Ghost hyperbolic surface polaritons in bulk anisotropic crystals

Hyperbolic-to-hyperbolic transition at exceptional Reststrahlen point in rare-earth oxyorthosilicates

Anisotropic optical crystals can exhibit a hyperbolic response within the Reststrahlen band (RB) and support directional polaritonic propagations when interacting with light. Most of the reported low-symmetry optical crystals showcase the evolution from hyperbolic to elliptic dispersion topologies, largely owing to their adjacent RBs being either overlapped or separated. Here, we report an exceptional Reststrahlen point (ERP) in rare-earth oxyorthosilicate Y2SiO5, at which two neighboring RBs almost kiss each other. Consequently, we observe the direct hyperbolic-to-hyperbolic topological transition: the hyperbolic branches close and reopen along with the rotating transverse axis (TA). At such ERP, the TA merges to the direction orthogonal to its proximate phonon mode, mainly due to the interplay between these two non-orthogonal phonon modes. We also find that even with the existence of only one single RB, the TA can rotate in-plane. Our findings are prevalent in isostructural rare-earth oxyorthosilicates, such as Lu2SiO5. The universally underlying physics of ERP and its corresponding special class of rare-earth oxyorthosilicates may offer playgrounds for continuously tuning phonon polariton propagation direction, and broadband controlling light dispersion of polaritonic nanodevices.

Hyperbolic Shear Metasurfaces

Polar dielectrics with low crystal symmetry and sharp phonon resonances can support hyperbolic shear polaritons, which are highly confined surface modes with frequency-dependent optical axes and asymmetric dissipation features. So far, these modes have been observed only in bulk natural materials at midinfrared frequencies, with properties limited by available crystal geometries and phonon resonance strength. Here, we introduce hyperbolic shear metasurfaces, which are ultrathin engineered surfaces supporting hyperbolic surface modes with symmetry-tailored axial dispersion and loss redistribution that can maximally enhance light-matter interactions. By engineering effective shear phenomena in these engineered surfaces, we demonstrate geometry-controlled, ultraconfined, low-loss hyperbolic surface waves with broadband Purcell enhancements applicable across a broad range of the electromagnetic spectrum.

Plasmons in the Kagome metal CsV3Sb5

Plasmon polaritons, or plasmons, are coupled oscillations of electrons and electromagnetic fields that can confine the latter into deeply subwavelength scales, enabling novel polaritonic devices. While plasmons have been extensively studied in normal metals or semimetals, they remain largely unexplored in correlated materials. In this paper, we report infrared (IR) nano-imaging of thin flakes of CsV3Sb5, a prototypical layered Kagome metal. We observe propagating plasmon waves in real-space with wavelengths tunable by the flake thickness. From their frequency-momentum dispersion, we infer the out-of-plane dielectric functionϵ c that is generally difficult to obtain in conventional far-field optics, and elucidate signatures of electronic correlations when compared to density functional theory (DFT). We propose correlation effects might have switched the real part ofϵ c from negative to positive values over a wide range of middle-IR frequencies, transforming the surface plasmons into hyperbolic bulk plasmons, and have dramatically suppressed their dissipation.

Van der Waals quaternary oxides for tunable low-loss anisotropic polaritonics

The discovery of ultraconfined polaritons with extreme anisotropy in a number of van der Waals (vdW) materials has unlocked new prospects for nanophotonic and optoelectronic applications. However, the range of suitable materials for specific applications remains limited. Here we introduce tellurite molybdenum quaternary oxides-which possess non-centrosymmetric crystal structures and extraordinary nonlinear optical properties-as a highly promising vdW family of materials for tunable low-loss anisotropic polaritonics. By employing chemical flux growth and exfoliation techniques, we successfully fabricate high-quality vdW layers of various compounds, including MgTeMoO6, ZnTeMoO6, MnTeMoO6 and CdTeMoO6. We show that these quaternary vdW oxides possess two distinct types of in-plane anisotropic polaritons: slab-confined and edge-confined modes. By leveraging metal cation substitutions, we establish a systematic strategy to finely tune the in-plane polariton propagation, resulting in the selective emergence of circular, elliptical or hyperbolic polariton dispersion, accompanied by ultraslow group velocities (0.0003c) and long lifetimes (5 ps). Moreover, Reststrahlen bands of these quaternary oxides naturally overlap that of α-MoO3, providing opportunities for integration. As an example, we demonstrate that combining α-MoO3 (an in-plane hyperbolic material) with CdTeMoO6 (an in-plane isotropic material) in a heterostructure facilitates collimated, diffractionless polariton propagation. Quaternary oxides expand the family of anisotropic vdW polaritons considerably, and with it, the range of nanophotonics applications that can be envisioned.

