Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial

Electrical spectroscopy of polaritonic nanoresonators

One of the most captivating properties of polaritons is their capacity to confine light at the nanoscale. This confinement is even more extreme in two-dimensional (2D) materials. 2D polaritons have been investigated by optical measurements using an external photodetector. However, their effective spectrally resolved electrical detection via far-field excitation remains unexplored. This hinders their exploitation in crucial applications such as sensing, hyperspectral imaging, and optical spectrometry, banking on their potential for integration with silicon technologies. Herein, we present the electrical spectroscopy of polaritonic nanoresonators based on a high-quality 2D-material heterostructure, which serves at the same time as the photodetector and the polaritonic platform. Subsequently, we electrically detect these mid-infrared resonators by near-field coupling to a graphene pn-junction. The nanoresonators simultaneously exhibit extreme lateral confinement and high-quality factors. This work opens a venue for investigating this tunable and complex hybrid system and its use in compact sensing and imaging platforms.

Resonant Metasurfaces with Van Der Waals Hyperbolic Nanoantennas and Extreme Light Confinement

This work reports on a metasurface based on optical nanoantennas made of van der Waals material hexagonal boron nitride. The optical nanoantenna made of hyperbolic material was shown to support strong localized resonant modes stemming from the propagating high-k waves in the hyperbolic material. An analytical approach was used to determine the mode profile and type of cuboid nanoantenna resonances. An electric quadrupolar mode was demonstrated to be associated with a resonant magnetic response of the nanoantenna, which resembles the induction of resonant magnetic modes in high-refractive-index nanoantennas. The analytical model accurately predicts the modes of cuboid nanoantennas due to the strong boundary reflections of the high-k waves, a capability that does not extend to plasmonic or high-refractive-index nanoantennas, where the imperfect reflection and leakage of the mode from the cavity complicate the analysis. In the reported metasurface, excitations of the multipolar resonant modes are accompanied by directional scattering and a decrease in the metasurface reflectance to zero, which is manifested as the resonant Kerker effect. Van der Waals nanoantennas are envisioned to support localized resonances and can become an important functional element of metasurfaces and transdimensional photonic components. By designing efficient subwavelength scatterers with high-quality-factor resonances, this work demonstrates that this type of nanoantenna made of naturally occurring hyperbolic material is a viable substitute for plasmonic and all-dielectric nanoantennas in developing ultra-compact photonic components.

Landau-phonon polaritons in Dirac heterostructures

Polaritons are light-matter quasiparticles that govern the optical response of quantum materials at the nanoscale, enabling on-chip communication and local sensing. Here, we report Landau-phonon polaritons (LPPs) in magnetized charge-neutral graphene encapsulated in hexagonal boron nitride (hBN). These quasiparticles emerge from the interaction of Dirac magnetoexciton modes in graphene with the hyperbolic phonon polariton modes in hBN. Using infrared magneto-nanoscopy, we reveal the ability to completely halt the LPP propagation in real space at quantized magnetic fields, defying the conventional optical selection rules. The LPP-based nanoscopy also tells apart two fundamental many-body phenomena: the Fermi velocity renormalization and field-dependent magnetoexciton binding energies. Our results highlight the potential of magnetically tuned Dirac heterostructures for precise nanoscale control and sensing of light-matter interaction.

A facile and scalable fabrication method of scrolled graphene/boron nitride-based van der Waals superlattice heterostructure materials for highly stable supercapacitor electrode application

Due to the increasing demand for the development of efficient renewable energy supply systems to reduce the mismatch between energy demand and utilization, supercapacitors have attracted increasing attention in the energy industry. However, the development of energy storage electrode materials to be applied at the industrial level is still challenging due to the unsatisfactory durability and scalable production issues. This study suggested a facile and scalable one-pot fabrication method of using graphene/hexagonal boron nitride (G/BN)-based one-dimensional (1D) van der Waals superlattice heterostructures (vdWSLs) as highly stable electrode materials to enhance the energy storage performance by improving the mesopore volume content, specific surface area, electrical properties, and interfacial interaction between the stacked G/BN layers. The G/BN-based vdWSLs were fabricated by a simple scrolling process through the electromagnetic interaction between the attached magnetic iron oxide nanoparticles (Fe3O4 NPs) on the surface of a G/BN vdW heterostructure (vdWH) and the applied magnetic field. The investigation results demonstrate that the changed morphology of the fabricated G/Fe/BN(NS) strongly affects the fine pore distribution, electrochemical performance, and electrical properties. Consequently, as a synergistic effect of an increased mesopore volume content, specific surface area, and C-B-N heterojunction interfacial area, the fabricated G/Fe/BN(NS) electrode showed a 100% enhancement of specific capacitance (207 F g-1 at 0.5 A g-1) and almost 7 times enhancement of electrical conductivity (800 S cm-1) with a nearly 2.3 times increase of carrier mobility (716 cm2 V-1 s-1) compared to that of the G/Fe/BN electrode. Furthermore, it exhibited outstanding long-term cycling stability with almost 119% capacitance retention even after 100 000 charge-discharge cycles. These results suggest that G/Fe/BN(NS) has tremendous potential as an electrode to fabricate high-performance supercapacitors with excellent cycling stability.

