within a reststrahlen band (≈820 cm-1 to ≈ 972 cm-1). Results show that isotopic enrichment (e.g., 92MoO3 and 100MoO3) alters the dielectric function, shifting the HPhP dispersion (HPhP angular wavenumber × thickness vs IR frequency) by ≈-7% and ≈ +9 %, respectively, and changes the HPhP group velocities by ≈ ±12 %, while the lifetimes (≈ 3 ps) in 92MoO3 were found to be slightly improved (≈ 20 %). The latter improvement is attributed to a decrease in isotopic disorder. Altogether, isotopic enrichment was found to offer fine control over the properties that determine the anisotropic in-plane propagation of HPhPs in α-MoO3, which is essential to its implementation in nanophotonic applications.

Hyperbolic Polaritonic Rulers Based on van der Waals α-MoO3 Waveguides and Resonators

Low-dimensional, strongly anisotropic nanomaterials can support hyperbolic phonon polaritons, which feature strong light-matter interactions that can enhance their capabilities in sensing and metrology tasks. In this work, we report hyperbolic polaritonic rulers, based on microscale α-phase molybdenum trioxide (α-MoO3) waveguides and resonators suspended over an ultraflat gold substrate, which exhibit near-field polaritonic characteristics that are exceptionally sensitive to device geometry. Using scanning near-field optical microscopy, we show that these systems support strongly confined image polariton modes that exhibit ideal antisymmetric gap polariton dispersion, which is highly sensitive to air gap dimensions and can be described and predicted using a simple analytic model. Dielectric constants used for modeling are accurately extracted using near-field optical measurements of α-MoO3 waveguides in contact with the gold substrate. We also find that for nanoscale resonators supporting in-plane Fabry-Perot modes, the mode order strongly depends on the air gap dimension in a manner that enables a simple readout of the gap dimension with nanometer precision.

Gate-Tuning Hybrid Polaritons in Twisted α-MoO3/Graphene Heterostructures

Modulating anisotropic phonon polaritons (PhPs) can open new avenues in infrared nanophotonics. Promising PhP dispersion engineering through polariton hybridization has been demonstrated by coupling gated graphene to single-layer α-MoO3. However, the mechanism underlying the gate-dependent modulation of hybridization has remained elusive. Here, using IR nanospectroscopic imaging, we demonstrate active modulation of the optical response function, quantified in measurements of gate dependence of wavelength, amplitude, and dissipation rate of the hybrid plasmon-phonon polaritons (HPPPs) in both single-layer and twisted bilayer α-MoO3/graphene heterostructures. Intriguingly, while graphene doping leads to a monotonic increase in HPPP wavelength, amplitude and dissipation rate show transition from an initially anticorrelated decrease to a correlated increase. We attribute this behavior to the intricate interplay of gate-dependent components of the HPPP complex momentum. Our results provide the foundation for active polariton control of integrated α-MoO3 nanophotonics devices.

Tunable heterostructural prism for planar polaritonic switch

The study of phonon polaritons in van der Waals materials at the nanoscale has gained significant attention in recent years due to its potential applications in nanophotonics. The unique properties of these materials, such as their ability to support sub-diffraction imaging, sensing, and hyperlenses, have made them a promising avenue for the development of new techniques in the field. Despite these advancements, there still exists a challenge in achieving dynamically reversible manipulation of phonon polaritons in these materials due to their insulating properties. In this study, we present experimental results on the reversible manipulation of anisotropic phonon polaritons in α-MoO3 on top of a VO2 film, a phase-change material known for its dramatic changes in dielectric properties between its insulating and metallic states. Our findings demonstrate that the engineered VO2 film enables a switch in the propagation of polaritons in the mid-infrared region by modifying the dielectric properties of the film through temperature changes. Our results represent a promising approach to effectively control the flow of light energy at the nanoscale and offer the potential for the design and fabrication of integrated, flat sub-diffraction polaritonic devices. This study adds to the growing body of work in the field of nanophotonics and highlights the importance of considering phase-change materials for the development of new techniques in this field.

Twist-Angle and Thickness-Ratio Tuning of Plasmon Polaritons in Twisted Bilayer van der Waals Films

Stacking bilayer structures is an efficient way to tune the topology of polaritons in in-plane anisotropic films, e.g., by leveraging the twist angle (TA). However, the effect of another geometric parameter, the film thickness ratio (TR), on manipulating the plasmon topology in bilayers is elusive. Here, we fabricate bilayer structures of WTe2 films, which naturally host in-plane hyperbolic plasmons in the terahertz range. Plasmon topology is successfully modified by changing the TR and TA synergistically, manifested by the extinction spectra of unpatterned films and the polarization dependence of the plasmon intensity measured in skew ribbon arrays. Such TR- and TA-tunable topological transitions can be well explained based on the effective sheet optical conductivity by adding up those of the two films. Our study demonstrates TR as another degree of freedom for the manipulation of plasmonic topology in nanophotonics, exhibiting promising applications in biosensing, heat transfer, and the enhancement of spontaneous emission.