Steering and cloaking of hyperbolic polaritons at deep-subwavelength scales

Polaritons are well-established carriers of light, electrical signals, and even heat at the nanoscale in the setting of on-chip devices. However, the goal of achieving practical polaritonic manipulation over small distances deeply below the light diffraction limit remains elusive. Here, we implement nanoscale polaritonic in-plane steering and cloaking in a low-loss atomically layered van der Waals (vdW) insulator, α-MoO3, comprising building blocks of customizable stacked and assembled structures. Each block contributes specific characteristics that allow us to steer polaritons along the desired trajectories. Our results introduce a natural materials-based approach for the comprehensive manipulation of nanoscale optical fields, advancing research in the vdW polaritonics domain and on-chip nanophotonic circuits.

Hybrid Ghost Phonon Polaritons in Thin-Film Heterostructure

Ghost phonon polaritons (g-PhPs), a unique class of phonon polaritons in the infrared, feature ultralong diffractionless propagation (>20 μm) across the surface and tilted wavefronts in the bulk. Here, we study hybrid g-PhPs in a heterostructure of calcite and an ultrathin film of the phase change material (PCM) In3SbTe2, where the optical field is bound in the PCM film with enhanced confinement compared with conventional g-PhPs. Near-field optical images for hybrid g-PhPs reveal a lemniscate pattern in the momentum distribution. We fabricated In3SbTe2 gratings and investigated how different orientations and periodicities of gratings impact the propagation of hybrid g-PhPs. As the grating period decreases to zero, the wavefront of hybrid g-PhPs can be dynamically steered by varying the grating orientation. Our results highlight the promise of hybrid g-PhPs with tunable functionalities for nanophotonic studies.

Twisted moiré conductive thermal metasurface

Extensive investigations on the moiré magic angle in twisted bilayer graphene have unlocked the emerging field-twistronics. Recently, its optics analogue, namely opto-twistronics, further expands the potential universal applicability of twistronics. However, since heat diffusion neither possesses the dispersion like photons nor carries the band structure as electrons, the real magic angle in electrons or photons is ill-defined for heat diffusion, making it elusive to understand or design any thermal analogue of magic angle. Here, we introduce and experimentally validate the twisted thermotics in a twisted diffusion system by judiciously tailoring thermal coupling, in which twisting an analog thermal magic angle would result in the function switching from cloaking to concentration. Our work provides insights for the tunable heat diffusion control, and opens up an unexpected branch for twistronics -- twisted thermotics, paving the way towards field manipulation in twisted configurations including but not limited to fluids.

Exploring van der Waals materials with high anisotropy: geometrical and optical approaches

The emergence of van der Waals (vdW) materials resulted in the discovery of their high optical, mechanical, and electronic anisotropic properties, immediately enabling countless novel phenomena and applications. Such success inspired an intensive search for the highest possible anisotropic properties among vdW materials. Furthermore, the identification of the most promising among the huge family of vdW materials is a challenging quest requiring innovative approaches. Here, we suggest an easy-to-use method for such a survey based on the crystallographic geometrical perspective of vdW materials followed by their optical characterization. Using our approach, we found As2S3 as a highly anisotropic vdW material. It demonstrates high in-plane optical anisotropy that is ~20% larger than for rutile and over two times as large as calcite, high refractive index, and transparency in the visible range, overcoming the century-long record set by rutile. Given these benefits, As2S3 opens a pathway towards next-generation nanophotonics as demonstrated by an ultrathin true zero-order quarter-wave plate that combines classical and the Fabry-Pérot optical phase accumulations. Hence, our approach provides an effective and easy-to-use method to find vdW materials with the utmost anisotropic properties.

Wandering principal optical axes in van der Waals triclinic materials

Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation - an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above π/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.