Tunable Phonon Polariton Hybridization in a Van der Waals Hetero-Bicrystal

Phonon polaritons, the hybrid quasiparticles resulting from the coupling of photons and lattice vibrations, have gained significant attention in the field of layered van der Waals heterostructures. Particular interest has been paid to hetero-bicrystals composed of molybdenum oxide (MoO3) and hexagonal boron nitride (hBN), which feature polariton dispersion tailorable via avoided polariton mode crossings. In this work, the polariton eigenmodes in MoO3-hBN hetero-bicrystals self-assembled on ultrasmooth gold are systematically studied using synchrotron infrared nanospectroscopy. It is experimentally demonstrated that the spectral gap in bicrystal dispersion and corresponding regimes of negative refraction can be tuned by material layer thickness, and these results are quantitatively matched with a simple analytic model. Polaritonic cavity modes and polariton propagation along "forbidden" directions are also investigated in microscale bicrystals, which arise from the finite in-plane dimension of the synthesized MoO3 micro-ribbons. The findings shed light on the unique dispersion properties of polaritons in van der Waals heterostructures and pave the way for applications leveraging deeply sub-wavelength mid-infrared light-matter interactions.

The Rise of Xene Hybrids

Xenes, mono-elemental atomic sheets, exhibit Dirac/Dirac-like quantum behavior. When interfaced with other 2D materials such as boron nitride, transition metal dichalcogenides, and metal carbides/nitrides/carbonitrides, it enables them with unique physicochemical properties, including structural stability, desirable bandgap, efficient charge carrier injection, flexibility/breaking stress, thermal conductivity, chemical reactivity, catalytic efficiency, molecular adsorption, and wettability. For example, BN acts as an anti-oxidative shield, MoS2 injects electrons upon laser excitation, and MXene provides mechanical flexibility. Beyond precise compositional modulations, stacking sequences, and inter-layer coupling controlled by parameters, achieving scalability and reproducibility in hybridization is crucial for implementing these quantum materials in consumer applications. However, realizing the full potential of these hybrid materials faces challenges such as air gaps, uneven interfaces, and the formation of defects and functional groups. Advanced synthesis techniques, a deep understanding of quantum behaviors, precise control over interfacial interactions, and awareness of cross-correlations among these factors are essential. Xene-based hybrids show immense promise for groundbreaking applications in quantum computing, flexible electronics, energy storage, and catalysis. In this timely perspective, recent discoveries of novel Xenes and their hybrids are highlighted, emphasizing correlations among synthetic parameters, structure, properties, and applications. It is anticipated that these insights will revolutionize diverse industries and technologies.

Highly Confined Hybridized Polaritons in Scalable van der Waals Heterostructure Resonators

The optimization of nanoscale optical devices and structures will enable the exquisite control of planar optical fields. Polariton manipulation is the primary strategy in play. In two-dimensional heterostructures, the ability to excite mixed optical modes offers an additional control in device design. Phonon polaritons in hexagonal boron nitride have been a common system explored for the control of near-infrared radiation. Their hybridization with graphene plasmons makes these mixed phonon polariton modes in hexagonal boron nitride more appealing in terms of enabling active control of electrodynamic properties with a reduction of propagation losses. Optical resonators can be added to confine these hybridized plasmon-phonon polaritons deeply into the subwavelength regime, with these structures featuring high quality factors. Here, we show a scalable approach for the design and fabrication of heterostructure nanodisc resonators patterned in chemical vapor deposition-grown monolayer graphene and h-BN sheets. Real-space mid-infrared nanoimaging reveals the nature of hybridized polaritons in the heterostructures. We simulate and experimentally demonstrate localized hybridized polariton modes in heterostructure nanodisc resonators and demonstrate that those nanodiscs can collectively couple to the waveguide. High quality factors for the nanodiscs are measured with nanoscale Fourier transform infrared spectroscopy. Our results offer practical strategies to realize scalable nanophotonic devices utilizing low-loss hybridized polaritons for applications such as on-chip optical components.

Photonics in Multimaterial Lateral Heterostructures Combining Group IV Chalcogenide van der Waals Semiconductors

Lateral heterostructures combining two multilayer group IV chalcogenide van der Waals semiconductors have attracted interest for optoelectronics, twistronics, and valleytronics, owing to their structural anisotropy, bulk-like electronic properties, enhanced optical thickness, and vertical interfaces enabling in-plane charge manipulation/separation, perpendicular to the trajectory of incident light. Group IV monochalcogenides support propagating photonic waveguide modes, but their interference gives rise to complex light emission patterns throughout the visible/near-infrared range both in uniform flakes and single-interface lateral heterostructures. Here, this work demonstrates the judicious integration of pure and alloyed monochalcogenide crystals into multimaterial heterostructures with unique photonic properties, notably the ability to select photonic modes with targeted discrete energies through geometric factors rather than band engineering. SnS-GeS1-xSex-GeSe-GeS1-xSex heterostructures with a GeS1-xSex active layer sandwiched laterally between GeSe and SnS, semiconductors with similar optical constants but smaller bandgaps, were designed and realized via sequential vapor transport synthesis. Raman spectroscopy, electron microscopy/diffraction, and energy-dispersive X-ray spectroscopy confirm a high crystal quality of the laterally stitched components with sharp interfaces. Nanometer-scale cathodoluminescence spectroscopy provides evidence for a facile transfer of electron-hole pairs across the lateral interfaces and demonstrates the selection of photon emission at discrete energies in the laterally embedded active (GeS1- xSex) part of the heterostructure.