Tunable optical topological transitions of plasmon polaritons in WTe2 van der Waals films

Naturally existing in-plane hyperbolic polaritons and the associated optical topological transitions, which avoid the nano-structuring to achieve hyperbolicity, can outperform their counterparts in artificial metasurfaces. Such plasmon polaritons are rare, but experimentally revealed recently in WTe2 van der Waals thin films. Different from phonon polaritons, hyperbolic plasmon polaritons originate from the interplay of free carrier Drude response and interband transitions, which promise good intrinsic tunability. However, tunable in-plane hyperbolic plasmon polariton and its optical topological transition of the isofrequency contours to the elliptic topology in a natural material have not been realized. Here we demonstrate the tuning of the optical topological transition through Mo doping and temperature. The optical topological transition energy is tuned over a wide range, with frequencies ranging from 429 cm-1 (23.3 microns) for pure WTe2 to 270 cm-1 (37.0 microns) at the 50% Mo-doping level at 10 K. Moreover, the temperature-induced blueshift of the optical topological transition energy is also revealed, enabling active and reversible tuning. Surprisingly, the localized surface plasmon resonance in skew ribbons shows unusual polarization dependence, accurately manifesting its topology, which renders a reliable means to track the topology with far-field techniques. Our results open an avenue for reconfigurable photonic devices capable of plasmon polariton steering, such as canaling, focusing, and routing, and pave the way for low-symmetry plasmonic nanophotonics based on anisotropic natural materials.

Hyperbolic polaritonic crystals with configurable low-symmetry Bloch modes

Photonic crystals (PhCs) are a kind of artificial structures that can mold the flow of light at will. Polaritonic crystals (PoCs) made from polaritonic media offer a promising route to controlling nano-light at the subwavelength scale. Conventional bulk PhCs and recent van der Waals PoCs mainly show highly symmetric excitation of Bloch modes that closely rely on lattice orders. Here, we experimentally demonstrate a type of hyperbolic PoCs with configurable and low-symmetry deep-subwavelength Bloch modes that are robust against lattice rearrangement in certain directions. This is achieved by periodically perforating a natural crystal α-MoO₃ that hosts in-plane hyperbolic phonon polaritons. The mode excitation and symmetry are controlled by the momentum matching between reciprocal lattice vectors and hyperbolic dispersions. We show that the Bloch modes and Bragg resonances of hyperbolic PoCs can be tuned through lattice scales and orientations while exhibiting robust properties immune to lattice rearrangement in the hyperbolic forbidden directions. Our findings provide insights into the physics of hyperbolic PoCs and expand the categories of PhCs, with potential applications in waveguiding, energy transfer, biosensing and quantum nano-optics.

Guided Polaritons along the Forbidden Direction in MoO3 with Geometrical Confinement

Highly anisotropic materials show great promise for spatial control and the manipulation of polaritons. In-plane hyperbolic phonon polaritons (HPhPs) supported by α-phase molybdenum trioxide (MoO₃) allow for wave propagation with a high directionality due to the hyperbola-shaped isofrequency contour (IFC). However, the IFC prohibits propagations along the [001] axis, hindering information or energy flow. Here, we illustrate a novel approach to manipulating the HPhP propagation direction. We experimentally demonstrate that geometrical confinement in the [100] axis can guide HPhPs along the forbidden direction with phase velocity becoming negative. We further developed an analytical model to provide insights into this transition. Moreover, as the guided HPhPs are formed in-plane, modal profiles were directly imaged to further expand our understanding of the formation of HPhPs. Our work reveals a possibility for manipulating HPhPs and paves the way for promising applications in metamaterials, nanophotonics, and quantum optics based on natural van der Waals materials.

Optical Effect Modulation in Polarized Raman Spectroscopy of Transparent Layered α-MoO3

Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO₃ as an excellent platform. It is found that there are several anomalous ARPRS responses in α-MoO₃ that cannot be reproduced by the real Raman tensor widely used in non-absorbing materials; however, they can be well explained by considering the birefringence-induced Raman selection rules. Moreover, the systematic thickness-dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α-MoO₃ is analyzed, giving the physical origins of the Raman anisotropy. This study employs α-MoO₃ as an example, although it is generally applicable to all transparent birefringent ALMs.