Manipulating hyperbolic transient plasmons in a layered semiconductor

Anisotropic materials with oppositely signed dielectric tensors support hyperbolic polaritons, displaying enhanced electromagnetic localization and directional energy flow. However, the most reported hyperbolic phonon polaritons are difficult to apply for active electro-optical modulations and optoelectronic devices. Here, we report a dynamic topological plasmonic dispersion transition in black phosphorus via photo-induced carrier injection, i.e., transforming the iso-frequency contour from a pristine ellipsoid to a non-equilibrium hyperboloid. Our work also demonstrates the peculiar transient plasmonic properties of the studied layered semiconductor, such as the ultrafast transition, low propagation losses, efficient optical emission from the black phosphorus's edges, and the characterization of different transient plasmon modes. Our results may be relevant for the development of future optoelectronic applications.

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.

Unique ghost surface phonon polaritons in biaxially hyperbolic materials

We predicted peculiar ghost surface phonon polaritons in biaxially hyperbolic materials, where the two hyperbolic principal axes lie in the plane of propagation. We took the biaxially-hyperbolic α-MoO3 as one example of the materials to numerically simulate the ghost surface phonon polaritons. We found three unique ghost surface polaritons to appear in three enclosed wavenumber-frequency regions, respectively. These ghost surface phonon polaritons have different features from the surface phonon polaritons found previously, i.e., they are some hybrid-polarization surface waves composed of two coherent evanescent branch-waves in the α-MoO3 crystal. The interference of branch-waves leads to that their Poynting vector and electromagnetic fields both exhibit the oscillation-attenuation behavior along the surface normal, or a series of rapidly attenuated fringes. We found that the in-plane hyperbolic anisotropy and low-symmetric geometry of surface are the two necessary conditions for the existence of these ghost surface 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.

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.

New Horizons in Near-Zero Refractive Index Photonics and Hyperbolic Metamaterials

The engineering of the spatial and temporal properties of both the electric permittivity and the refractive index of materials is at the core of photonics. When vanishing to zero, those two variables provide efficient knobs to control light-matter interactions. This Perspective aims at providing an overview of the state of the art and the challenges in emerging research areas where the use of near-zero refractive index and hyperbolic metamaterials is pivotal, in particular, light and thermal emission, nonlinear optics, sensing applications, and time-varying photonics.

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.

Visible to mid-infrared giant in-plane optical anisotropy in ternary van der Waals crystals

Birefringence is at the heart of photonic applications. Layered van der Waals materials inherently support considerable out-of-plane birefringence. However, funnelling light into their small nanoscale area parallel to its out-of-plane optical axis remains challenging. Thus far, the lack of large in-plane birefringence has been a major roadblock hindering their applications. Here, we introduce the presence of broadband, low-loss, giant birefringence in a biaxial van der Waals materials Ta2NiS5, spanning an ultrawide-band from visible to mid-infrared wavelengths of 0.3-16 μm. The in-plane birefringence Δn ≈ 2 and 0.5 in the visible and mid-infrared ranges is one of the highest among van der Waals materials known to date. Meanwhile, the real-space propagating waveguide modes in Ta2NiS5 show strong in-plane anisotropy with a long propagation length (>20 μm) in the mid-infrared range. Our work may promote next-generation broadband and ultracompact integrated photonics based on van der Waals materials.

In-plane dipolar nano-antenna steers polariton waves at nanoscale

Hyperbolic polaritons can be launched and guided into mirror-symmetric-broken trajectories using an in-plane dipolar nano-antenna, and this asymmetry can be configured by adjusting the polarization direction of the in-plane dipole moment.

Terahertz Twistoptics-Engineering Canalized Phonon Polaritons

The terahertz (THz) frequency range is key to studying collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecule arrangements requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confined radiation. For this purpose, in recent years, taking advantage of hyperbolic phonon polaritons (HPhPs) in highly anisotropic van der Waals (vdW) materials has emerged as a promising approach, offering a multitude of manipulation options, such as control over the wavefront shape and propagation direction. Here, we demonstrate the THz application of twist-angle-induced HPhP manipulation, designing the propagation of confined THz radiation between 8.39 and 8.98 THz in the vdW material α-molybdenum trioxide (α-MoO3), hence extending twistoptics to this intriguing frequency range. Our images, recorded by near-field optical microscopy, show the frequency- and twist-angle-dependent changes between hyperbolic and elliptic polariton propagation, revealing a polaritonic transition at THz frequencies. As a result, we are able to allocate canalization (highly collimated propagation) of confined THz radiation by carefully adjusting these two parameters, i.e. frequency and twist angle. Specifically, we report polariton canalization in α-MoO3 at 8.67 THz for a twist angle of 50°. Our results demonstrate the precise control and manipulation of confined collective excitations at THz frequencies, particularly offering possibilities for nanophotonic applications.