Direct programming of confined surface phonon polariton resonators with the plasmonic phase-change material In3SbTe2

Tailoring light-matter interaction is essential to realize nanophotonic components. It can be achieved with surface phonon polaritons (SPhPs), an excitation of photons coupled with phonons of polar crystals, which also occur in 2d materials such as hexagonal boron nitride or anisotropic crystals. Ultra-confined resonances are observed by restricting the SPhPs to cavities. Phase-change materials (PCMs) enable non-volatile programming of these cavities based on a change in the refractive index. Recently, the plasmonic PCM In3SbTe2 (IST) was introduced which can be reversibly switched from an amorphous dielectric state to a crystalline metallic one in the entire infrared to realize numerous nanoantenna geometries. However, reconfiguring SPhP resonators to modify the confined polaritons modes remains elusive. Here, we demonstrate direct programming of confined SPhP resonators by phase-switching IST on top of a polar silicon carbide crystal and investigate the strongly confined resonance modes with scanning near-field optical microscopy. Reconfiguring the size of the resonators themselves result in enhanced mode confinements up to a value of λ / 35 . Finally, unconventional cavity shapes with complex field patterns are explored as well. This study is a first step towards rapid prototyping of reconfigurable SPhP resonators that can be easily transferred to hyperbolic and anisotropic 2d materials.

A high figure of merit of phonon-polariton waveguide modes with hbn/SiO2/graphene /hBN ribs waveguide in mid-infrared range

Natural hyperbolic materials can confine electromagnetic waves at the nanoscale. In this study, we propose a waveguide design that combines a high quality factor (FOM) with low loss, utilizing hexagonal boron nitride and graphene and gold substrate. The waveguide consists of a dielectric rib with a graphene layer sandwiched between two hBN ribs. Numerical simulations demonstrate the existence of two guided modes in the proposed waveguide within the second reststrahlen band (1360.0 cm-1<ω < 1609.8 cm-1) of hBN. These modes are formed by coupling the hyperbolic phonon polariton (HPhP) of two hBN rib in the middle dielectric rib and are subsequently modulated by a graphene layer. Interestingly, we observe variations in four transmission parameters, namely effective length, figure of merit, device length, and propagation loss of the guided modes, with respect to the operation frequency and gate voltage. By optimizing the waveguide's geometry parameters and dielectric permittivity, the modal properties were analyzed. Simulation results indicate that optimizing the waveguide size parameters enables us to achieve a high FOM of 4.0 × 107. The proposed waveguide design offers a promising approach for designing tunable mid infrared range waveguides on photonic chips, and this concept can be extended to other 2D materials and hyperbolic materials.

Accelerated Nano-Optical Imaging through Sparse Sampling

The integration time and signal-to-noise ratio are inextricably linked when performing scanning probe microscopy based on raster scanning. This often yields a large lower bound on the measurement time, for example, in nano-optical imaging experiments performed using a scanning near-field optical microscope (SNOM). Here, we utilize sparse scanning augmented with Gaussian process regression to bypass the time constraint. We apply this approach to image charge-transfer polaritons in graphene residing on ruthenium trichloride (α-RuCl3) and obtain key features such as polariton damping and dispersion. Critically, nano-optical SNOM imaging data obtained via sparse sampling are in good agreement with those extracted from traditional raster scans but require 11 times fewer sampled points. As a result, Gaussian process-aided sparse spiral scans offer a major decrease in scanning time.

Advances in boron nitride-based nanomaterials for environmental remediation and water splitting: a review

Boron nitride has gained wide-spread attention globally owing to its outstanding characteristics, such as a large surface area, high thermal resistivity, great mechanical strength, low density, and corrosion resistance. This review compiles state-of-the-art synthesis techniques, including mechanical exfoliation, chemical exfoliation, chemical vapour deposition (CVD), and green synthesis for the fabrication of hexagonal boron nitride and its composites, their structural and chemical properties, and their applications in hydrogen production and environmental remediation. Additionally, the adsorptive and photocatalytic properties of boron nitride-based nanocomposites for the removal of heavy metals, dyes, and pharmaceuticals from contaminated waters are discussed. Lastly, the scope of future research, including the facile synthesis and large-scale applicability of boron nitride-based nanomaterials for wastewater treatment, is presented. This review is expected to deliver preliminary knowledge of the present state and properties of boron nitride-based nanomaterials, encouraging the future study and development of these materials for their applications in various fields.

Planar hyperbolic polaritons in 2D van der Waals materials

Anisotropic planar polaritons - hybrid electromagnetic modes mediated by phonons, plasmons, or excitons - in biaxial two-dimensional (2D) van der Waals crystals have attracted significant attention due to their fundamental physics and potential nanophotonic applications. In this Perspective, we review the properties of planar hyperbolic polaritons and the variety of methods that can be used to experimentally tune them. We argue that such natural, planar hyperbolic media should be fairly common in biaxial and uniaxial 2D and 1D van der Waals crystals, and identify the untapped opportunities they could enable for functional (i.e. ferromagnetic, ferroelectric, and piezoelectric) polaritons. Lastly, we provide our perspectives on the technological applications of such planar hyperbolic polaritons.

Polariton design and modulation via van der Waals/doped semiconductor heterostructures

Hyperbolic phonon polaritons (HPhPs) can be supported in materials where the real parts of their permittivities along different directions are opposite in sign. HPhPs offer confinements of long-wavelength light to deeply subdiffractional scales, while the evanescent field allows for interactions with substrates, enabling the tuning of HPhPs by altering the underlying materials. Yet, conventionally used noble metal and dielectric substrates restrict the tunability of this approach. To overcome this challenge, here we show that doped semiconductor substrates, e.g., InAs and CdO, enable a significant tuning effect and dynamic modulations. We elucidated HPhP tuning with the InAs plasma frequency in the near-field, with a maximum difference of 8.3 times. Moreover, the system can be dynamically modulated by photo-injecting carriers into the InAs substrate, leading to a wavevector change of ~20%. Overall, the demonstrated hBN/doped semiconductor platform offers significant improvements towards manipulating HPhPs, and potential for engineered and modulated polaritonic systems.