Polaritons in Van der Waals Heterostructures

2D monolayers supporting a wide variety of highly confined plasmons, phonon polaritons, and exciton polaritons can be vertically stacked in van der Waals heterostructures (vdWHs) with controlled constituent layers, stacking sequence, and even twist angles. vdWHs combine advantages of 2D material polaritons, rich optical structure design, and atomic scale integration, which have greatly extended the performance and functions of polaritons, such as wide frequency range, long lifetime, ultrafast all-optical modulation, and photonic crystals for nanoscale light. Here, the state of the art of 2D material polaritons in vdWHs from the perspective of design principles and potential applications is reviewed. Some fundamental properties of polaritons in vdWHs are initially discussed, followed by recent discoveries of plasmons, phonon polaritons, exciton polaritons, and their hybrid modes in vdWHs. The review concludes with a perspective discussion on potential applications of these polaritons such as nanophotonic integrated circuits, which will benefit from the intersection between nanophotonics and materials science.

Broadband hyperbolic thermal metasurfaces based on the plasmonic phase-change material In3SbTe2

Thermal radiation modulation facilitated by phase change materials (PCMs) needs a large thermal radiation contrast in broadband as well as in a non-volatile phase transition, which are only partially satisfied by conventional PCMs. In contrast, the emerging plasmonic PCM In₃SbTe₂ (IST) that undergoes a non-volatile dielectric-to-metal phase transition during crystallization offers a fitting solution. Here, we have prepared IST-based hyperbolic thermal metasurfaces and demonstrated their capabilities to modulate thermal radiation. By laser-printing crystalline IST gratings with different fill factors on amorphous IST films, we have achieved multilevel, large-range, and polarization-dependent control of the emissivity modulation (0.07 for the crystalline phase and 0.73 for the amorphous phase) over a broad bandwidth (8-14 μm). With the convenient direct laser writing technique that supports large-scale surface patterning, we have also demonstrated promising applications of thermal anti-counterfeiting with hyperbolic thermal metasurfaces.

Magnetical Manipulation of Hyperbolic Phonon Polaritons in Twisted Double-Layers of Molybdenum Trioxide

Controlling the twist angle between double stacked van der Waals (vdW) crystals holds great promise for nanoscale light compression and manipulation in the mid-infrared (MIR) range. A lithography-free geometry has been proposed to mediate the coupling of phonon polaritons (PhPs) in double-layers of vdW α-MoO3. The anisotropic hyperbolic phonon polaritons (AHPhPs) are further hybridized by the anisotropic substrate environment of magneto-optic indium arsenide (InAs). The AHPhPs can be tuned by twisting the angle between the optical axes of the two separated layers and realize a topological transition from open to closed dispersion contours. Moreover, in the presence of external magnetic field, an alteration of the hybridization of PhPs will be met, which enable an efficient way for the control of light-matter interaction at nanoscale in the MIR region.

Selective excitation of hyperbolic phonon polaritons-induced broadband absorption via α-MoO3 square pyramid arrays

Optical anisotropy of α-MoO₃ in its reststrahlen (RS) bands provides exciting opportunities for constructing the polarization-dependent devices. However, achieving broadband anisotropic absorptions through the same α-MoO₃ arrays is still challenging. In this study, we demonstrate that selective broadband absorption can be achieved by using the same α-MoO₃ square pyramid arrays (SPAs). For both the x and y polarizations, the absorption responses of the α-MoO₃ SPAs calculated by using the effective medium theory (EMT) agreed well with those of the FDTD, indicating the excellent selective broadband absorption of the α-MoO₃ SPAs are associated with the resonant hyperbolic phonon polaritons (HPhPs) modes assisted by the anisotropic gradient antireflection (AR) effect of the structure. The near-field distribution of the absorption wavelengths of the α-MoO₃ SPAs shows that the magnetic-field enhancement of the lager absorption wavelength tends to shift to the bottom of the α-MoO₃ SPAs due to the lateral Fabry-Pérot (F-P) resonance, and the electric-field distribution exhibits the ray-like light propagation trails due to the resonance nature of the HPhPs modes. In addition, broadband absorption of the α-MoO₃ SPAs can be maintained if the width of the bottom edge of the α-MoO₃ pyramid is large than 0.8 μm, and excellent anisotropic absorption performances are almost immune to the variations of the thickness of the spacer and the height of the α-MoO₃ pyramid.

Germanium Monosulfide as a Natural Platform for Highly Anisotropic THz Polaritons

Terahertz (THz) electromagnetic radiation is key to access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging topics 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 circumventing the large footprint and inherently low spectral power density of far-field THz radiation. For that purpose, phonon polaritons (PhPs) in anisotropic van der Waals (vdW) materials have recently emerged as a promising platform for THz nanooptics. Hence, there is a demand for the exploration of materials that feature not only THz PhPs at different spectral regimes but also host anisotropic (directional) electrical, thermoelectric, and vibronic properties. To that end, we introduce here the semiconducting vdW-material alpha-germanium(II) sulfide (GeS) as an intriguing candidate. By employing THz nanospectroscopy supported by theoretical analysis, we provide a thorough characterization of the different in-plane hyperbolic and elliptical PhP modes in GeS. We find not only PhPs with long lifetimes (τ > 2 ps) and excellent THz light confinement (λ0/λ > 45) but also an intrinsic, phonon-induced anomalous dispersion as well as signatures of naturally occurring, substrate-mediated PhP canalization within a single GeS slab.