Colossal Optical Anisotropy from Atomic-Scale Modulations

Materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light-matter coupling, such as hyperbolic phonon polaritons. Layered 2D materials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure-Sr9/8 TiS3 -is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3 , results in the formation of TiS6 trigonal-prismatic units that break the chains of face-sharing TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary index ne and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation.

Controlling the propagation asymmetry of hyperbolic shear polaritons in beta-gallium oxide

Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO3 has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga2O3 (bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals.

Ultrafast anisotropic dynamics of hyperbolic nanolight pulse propagation

Polariton pulses-transient light-matter hybrid excitations-traveling through anisotropic media can lead to unusual optical phenomena in space and time. However, studying these pulses presents challenges with their anisotropic, ultrafast, and nanoscale field variations. Here, we demonstrate the creation, observation, and control of polariton pulses, with in-plane hyperbolic dispersion, on anisotropic crystal surfaces by using a time-resolved nanoimaging technique and our developed high-dimensional data processing. We capture and analyze movies of distinctive pulse spatiotemporal dynamics, including curved ultraslow energy flow trajectories, anisotropic dissipation, and dynamical misalignment between phase and group velocities. Our approach enables analysis of polariton pulses in the wave vector time domain, demonstrating a time-domain polaritonic topological transition from lenticular to hyperbolic dispersion contours and the ability to study the polariton-induced time-varying optical forces. Our findings promise to facilitate the study of diverse space-time phenomena at extreme scales and drive advances in ultrafast nanoimaging.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

Optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3

Phonon polaritons (PhPs), collective modes hybridizing photons with lattice vibrations in polar insulators, enable nanoscale control of light. In recent years, the exploration of in-plane anisotropic PhPs has yielded new levels of confinement and directional manipulation of nano-light. However, the investigation of in-plane anisotropic PhPs at the atomic layer limit is still elusive. Here, we report the optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3 (5 atomic layers). We show that narrow α-MoO3 nanoribbons as thin as a few atomic layers can support anisotropic PhPs modes with a high confinement ratio (∼133 times smaller wavelength than that of light). The anisotropic PhPs interference fringe patterns in atomic layers are tunable depending on the PhP wavelength via changing the illumination frequency. Moreover, spatial control over the PhPs interference patterns is also achieved by varying the nanostructures' shape or nanoribbon width of atomically-thin α-MoO3. Our work may serve as an empirical reference point for other anisotropic PhPs that approach the thickness limit and pave the way for applications such as atomically integrated nano-photonics and sensing.

Ultrafast and Nanoscale Energy Transduction Mechanisms and Coupled Thermal Transport across Interfaces

The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.

Microenvironment Restruction of Emerging 2D Materials and their Roles in Therapeutic and Diagnostic Nano-Bio-Platforms

Engineering advanced therapeutic and diagnostic nano-bio-platforms (NBPFs) have emerged as rapidly-developed pathways against a wide range of challenges in antitumor, antipathogen, tissue regeneration, bioimaging, and biosensing applications. Emerged 2D materials have attracted extensive scientific interest as fundamental building blocks or nanostructures among material scientists, chemists, biologists, and doctors due to their advantageous physicochemical and biological properties. This timely review provides a comprehensive summary of creating advanced NBPFs via emerging 2D materials (2D-NBPFs) with unique insights into the corresponding molecularly restructured microenvironments and biofunctionalities. First, it is focused on an up-to-date overview of the synthetic strategies for designing 2D-NBPFs with a cross-comparison of their advantages and disadvantages. After that, the recent key achievements are summarized in tuning the biofunctionalities of 2D-NBPFs via molecularly programmed microenvironments, including physiological stability, biocompatibility, bio-adhesiveness, specific binding to pathogens, broad-spectrum pathogen inhibitors, stimuli-responsive systems, and enzyme-mimetics. Moreover, the representative therapeutic and diagnostic applications of 2D-NBPFs are also discussed with detailed disclosure of their critical design principles and parameters. Finally, current challenges and future research directions are also discussed. Overall, this review will provide cutting-edge and multidisciplinary guidance for accelerating future developments and therapeutic/diagnostic applications of 2D-NBPFs.