Thermal and electrostatic tuning of surface phonon-polaritons in LaAlO3/SrTiO3 heterostructures

Phonon polaritons are promising for infrared applications due to a strong light-matter coupling and subwavelength energy confinement they offer. Yet, the spectral narrowness of the phonon bands and difficulty to tune the phonon polariton properties hinder further progress in this field. SrTiO3 - a prototype perovskite oxide - has recently attracted attention due to two prominent far-infrared phonon polaritons bands, albeit without any tuning reported so far. Here we show, using cryogenic infrared near-field microscopy, that long-propagating surface phonon polaritons are present both in bare SrTiO3 and in LaAlO3/SrTiO3 heterostructures hosting a two-dimensional electron gas. The presence of the two-dimensional electron gas increases dramatically the thermal variation of the upper limit of the surface phonon polariton band due to temperature dependent polaronic screening of the surface charge carriers. Furthermore, we demonstrate a tunability of the upper surface phonon polariton frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that oxide interfaces are a new platform bridging unconventional electronics and long-wavelength nanophotonics.

Topological Transitions and Surface Umklapp Scattering in Weakly Modulated Periodic Metasurfaces

Controlling and manipulating surface waves is highly beneficial for imaging applications, nanophotonic device design, and light-matter interactions. While deep-subwavelength structuring of the metal-dielectric interface can influence surface waves by forming strong effective anisotropy, it disregards important structural degrees of freedom such as the interplay between corrugation periodicity and depth and its effect on the beam transport. Here, we unlock these degrees of freedom, introducing weakly modulated metasurfaces, structured metal-dielectric surfaces beyond effective medium. We utilize groove-structuring with varying depths and periodicities to demonstrate control over the transport of surface waves, dominated by the depth-period interplay. We show unique backward focusing of surface waves driven by an umklapp process-momentum relaxation empowered by the periodic nature of the structure and discover a yet unexplored, dual-stage topological transition. Our findings can be applied to any type of guided wave, introducing a simple and versatile approach for controlling wave propagation in artificial media.

Comparative analysis of two models for phonon polaritons in van der Waals materials: 2D and 3D

Phonon polaritons with ultralow losses and high confinement in extremely anisotropic media have opened up new avenues for manipulating the flow of light at the nanoscale. Recent advances in var der Waals (vdW) materials reveal unprecedented dispersion characteristics of polaritons using a two-dimensional (2D) model, treating the slab as a surface without thickness. However, the difference between the 2D and three-dimensional (3D) models of hyperbolic polaritons remains largely unexplored. Herein, we compare the polaritonic difference between these two models for biaxial vdW slabs. In addition, we demonstrate that the fundamental mode in slab configuration corresponds to the polaritonic mode in surface sheet and higher-order modes vanish in the latter configuration. In particular, we reveal that the difference in in-plane polaritons along the [100] and [001] crystal directions between the two models is associated with the inverse of the dielectric function along these two directions. For example, we compare the near-field radiative heat transfer (NFRHT) between two vdW slabs based on these two models. It is found that when the attenuation length of the higher-order hyperbolic mode is less than the gap distance, the enhancement achieved using the 3D model comes from only the fundamental mode, resulting in a negligible difference between these two models. Therefore, our findings may help to understand in-plane anisotropic polaritons and provide guidance for the application of the 2D model in the analysis of vdW materials.

Polaritonic Probe of an Emergent 2D Dipole Interface

The use of work-function-mediated charge transfer has recently emerged as a reliable route toward nanoscale electrostatic control of individual atomic layers. Using α-RuCl3 as a 2D electron acceptor, we are able to induce emergent nano-optical behavior in hexagonal boron nitride (hBN) that arises due to interlayer charge polarization. Using scattering-type scanning near-field optical microscopy (s-SNOM), we find that a thin layer of α-RuCl3 adjacent to an hBN slab reduces the propagation length of hBN phonon polaritons (PhPs) in significant excess of what can be attributed to intrinsic optical losses. Concomitant nano-optical spectroscopy experiments reveal a novel resonance that aligns energetically with the region of excess PhP losses. These experimental observations are elucidated by first-principles density-functional theory and near-field model calculations, which show that the formation of a large interfacial dipole suppresses out-of-plane PhP propagation. Our results demonstrate the potential utility of charge-transfer heterostructures for tailoring optoelectronic properties of 2D insulators.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

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.

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.

Van der Waals isotope heterostructures for engineering phonon polariton dispersions

Element isotopes are characterized by distinct atomic masses and nuclear spins, which can significantly influence material properties. Notably, however, isotopes in natural materials are homogenously distributed in space. Here, we propose a method to configure material properties by repositioning isotopes in engineered van der Waals (vdW) isotopic heterostructures. We showcase the properties of hexagonal boron nitride (hBN) isotopic heterostructures in engineering confined photon-lattice waves-hyperbolic phonon polaritons. By varying the composition, stacking order, and thicknesses of h10BN and h11BN building blocks, hyperbolic phonon polaritons can be engineered into a variety of energy-momentum dispersions. These confined and tailored polaritons are promising for various nanophotonic and thermal functionalities. Due to the universality and importance of isotopes, our vdW isotope heterostructuring method can be applied to engineer the properties of a broad range of materials.

Flatband polaritonic router in twisted bilayer van der Waals materials

In recent years, van der Waals (vdW) polaritons excited by the hybrid of matter and photons have shown great promise for applications in nanoimaging, biosensing, and on-chip light guiding. In particular, polaritons with a flatband dispersion allow for mode canalization and diffractionless propagation, which showcase advantages for on-chip technologies requiring long-range transportation of optical information. Here, we propose a flatband polaritonic router based on twisted α-MoO₃ bilayers, which allows for on-chip routing of highly confined and low-loss phonon polaritons (PhPs) along multichannel propagating paths under different circular polarized dipole excitations. Our work combines flatband physics and optical spin- orbit coupling, with potential applications in nanoscale light propagation, on-chip optical switching, and communication.