Criterion for photonic topological transition in two-dimensional heterostructures

The anisotropic van der Waals material α-MoO₃ has recently attracted considerable attention because of the ability to support ellipse and hyperbolic phonon polaritons with extreme field confinement and long lifetimes, which can be used in topological transition and transformation polaritonics. However, the dispersion theory of some phonon polaritons in complex heterojunctions often requires tedious computation, which makes it difficult to simply judge and analyze the physical process of the photonic topological transition. Here we obtain the equivalent permittivity distribution of two-dimensional (2D) heterostructures by the effective medium theory and analyze the rotation-induced topological transitions and stack-dependent topological transitions of phonon polaritons. Unlike the previous discussion, we can predict the topological transition points by a parameter ɛ_x/y(i.e., the permittivity ratio along the in-plane crystal axis of the equivalent medium) and design precisely the phonon polaritons in the stacked materials by controlling the equivalent permittivity after simple calculation. The feasibility of the effective medium theory is verified based on the 2D approximation model and the non-2D approximation model under the limit of an ultrathin slab. Meanwhile, we compare the field distributions and dispersions of the 2D heterostructures and the corresponding equivalent structure. The simulation suggests that the elliptic/hyperbolic responses of the stacked materials depend on the sign of ɛ_x/y. The new, to the best of our knowledge, method not only provides an easier and clearer criterion for the study of photonic topological transition in anisotropic polaritons, but also shows great potential in designing some multilayer 2D heterostructures.

Doping-driven topological polaritons in graphene/α-MoO3 heterostructures

Control over charge carrier density provides an efficient way to trigger phase transitions and modulate the optoelectronic properties of materials. This approach can also be used to induce topological transitions in the optical response of photonic systems. Here we report a topological transition in the isofrequency dispersion contours of hybrid polaritons supported by a two-dimensional heterostructure consisting of graphene and α-phase molybdenum trioxide. By chemically changing the doping level of graphene, we observed that the topology of polariton isofrequency surfaces transforms from open to closed shapes as a result of doping-dependent polariton hybridization. Moreover, when the substrate was changed, the dispersion contour became dominated by flat profiles at the topological transition, thus supporting tunable diffractionless polariton propagation and providing local control over the optical contour topology. We achieved subwavelength focusing of polaritons down to 4.8% of the free-space light wavelength by using a 1.5-μm-wide silica substrate as an in-plane lens. Our findings could lead to on-chip applications in nanoimaging, optical sensing and manipulation of energy transfer at the nanoscale.

Molding Broadband Dispersion in Twisted Trilayer Hyperbolic Polaritonic Surfaces

Recent advancement in twisted layered metasurfaces can be employed to control the nanoscale flow of light, including the exotic hyperbolic-to-elliptic topological transitions in twisted bilayers (tBL). Such topological transitions can only occur to limited frequency ranges, restricted by the intrinsic in-plane dispersion of individual hyperbolic surfaces. Here, we report that, by controlling interlayer evanescent coupling in twisted polaritonic trilayers, moldable topological transitions of light can be achieved in broadband. We reveal that the required minimum open angle of the individual hyperbolic polaritonic surface for open-to-close topological transitions can be significantly lowered compared to that of the twisted bilayer counterpart. This increases the degree of freedom to enhance and control near-field light-matter interactions and energy management. As an example, we demonstrate a knob to manipulate near-field radiative heat transfer (NFRHT). By rotating the relative angles of trilayers, exotic and tunable thermal conductance can be achieved. Our findings enrich the controllability of light at the nanoscale in broadband, bringing twisted optical materials one step closer to practical applications.

Reconfigurable hyperbolic polaritonics with correlated oxide metasurfaces

Polaritons enable subwavelength confinement and highly anisotropic flows of light over a wide spectral range, holding the promise for applications in modern nanophotonic and optoelectronic devices. However, to fully realize their practical application potential, facile methods enabling nanoscale active control of polaritons are needed. Here, we introduce a hybrid polaritonic-oxide heterostructure platform consisting of van der Waals crystals, such as hexagonal boron nitride (hBN) or alpha-phase molybdenum trioxide (α-MoO₃), transferred on nanoscale oxygen vacancy patterns on the surface of prototypical correlated perovskite oxide, samarium nickel oxide, SmNiO₃ (SNO). Using a combination of scanning probe microscopy and infrared nanoimaging techniques, we demonstrate nanoscale reconfigurability of complex hyperbolic phonon polaritons patterned at the nanoscale with high resolution. Hydrogenation and temperature modulation allow spatially localized conductivity modulation of SNO nanoscale patterns, enabling robust real-time modulation and nanoscale reconfiguration of hyperbolic polaritons. Our work paves the way towards nanoscale programmable metasurface engineering for reconfigurable nanophotonic applications.

Phonon polaritons in anisotropic van der Waals materials enable subwavelength confinement and controllable flow of light at the nanoscale. Here, the authors exploit correlated perovskite oxide (SmNiO₃) substrates with tunable conductivity to obtain real-time modulation and nanoscale reconfiguration of hyperbolic polaritons in hBN and α-MoO₃ crystals.

Periodic evolution of the out-of-phase dipole and the single-charged vortex solitons in periodic photonic moiré lattice with saturable self-focusing nonlinearity media

We survey the propagation properties of the out-of-phase (OOP) dipole solitons and the single-charged vortex (SCV) soliton in a periodic photonic moiré lattice with θ=arctan⁡(3/4) under self-focusing nonlinearity media. Since the rotation angle, periodic photonic moiré lattices have peculiar energy band structures, with highly flat bands and the bandgaps being much more extensive, which is very favorable for the realization and stability of the solitons. When exciting a single point on-site with the OOP dipole beam, its evolution shows a periodic rollover around the lattice axis. Whereas, when exciting a single point on-site with the SCV beam, it transmits counterclockwise rotating periodically. Both the OOP dipole solitons and the SVC soliton maintain the local state, but their phase exhibits different variations. The phase of the OOP dipole solitons is flipped, while that of the SCV is rotated counterclockwise. Our work further complements the exploration of solitons in photonic moiré lattice with nonlinearity.

Unidirectionally excited phonon polaritons in high-symmetry orthorhombic crystals

Advanced control over the excitation of ultraconfined polaritons-hybrid light and matter waves-empowers unique opportunities for many nanophotonic functionalities, e.g., on-chip circuits, quantum information processing, and controlling thermal radiation. Recent work has shown that highly asymmetric polaritons are directly governed by asymmetries in crystal structures. Here, we experimentally demonstrate extremely asymmetric and unidirectional phonon polariton (PhP) excitation via directly patterning high-symmetry orthorhombic van der Waals (vdW) crystal α-MoO₃. This phenomenon results from symmetry breaking of momentum matching in polaritonic diffraction in vdW materials. We show that the propagation of PhPs can be versatile and robustly tailored via structural engineering, while PhPs in low-symmetry (e.g., monoclinic and triclinic) crystals are largely restricted by their naturally occurring permittivities. Our work synergizes grating diffraction phenomena with the extreme anisotropy of high-symmetry vdW materials, enabling unexpected control of infrared polaritons along different pathways and opening opportunities for applications ranging from on-chip photonics to directional heat dissipation.

Monolayer NaW2O2Br6: a gate tunable near-infrared hyperbolic plasmonic surface

Electrically tunable hyperbolic polaritons in two dimensional (2D) materials can offer unexplored opportunities in integrating photonics and nano-optoelectronics into a single chip. Here, we suggest that monolayer NaW₂O₂Br₆ can host electrically tunable hyperbolic plasmon polaritons for infrared light via first-principles calculations. 2D monolayer NaW₂O₂Br₆ exhibits an extremely anisotropic metallic property: conducting for one direction but almost insulating for the other direction, which could be considered as a 2D analogue of metal/dielectric multilayers, a typical structure for hyperbolic metamaterials. More interestingly, we also demonstrate that the hyperbolic properties in the near-infrared range, including the hyperbolic windows, figure of merit, and propagation directions of plasmon beams, can be effectively modulated by carrier doping at the order of 10¹³ cm^-2, which even can be accessed by solid-gated field effect transistors. Thus, it is anticipated that monolayer NaW₂O₂Br₆ has a great potential in constructing field programmable polariton nanodevices for emerging and diverse photonic applications.

Negative Reflection and Negative Refraction in Biaxial van der Waals Materials

Negative reflection and negative refraction are exotic phenomena that can be achieved by platforms such as double-negative metamaterial, hyperbolic metamaterial, and phase-discontinuity metasurface. Recently, natural biaxial van der Waals (vdW) materials, which support extremely anisotropic, low-loss, and highly confined polaritons from infrared to visible regime, are emerging as promising candidates for planar reflective and refractive optics. Here, we introduce three degrees of freedom, namely interface, crystal direction, and electric tunability, to manipulate the reflection and refraction of the polaritons. With broken in-plane symmetry contributed by the interface and crystal direction, distinguished reflection, and refraction such as negative and backward reflection, positive and negative refraction could exist simultaneously and exhibit high tunability. The numerical simulations show good consistency with the theoretical analysis. Our findings provide a robust recipe for the realization of negative reflection and refraction in biaxial vdW materials, paving the way for the polaritonics and on-chip integrated circuits.