Electron cooling in graphene enhanced by plasmon-hydron resonance

Evidence is accumulating for the crucial role of a solid's free electrons in the dynamics of solid-liquid interfaces. Liquids induce electronic polarization and drive electric currents as they flow; electronic excitations, in turn, participate in hydrodynamic friction. Yet, the underlying solid-liquid interactions have been lacking a direct experimental probe. Here we study the energy transfer across liquid-graphene interfaces using ultrafast spectroscopy. The graphene electrons are heated up quasi-instantaneously by a visible excitation pulse, and the time evolution of the electronic temperature is then monitored with a terahertz pulse. We observe that water accelerates the cooling of the graphene electrons, whereas other polar liquids leave the cooling dynamics largely unaffected. A quantum theory of solid-liquid heat transfer accounts for the water-specific cooling enhancement through a resonance between the graphene surface plasmon mode and the so-called hydrons-water charge fluctuations-particularly the water libration modes, which allows for efficient energy transfer. Our results provide direct experimental evidence of a solid-liquid interaction mediated by collective modes and support the theoretically proposed mechanism for quantum friction. They further reveal a particularly large thermal boundary conductance for the water-graphene interface and suggest strategies for enhancing the thermal conductivity in graphene-based nanostructures.

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.

Observation of directional leaky polaritons at anisotropic crystal interfaces

Extreme anisotropy in some polaritonic materials enables light propagation with a hyperbolic dispersion, leading to enhanced light-matter interactions and directional transport. However, these features are typically associated with large momenta that make them sensitive to loss and poorly accessible from far-field, being bound to the material interface or volume-confined in thin films. Here, we demonstrate a new form of directional polaritons, leaky in nature and featuring lenticular dispersion contours that are neither elliptical nor hyperbolic. We show that these interface modes are strongly hybridized with propagating bulk states, sustaining directional, long-range, sub-diffractive propagation at the interface. We observe these features using polariton spectroscopy, far-field probing and near-field imaging, revealing their peculiar dispersion, and - despite their leaky nature - long modal lifetime. Our leaky polaritons (LPs) nontrivially merge sub-diffractive polaritonics with diffractive photonics onto a unified platform, unveiling opportunities that stem from the interplay of extreme anisotropic responses and radiation leakage.

Graphene Nano-Optics in the Terahertz Gap

Graphene nano-optics at terahertz (THz) frequencies (ν) is theoretically anticipated to feature extraordinary effects. However, interrogating such phenomena is nontrivial, since the atomically thin graphene dimensionally mismatches the THz radiation wavelength reaching hundreds of micrometers. Greater challenges happen in the THz gap (0.1-10 THz) wherein light sources are scarce. To surpass these barriers, we use a nanoscope illuminated by a highly brilliant and tunable free-electron laser to image the graphene nano-optical response from 1.5 to 6.0 THz. For ν < 2 THz, we observe a metal-like behavior of graphene, which screens optical fields akin to noble metals, since this excitation range approaches its charge relaxation frequency. At 3.8 THz, plasmonic resonances cause a field-enhancement effect (FEE) that improves the graphene imaging power. Moreover, we show that the metallic behavior and the FEE are tunable upon electrical doping, thus providing further control of these graphene nano-optical properties in the THz gap.

Mid-infrared analogue polaritonic reversed Cherenkov radiation in natural anisotropic crystals

Cherenkov radiation (CR) excited by fast charges can serve as on-chip light sources with a nanoscale footprint and broad frequency range. The reversed CR, which usually occurs in media with the negative refractive index or negative group-velocity dispersion, is highly desired because it can effectively separate the radiated light from fast charges thanks to the obtuse radiation angle. However, reversed CR at the mid-infrared remains challenging due to the significant loss of conventional artificial structures. Here we observe mid-infrared analogue polaritonic reversed CR in a natural van der Waals (vdW) material (i.e., α-MoO₃), whose hyperbolic phonon polaritons exhibit negative group velocity. Further, the real-space image results of analogue polaritonic reversed CR indicate that the radiation distributions and angles are closely related to the in-plane isofrequency contours of α-MoO₃, which can be further tuned in the heterostructures based on α-MoO₃. This work demonstrates that natural vdW heterostructures can be used as a promising platform of reversed CR to design on-chip mid-infrared nano-light sources.

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.

2D Material Infrared Photonics and Plasmonics

Two-dimensional (2D) materials including graphene, transition metal dichalcogenides, black phosphorus, MXenes, and semimetals have attracted extensive and widespread interest over the past years for their many intriguing properties and phenomena, underlying physics, and great potential for applications. The vast library of 2D materials and their heterostructures provides a diverse range of electrical, photonic, mechanical, and chemical properties with boundless opportunities for photonics and plasmonic devices. The infrared (IR) regime, with wavelengths across 0.78 μm to 1000 μm, has particular technological significance in industrial, military, commercial, and medical settings while facing challenges especially in the limit of materials. Here, we present a comprehensive review of the varied approaches taken to leverage the properties of the 2D materials for IR applications in photodetection and sensing, light emission and modulation, surface plasmon and phonon polaritons, non-linear optics, and Smith-Purcell radiation, among others. The strategies examined include the growth and processing of 2D materials, the use of various 2D materials like semiconductors, semimetals, Weyl-semimetals and 2D heterostructures or mixed-dimensional hybrid structures, and the engineering of light-matter interactions through nanophotonics, metasurfaces, and 2D polaritons. Finally, we give an outlook on the challenges in realizing high-performance and ambient-stable devices and the prospects for future research and large-scale commercial applications.

Carrier transport and photoconductivity properties of BN50/NiO50 nanocomposite films

BN₅₀/NiO₅₀ and Au-loaded BN₅₀/NiO₅₀ nanocomposite films were separately fabricated on the glass substrates for carrier transport and photoconductivity properties. X-ray diffraction pattern of the films show the hexagonal structure of BN and presence of defect states by Nelson Riley factor analysis. Morphological images show spherical shaped particles with highly porous structure. The incorporation of NiO may hindered growth of BN layers and resulted in spherical particles. Temperature-dependent conductivity describes semiconductor transport behaviour for deposited nanocomposite films. Thermal activation conduction with low activation energy (∼0.308 eV) may be responsible for the resulting conductivity. Further, the light intensity dependent photoelectrical properties of BN₅₀/NiO₅₀ and Au-loaded BN₅₀/NiO₅₀ nanocomposites have been explored. The effect of Au nanoparticles loading on enhanced photo-conductivities (∼22% increase) than bare nanocomposite film has been elaborated by proposed mechanism. This study provided the insightful information for carrier transport and photoconductivity of BN-based nanocomposites.

Hexagonal Boron Nitride for Next-Generation Photonics and Electronics

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Ultralow-Loss Phonon Polaritons in the Isotope-Enriched α-MoO3

α-MoO₃, a natural van der Waals (vdWs) material, has received wide attention in nano-optics for supporting highly confined anisotropic phonon polaritons (PhPs) from the mid-infrared to the terahertz region, which opens a new route for manipulating light at the nanoscale. However, its optical loss hinders light manipulation with high efficiency. This work demonstrates that the isotope-enriched Mo element enables ultralow-loss PhPs in the α-MoO₃. Raman spectra reveal that the isotope-enriched Mo element in the α-MoO₃ allows different optical phonon frequencies by efficiently altering the Reststrahlen band's dispersion. The Mo isotope-enriched α-MoO₃ significantly reduces the PhPs' optical loss due to efficient optical coherence, which enhances the propagation length revealed by infrared nanoimaging. These findings suggest that the isotope-enriched α-MoO₃ is a new feasible 2D material with an ultralow optical loss for possible high-performance integrated photonics and quantum optics devices.

Curvature-Induced One-Dimensional Phonon Polaritons at Edges of Folded Boron Nitride Sheets

Generation and manipulation of phonon polaritons are of paramount importance for understanding the interaction between an electromagnetic field and dielectric materials and furthering their application in mid-infrared optical communication. However, the formation of tunable one-dimensional phonon polaritons has been rarely realized in van der Waals layered structures. Here we report the discovery of curvature-induced phonon polaritons localized at the crease of folded hexagonal boron nitrides (h-BNs) with a few atomic layers using monochromated electron energy-loss spectroscopy. Compared to bulk regions, the creased-localized signals undergo an abnormal blue-shift of 1.4 meV. First-principles calculations reveal that the energy shift arises from the optical phonon hardening in the curled region. Interestingly, the curvature-induced phonon polariton can also be controllably achieved via an electron-beam etching approach. This work opens an avenue of tailoring local electromagnetic response and creating unique phonon polariton modes in van der Waals layered materials for diverse applications.

Plasmonic gain in current biased tilted Dirac nodes

Surface plasmons, which allow tight confinement of light, suffer from high intrinsic electronic losses. It has been shown that stimulated emission from excited electrons can transfer energy to plasmons and compensate for the high intrinsic losses. To-date, these realizations have relied on introducing an external gain media coupled to the surface plasmon. Here, we propose that plasmons in two-dimensional materials with closely located electron and hole Fermi pockets can be amplified, when an electrical current bias is applied along the displaced electron-hole pockets, without the need for an external gain media. As a prototypical example, we consider WTe₂ from the family of 1T[Formula: see text]-MX₂ materials, whose electronic structure can be described within a type-II tilted massive Dirac model. We find that the nonlocal plasmonic response experiences prominent gain for experimentally accessible currents on the order of mAμm^-1. Furthermore, the group velocity of the plasmon found from the isofrequency curves imply that the amplified plasmons are highly collimated along a direction perpendicular to the Dirac node tilt when the electrical current is applied along it.

Electric Field-Controlled Damping Switches of Coupled Dirac Plasmons

For quasiparticle systems, the control of the quasiparticle lifetime is an important goal, determining whether the related fascinating physics can be revealed in fundamental research and utilized in practical applications. Here, we use double-layer graphene with a boron nitride spacer as a model system to demonstrate that the lifetime of coupled Dirac plasmons can be remotely tuned by electric field-controlled damping pathways. Essentially, one of the graphene layers serves as an external damping amplifier whose efficiency can be controlled by the corresponding doping level. Through this damping switch, the damping rate of the plasmon can be actively tuned up to 1.7 fold. This Letter provides a prototype design to actively control the lifetime of graphene plasmons and also broadens our horizon for the damping control of other quasiparticle systems.

Raman Fingerprint of Interlayer Coupling in 2D TMDCs

Vertical stacking of two-dimensional (2D) homo- and heterostructures are intriguing research objects, as they are essential for fundamental studies and a key towards 2D device applications. It is paramount to understand the interlayer coupling in 2D materials and to find a fast yet precise characteristic signature. In this work, we report on a Raman fingerprint of interlayer coupling in 2D transition metal dichalcogenides (TMDCs). We observed that the out-of-plane B2g vibrational mode is absent when two monolayers form a vertical stack yet remain uncoupled but emerges after strong coupling. Using systematic Raman, photoluminescence (PL), and atomic force microscopy (AFM) studies of WSe2/WSe2 homo-bilayers and MoSe2/WSe2 hetero-bilayers, we conclude that the B2g vibrational mode is a distinct Raman fingerprint of interlayer coupling in 2D TMDCs. Our results propose an easy, fast, precise, and reliable measure to evaluate the interlayer coupling in 2D TMDCs.