Surface plasmons induce topological transition in graphene/α-MoO3 heterostructures

Polaritons in hyperbolic van der Waals materials-where principal axes have permittivities of opposite signs-are light-matter modes with unique properties and promising applications. Isofrequency contours of hyperbolic polaritons may undergo topological transitions from open hyperbolas to closed ellipse-like curves, prompting an abrupt change in physical properties. Electronically-tunable topological transitions are especially desirable for future integrated technologies but have yet to be demonstrated. In this work, we present a doping-induced topological transition effected by plasmon-phonon hybridization in graphene/α-MoO3 heterostructures. Scanning near-field optical microscopy was used to image hybrid polaritons in graphene/α-MoO3. We demonstrate the topological transition and characterize hybrid modes, which can be tuned from surface waves to bulk waveguide modes, traversing an exceptional point arising from the anisotropic plasmon-phonon coupling. Graphene/α-MoO3 heterostructures offer the possibility to explore dynamical topological transitions and directional coupling that could inspire new nanophotonic and quantum devices.

Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in α-MoO3

Manipulation of the propagation and energy-transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonic devices in photocatalysis, biosensing, and thermal management. In this context, metamaterials are useful composite materials, although traditional metal-based structures are constrained by their weak mid-IR response, while their associated capabilities for optical propagation and focusing are limited by the size of attainable artificial optical structures and the poor performance of the available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region is reported by exploiting highly oriented in-plane hyperbolic phonon polaritons in α-MoO₃ . Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 μm is demonstrated by the following three different means of control: the dimension of the device, the employed light frequency, and engineering of phonon-plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO₃ permit the focal length and focal spot size to be reduced to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons supported in α-MoO₃ are combined with tunable surface-plasmon polaritons in graphene to realize in situ and dynamical control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications.

Tailoring Topological Transitions of Anisotropic Polaritons by Interface Engineering in Biaxial Crystals

Polaritons in polar biaxial crystals with extreme anisotropy offer a promising route to manipulate nanoscale light-matter interactions. The dynamic modulation of their dispersion is of great significance for future integrated nano-optics but remains challenging. Here, we report tunable topological transitions in biaxial crystals enabled by interface engineering. We theoretically demonstrate such tailored polaritons at the interface of heterostructures between graphene and α-phase molybdenum trioxide (α-MoO₃). The interlayer coupling can be modulated by both the stack of graphene and α-MoO₃ and the magnitude of the Fermi level in graphene enabling a dynamic topological transition. More interestingly, we found that the wavefront transition occurs at a constant Fermi level when the thickness of α-MoO₃ is tuned. Furthermore, we also experimentally verify the hybrid polaritons in the graphene/α-MoO₃ heterostructure with different thicknesses of α-MoO₃. The interface engineering offers new insights into optical topological transitions, which may shed new light on programmable polaritonics, energy transfer, and neuromorphic photonics.

Enhanced near-field coupling and tunable topological transitions in hyperbolic van der Waals metasurfaces for optical nanomanipulation

Hyperbolic metasurfaces based on van der Waals (vdW) materials support propagation of extremely anisotropic polaritons towards nanoscale light compression and manipulation, and thus have great potential in the applications of planar hyperlenses, nanolasing, quantum optics, and ultrasensitive infrared spectroscopy. Two-dimensional hexagonal boron nitride (h-BN) subwavelength gratings as vdW metasurfaces can manipulate the propagation of hyperbolic polaritons at the level of single atomic layers, possessing a higher degree of field confinement and lower losses than conventional media. However, active manipulation of hyperbolic polaritonic waves in h-BN midinfrared metasurfaces remains elusive. Herein, we provide an effective strategy for tunable topological transitions in mid-infrared hyperbolic vdW metasurfaces (HMSs) via enhanced plasmon-phonon polaritons coupling. They are composed of in-plane heterostructures of thin-layer h-BN and monolayer graphene strips (iHBNG) as meta-atoms. The graphene-plasmon-enhanced near-field coupling enables a large tunability of light fields by tailoring the chemical potentials of graphene without frequency shift, which involves topological transitions of polaritonic modes, unidirectional polariton propagation, and local-density-of-state enhancement. Simulated visual near-field distributions of iHBNG metasurfaces reveal the unique transformations of hyperbolic polariton propagations, distinguished from that of individual h-BN and graphene metasurfaces. Our findings provide a platform of optical nanomanipulation towards emerging on-chip polaritonic devices.