Stability of Wafer-Scale Thin Films of Vertically Aligned Hexagonal BN Nanosheets Exposed to High-Energy Ions and Reactive Atomic Oxygen

Stability of advanced functional materials subjected to extreme conditions involving ion bombardment, radiation, or reactive chemicals is crucial for diverse applications. Here we demonstrate the excellent stability of wafer-scale thin films of vertically aligned hexagonal BN nanosheets (hBNNS) exposed to high-energy ions and reactive atomic oxygen representative of extreme conditions in space exploration and other applications. The hBNNS are fabricated catalyst-free on wafer-scale silicon, stainless steel, copper and glass panels at a lower temperature of 400 °C by inductively coupled plasma (ICP) assisted chemical vapor deposition (CVD) and subsequently characterized. The resistance of BNNS to high-energy ions was tested by immersing the samples into the plasma plume at the anode of a 150 W Hall Effect Thruster with BNNS films facing Xenon ions, revealing that the etching rate of BNNS is 20 times less than for a single-crystalline silicon wafer. Additionally, using O₂/Ar/H₂ plasmas to simulate the low Earth orbit (LEO) environment, it is demonstrated that the simulated plasma had very weak influence on the hBNNS surface structure and thickness. These results validate the strong potential of BNNS films for applications as protective, thermally conductive and insulating layers for spacecrafts, electric plasma satellite thrusters and semiconductor optoelectronic devices.

Enhanced Performance of Li-S Batteries due to Synergistic Adsorption and Catalysis Activity within a Separation Coating Made of Hybridized BNNSs/N-Doping Porous Carbon Fibers

Lithium-sulfur (Li-S) batteries with high theoretical energy density are considered as the most promising devices for rechargeable energy-storage systems. However, their actual applications are rather limited by the shuttle effect of lithium polysulfides (LiPSs) and the sluggish redox kinetics. Here, the boron nitride nanosheets are homodispersedly embedded into N-doping porous carbon fibers (BNNSs/CHFs) by an electrospinning technique and a subsequent in situ pyrolysis process. The hybridized BNNSs/CHFs can be smartly designed as a multifunctional separation coating onto the commercial PP membrane to enhance the electrochemical performance of Li-S batteries. As a result, the Li-S batteries with extra BNNSs/CHF modification deliver a highly reversible discharge capacity of 830.4 mA h g^-1 at a current density of 1 C. Even under 4 C, the discharge specific capacity can reach up to 609.9 mA h g^-1 and maintain at 553.9 mA h g^-1 after 500 cycles, showing a low capacity decay of 0.01836% per cycle. It is considered that the excellent performance is attributed to the synergistic effect of adsorption and catalysis of the BNNSs/CHF coating used. First, this coating can efficiently reduce the charge transfer resistance and enhance Li-ion diffusion, due to increased catalytic activity from strong electronic interactions between BNNSs and N-doping CHFs. Second, the combination of polar BNNSs and abundant pore structures within the hybridized BNNSs/CHF networks can highly facilitate an adsorption for LiPSs. Here, we believed that this work would provide a promising strategy to increase the Li-S batteries' performance by introducing hybridized BNNSs/N-doping carbon networks, which could efficiently suppress the LiPSs' shuttle effect and improve the electrochemical kinetics of Li-S batteries.

Two-dimensional Dirac plasmon-polaritons in graphene, 3D topological insulator and hybrid systems

Collective oscillations of massless particles in two-dimensional (2D) Dirac materials offer an innovative route toward implementing atomically thin devices based on low-energy quasiparticle interactions. Strong confinement of near-field distribution on the 2D surface is essential to demonstrate extraordinary optoelectronic functions, providing means to shape the spectral response at the mid-infrared (IR) wavelength. Although the dynamic polarization from the linear response theory has successfully accounted for a range of experimental observations, a unified perspective was still elusive, connecting the state-of-the-art developments based on the 2D Dirac plasmon-polaritons. Here, we review recent works on graphene and three-dimensional (3D) topological insulator (TI) plasmon-polariton, where the mid-IR and terahertz (THz) radiation experiences prominent confinement into a deep-subwavelength scale in a novel optoelectronic structure. After presenting general light-matter interactions between 2D Dirac plasmon and subwavelength quasiparticle excitations, we introduce various experimental techniques to couple the plasmon-polaritons with electromagnetic radiations. Electrical and optical controls over the plasmonic excitations reveal the hybridized plasmon modes in graphene and 3D TI, demonstrating an intense near-field interaction of 2D Dirac plasmon within the highly-compressed volume. These findings can further be applied to invent optoelectronic bio-molecular sensors, atomically thin photodetectors, and laser-driven light sources.

Introduction of Graphene/h-BN Metamaterial as Neutron Radiation Shielding by Implementing Monte Carlo Simulation

In this paper, graphene/h-BN metamaterial was investigated as a new neutron radiation shielding (NRS) material by Monte Carlo N-Particle X version (MCNPX) Transport Code. The graphene/h-BN metamaterial are capable of both thermal and fast neutron moderator and neutron absorber process. The constituent phases in graphene/h-BN metamaterial are chosen to be hexagonal boron nitride (h-BN) and graphene. The introduced target was irradiated by an Am-Be neutron source with an energy spectrum of 100 keV to 15 MeV in a Monte Carlo simulation input file. The resulting current transmission rate (CTR) was investigated by the MCNPX code. Due to concrete's widespread use as a radiation shielding material, the results of this design were also compared with concrete targets. The results show a significant increase in NRS compared to concrete. Therefore, metamaterial with constituent phase's graphene/h-BN can be a suitable alternative to concrete for NRS.