Tunable Cherenkov radiation based on a van der Waals semiconductor α-MoO3 and graphene hybrid

In this Letter, we explore the Cherenkov radiation properties of α-phase molybdenum trioxide (α-MoO₃). We demonstrate that the asymmetric, forward, and reverse Cherenkov radiation can simultaneously exist by rotating the α-MoO₃ slab at the same working frequency and structure. In addition, thanks to the tunable functionalities of graphene, the conversion of forward and reverse Cherenkov radiation can be actualized by altering the Fermi level of graphene. These dynamically adjustable features provide a novel, to the best of our knowledge, and intuitive way for tunable Cherenkov radiation in the mid-infrared range, which opens up new opportunities in designing and manufacturing tunable radiation sources in future.

Tuning and hybridization of surface phonon polaritons in α-MoO3 based metamaterials

We propose an effective medium approach to tune and control surface phonon polariton dispersion relations along the three main crystallographic directions of α-phase molybdenum trioxide. We show that a metamaterial consisting of subwavelength air inclusions into the α-MoO₃ matrix displays new absorption modes producing a split of the Reststrahlen bands of the crystal and creating new branches of phonon polaritons. In particular, we report hybridization of bulk and surface polariton modes by tailoring metamaterials' structural parameters. Theoretical predictions obtained with the effective medium approach are validated by full-field electromagnetic simulations using finite difference time domain method. Our study sheds light on the use of effective medium theory for modeling and predicting wavefront polaritons. Our simple yet effective approach could potentially enable different functionalities for hyperbolic infrared metasurface devices and circuits on a single compact platform for on-chip infrared photonics.

Imaging Anisotropic Waveguide Exciton Polaritons in Tin Sulfide

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a great deal 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 (near-IR) region using 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 that both the polariton wavelength and propagation length are anisotropic in the sample plane. In particular, in a narrow spectral range from 1.32 to 1.44 eV, the EPs demonstrate quasi-one-dimensional propagation, which is rarely seen in natural polaritonic materials. A further analysis indicates that the observed polariton anisotropy originates from the different optical band gaps and exciton binding energies along the two principal crystal axes of SnS.

Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links

The forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.

Collective Phonon-Polaritonic Modes in Silicon Carbide Subarrays

Localized surface phonon polaritons (LSPhPs) can be implemented to engineer light-matter interactions through nanoscale patterning for a range of midinfrared application spaces. However, the polar material systems studied to date have mainly focused on simple designs featuring a single element in the periodic unit cell. Increasing the complexity of the unit cell can serve to modify the resonant near-fields and intra- and inter-unit-cell coupling as well as to dictate spectral tuning in the far-field. In this work, we exploit more complicated unit-cell structures to realize LSPhP modes with additional degrees of design freedom, which are largely unexplored. Collectively excited LSPhP modes with distinctly symmetric and antisymmetric near-fields are supported in these subarray designs, which are based on nanopillars that are scaled by the number of subarray elements to ensure a constant unit-cell size. Moreover, we observe an anomalous mode-matching of the collective symmetric mode in our fabricated subarrays that is robust to changing numbers of pillars within the subarrays as well as to defects intentionally introduced in the form of missing pillars. This work therefore illustrates the hierarchical design of tailored LSPhP resonances and modal near-field profiles simultaneously for a variety of IR applications such as surface-enhanced spectroscopies and biochemical sensing.

Optimized Colossal Near-Field Thermal Radiation Enabled by Manipulating Coupled Plasmon Polariton Geometry

Collective optoelectronic phenomena such as plasmons and phonon polaritons can drive processes in many branches of nanoscale science. Classical physics predicts that a perfect thermal emitter operates at the black body limit. Numerous experiments have shown that surface phonon polaritons allow emission two orders of magnitude above the limit at a gap distance of ≈50 nm. This work shows that a supported multilayer graphene structure improves the state of the art by around one order of magnitude with a ≈1129-fold-enhancement at a gap distance of ≈55 nm. Coupled surface plasmon polaritons at mid- and far-infrared frequencies allow for near-unity photon tunneling across a broad swath of k-space enabling the improved result. Electric tuning of the Fermi-level allows for the detailed characterization and optimization of the colossal nanoscale heat transfer.

Surface and volume phonon polaritons in a uniaxial hyperbolic material: optic axis parallel versus perpendicular to the surface

Uniaxial hyperbolic materials enable excitation of phonon polaritons with utrahigh wavevectors that have been shown to be promising for many optical and thermal radiative applications and thus have attracted much attention recently. However, the characteristics of surface and volume phonon polaritons excited with uniaxial hyperbolic materials that exhibit in-plane anisotropy or in-plane isotropy have not been discussed thoroughly and some issues have so far remained elusive. In this paper, we conducted a comprehensive investigation on surface and volume phonon polaritons in a bulk or a thin slab of hexagonal boron nitride (hBN). We clarified the excitation, characteristics and topology of surface and volume phonon polaritons in such a uniaxial hyperbolic material. In particular, we showed that hyperbolic surface phonon polaritons (HSPhPs) can exist in the Type I hyperbolic band of hBN with confined wavevectors when the optic axis (OA) is parallel to the surface. For a thin hBN slab, we revealed a split of HSPhPs and a smooth transition between HSPhPs and HVPhPs in the Type II hyperbolic band. Furthermore, we also identified non-Dyakonov surface phonon polaritons excited without evanescent ordinary waves. These findings may extend the understanding of phonon polaritons in hyperbolic materials and offer new theoretical guidance for the design of infrared optical devices with hyperbolic materials.

Broadband Achromatic Metasurfaces for Longwave Infrared Applications

Longwave infrared (LWIR) optics are essential for several technologies, such as thermal imaging and wireless communication, but their development is hindered by their bulk and high fabrication costs. Metasurfaces have recently emerged as powerful platforms for LWIR integrated optics; however, conventional metasurfaces are highly chromatic, which adversely affects their performance in broadband applications. In this work, the chromatic dispersion properties of metasurfaces are analyzed via ray tracing, and a general method for correcting chromatic aberrations of metasurfaces is presented. By combining the dynamic and geometric phases, the desired group delay and phase profiles are imparted to the metasurfaces simultaneously, resulting in good achromatic performance. Two broadband achromatic metasurfaces based on all-germanium platforms are demonstrated in the LWIR: a broadband achromatic metalens with a numerical aperture of 0.32, an average intensity efficiency of 31%, and a Strehl ratio above 0.8 from 9.6 μm to 11.6 μm, and a broadband achromatic metasurface grating with a constant deflection angle of 30° from 9.6 μm to 11.6 μm. Compared with state-of-the-art chromatic-aberration-restricted LWIR metasurfaces, this work represents a substantial advance and brings the field a step closer to practical applications.

Tunable phonon-plasmon hybridization in α-MoO3-graphene based van der Waals heterostructures

The plasmon-phonon hybridization behavior between anisotropic phonon polaritons (APhP) of orthorhombic phase Molybdenum Trioxide (α - MoO₃) and the plasmon-polaritons of Graphene layer - forming a van der Waals (vdW) heterostructure is investigated theoretically in this paper. It is found that in-plane APhP shows strong interaction with graphene plasmons lying in their close vicinity, leading to large Rabi splitting. Anisotropic behavior of biaxial MoO₃ shows the polarization-dependent response with strong anti-crossing behavior at 0.55 eV and 0.3 eV of graphene's Fermi potential for [100] and [001] crystalline directions, respectively. Numerical results reveal unusual electric field confinement for the two arms of enhanced hybrid modes: the first being confined in the graphene layer representing plasmonic-like behavior. The second shows volume confined zigzag pattern in hyperbolic MoO₃. It is also found that the various plasmon-phonon hybridized modes could be wavelength tuned, simply by varying the Fermi potential of the graphene layer. The coupling response of the hybrid structure is studied analytically using the coupled oscillator model. Furthermore, we also infer upon the coupling strength and frequency splitting between the two layers with respect to their structural parameters and interlayer spacing. Our work will provide an insight into the active tunable property of hybrid van der Waals (vdW) structure for their potential application in sensors, detectors, directional spontaneous emission, as well as for the tunable control of the propagating polaritons in fields of flat dispersion where strong localization of photons can be achieved, popularly known as the flatband optics.

Localization-to-delocalization transition of light in frequency-tuned photonic moiré lattices

We demonstrate in a numerical manner the intriguing localization-to-delocalization transition of light in frequency-tuned photonic moiré lattices, both in the zero-order and the higher-order regimes of light waves. We present a different technique to realize the composite photonic lattices, by means of two relatively twisted sublattices with different modulated lattice constants. Even though various kinds of photonic patterns including the commensurable and the incommensurable lattices can be well constructed, the observed transition between the localization and the delocalization of light field is moiré angle-independent. This angle-insensitive property was not reported before, and cannot be achieved by those photonic moiré lattices that are all moiré angle-dependent. We reveal that the obtained phase transition of light is robust to the changes of refractive index modulation of the photonic lattices. Moreover, we reveal that the effect of moiré angle-independent transition of light can be extended to the higher-order vortex light field, hence allowing prediction, for the first time to our knowledge, of both the localization and the delocalization of the vortex light field in the photonic lattices.