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.

Resonance fluorescence engineering in hybrid systems consist of biexciton quantum dots and anisotropic metasurfaces

The resonance fluorescence properties in the steady-state regime are investigated for a driven cascaded exciton-biexciton quantum dot coupled to the two-dimensional black phosphorus metasurfaces. It is shown that for the material parameters under consideration, both the elliptic and hyperbolic dispersion patterns of the surface plasmon modes are achievable according to the variation of the carrier concentration. Further study on the Purcell factor indicates unequal enhancements in the spontaneous decay of the orthogonal in-plane dipoles. Motivated by this intriguing phenomenon, we then investigate the steady-state properties of the driven quantum dot, where the populations of the dressed levels are highly tunable by engineering the anisotropy of the surfaces. As a result, the manipulation of the carrier concentration will lead to strong modifications in the resonance fluorescence. Under certain conditions, one can observe the squeezing of two-mode noise spectra with different resonances and polarizations. Although at the expense of declines in the photon-sideband detunings, it is feasible to enhance the two-mode squeezing by gate doping. Our proposal can be easily extended to other hybrid systems containing anisotropic metasurfaces, which are important for the development of quantum information science.

Enhancement and Manipulation of Near-Field Thermal Radiation Using Hybrid Hyperbolic Polaritons

Owing to a high electromagnetic confinement and a strong photonic density of states, hyperbolic surface plasmon polaritons (HSPPs) provide a fascinating promise for applications in thermal photonics. In this work, we theoretically predict a possibility for the improvement of the near-field radiative heat transfer on the basis of tailoring the electromagnetic state of hyperbolic metasurfaces by the uniaxial hyperbolic substrate. By using the photonic tunneling coefficient and the polaritons dispersion, we present a comprehensive study of the hybrid effect of the hyperbolic substrate on HSPPs. We find that due to the hybrid effect of the hyperbolic substrate, the anisotropy surface state of hyperbolic metasurfaces would undergo significant deformations and even topological transition. Moreover, we systematically exhibit the evolution of such hybrid hyperbolic mode with different thicknesses of the hyperbolic substrate and analyze the thickness effect on radiative properties of the hybrid system. It is shown that the resulting heat transfer with the assistance of the hybrid hyperbolic mode by optimizing the substrate parameters is many times stronger than that of monolayer hyperbolic metasurface at the same vacuum gap. Taken together, our results provide a platform to tailor 2D hyperbolic plasmons as a potential strategy toward passive or active control of the near-field heat transfer, and the hybrid hyperbolic mode presented here may facilitate the system design for near-field energy harvesting, thermal imaging, and radiative cooling applications.

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.

Plasmon mode manipulation based on multi-layer hyperbolic metamaterials

Metamaterial with hyperbolic dispersion properties can effectively manipulate plasmonic resonances. Here, we designed a hyperbolic metamaterial (HMM) substrate with a near-zero dielectric constant in the near-infrared region to manipulate the plasmon resonance of the nano-antenna (NA). For NA arrays, tuning the equivalent permittivity of HMM substrate by modifying the thickness of Au/diamond, the wavelength range of plasmon resonance can be manipulated. When the size of the NA changes within a certain range, the spectral position of the plasmon resonance will be fixed in a narrow band close to the epsilon-near-zero (ENZ) wavelength and produce a phenomenon similar to "pinning effect." In addition, since the volume plasmon polaritons (VPP) mode is excited, it will couple with the localized surface plasmon (LSP) mode to generate a spectrum splitting. Therefore, the plasmon resonance is significantly affected and can be precisely controlled by designing the HMM substrate.

Plasmonic sensors based on graphene and graphene hybrid materials

The past decade has witnessed a rapid growth of graphene plasmonics and their applications in different fields. Compared with conventional plasmonic materials, graphene enables highly confined plasmons with much longer lifetimes. Moreover, graphene plasmons work in an extended wavelength range, i.e., mid-infrared and terahertz regime, overlapping with the fingerprints of most organic and biomolecules, and have broadened their applications towards plasmonic biological and chemical sensors. In this review, we discuss intrinsic plasmonic properties of graphene and strategies both for tuning graphene plasmons as well as achieving higher performance by integrating graphene with plasmonic nanostructures. Next, we survey applications of graphene and graphene-hybrid materials in biosensors, chemical sensors, optical sensors, and sensors in other fields. Lastly, we conclude this review by providing a brief outlook and challenges of the field. Through this review, we aim to provide an overall picture of graphene plasmonic sensing and to suggest future trends of development of graphene plasmonics.

Influence of Hexagonal Boron Nitride on Electronic Structure of Graphene

By performing first-principles calculations, we studied hexagonal-boron-nitride (hBN)-supported graphene, in which moiré structures are formed due to lattice mismatch or interlayer rotation. A series of graphene/hBN systems has been studied to reveal the evolution of properties with respect to different twisting angles (21.78°, 13.1°, 9.43°, 7.34°, 5.1°, and 3.48°). Although AA- and AB-stacked graphene/hBN are gapped at the Dirac point by about 50 meV, the energy gap of the moiré graphene/hBN, which is much more asymmetric, is only about several meV. Although the Dirac cone of graphene residing in the wide gap of hBN is not much affected, the calculated Fermi velocity is found to decrease with the increase in the moiré super lattice constant due to charge transfer. The periodic potential imposed by hBN modulated charge distributions in graphene, leading to the shift of graphene bands. In agreement with experiments, there are dips in the calculated density of states, which get closer and closer to the Fermi energy as the moiré lattice grows larger.

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.