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.

Launching and Manipulation of Higher-Order In-Plane Hyperbolic Phonon Polaritons in Low-Dimensional Heterostructures

Hyperbolic phonon polaritons (HPhPs) are stimulated by coupling infrared (IR) photons with the polar lattice vibrations. Such HPhPs offer low-loss, highly confined light propagation at subwavelength scales with out-of-plane or in-plane hyperbolic wavefronts. For HPhPs, while a hyperbolic dispersion implies multiple propagating modes with a distribution of wavevectors at a given frequency, so far it has been challenging to experimentally launch and probe the higher-order modes that offer stronger wavelength compression, especially for in-plane HPhPs. In this work, the experimental observation of higher-order in-plane HPhP modes stimulated on a 3C-SiC nanowire (NW)/α-MoO₃ heterostructure is reported where leveraging both the low-dimensionality and low-loss nature of the polar NWs, higher-order HPhPs modes within 2D α-MoO₃ crystal are launched by the 1D 3C-SiC NW. The launching mechanism is further studied and the requirements for efficiently launching of such higher-order modes are determined. In addition, by altering the geometric orientation between the 3C-SiC NW and α-MoO₃ crystal, the manipulation of higher-order HPhP dispersions as a method of tuning is demonstrated. This work illustrates an extremely anisotropic low dimensional heterostructure platform to confine and configure electromagnetic waves at the deep-subwavelength scales for a range of IR applications including sensing, nano-imaging, and on-chip photonics.

Topological Metamaterials

The topological properties of an object, associated with an integer called the topological invariant, are global features that cannot change continuously but only through abrupt variations, hence granting them intrinsic robustness. Engineered metamaterials (MMs) can be tailored to support highly nontrivial topological properties of their band structure, relative to their electronic, electromagnetic, acoustic and mechanical response, representing one of the major breakthroughs in physics over the past decade. Here, we review the foundations and the latest advances of topological photonic and phononic MMs, whose nontrivial wave interactions have become of great interest to a broad range of science disciplines, such as classical and quantum chemistry. We first introduce the basic concepts, including the notion of topological charge and geometric phase. We then discuss the topology of natural electronic materials, before reviewing their photonic/phononic topological MM analogues, including 2D topological MMs with and without time-reversal symmetry, Floquet topological insulators, 3D, higher-order, non-Hermitian and nonlinear topological MMs. We also discuss the topological aspects of scattering anomalies, chemical reactions and polaritons. This work aims at connecting the recent advances of topological concepts throughout a broad range of scientific areas and it highlights opportunities offered by topological MMs for the chemistry community and beyond.

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.

In-plane hyperbolic polariton tuners in terahertz and long-wave infrared regimes

One of the main bottlenecks in the development of terahertz (THz) and long-wave infrared (LWIR) technologies is the limited intrinsic response of traditional materials. Hyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple strongly with THz and LWIR radiation. However, the mismatch of photon - polariton momentum makes far-field excitation of HPhPs challenging. Here, we propose an In-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals semiconductors, here α-MoO₃, into ribbon arrays. We demonstrate that such tuners respond directly to far-field excitation and give rise to LWIR and THz resonances with high quality factors up to 300, which are strongly dependent on in-plane hyperbolic polariton of the patterned α-MoO₃. We further show that with this tuner, intensity regulation of reflected and transmitted electromagnetic waves, as well as their wavelength and polarization selection can be achieved. Our results can help the development of THz and LWIR miniaturized devices.

Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons

Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)-which exist in much broader spectral ranges than phonon polaritons-has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag₂Te platelets. The hybridization of the PPs with their mirror image-by placing the platelets above a Au layer-increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

Giant and Tunable Excitonic Optical Anisotropy in Single-Crystal Halide Perovskites

During the last years, giant optical anisotropy has demonstrated its paramount importance for light manipulation. In spite of recent advances in the field, the achievement of continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through the chemical alteration of halogen atoms in single-crystal halide perovskites. As a result, we manage to continually modify the optical anisotropy by 0.14. We also discover that the halide perovskite can demonstrate optical anisotropy up to 0.6 in the visible range─the largest value among non-van der Waals materials. Moreover, our results reveal that this anisotropy could be in-plane and out-of-plane depending on perovskite shape─rectangular and square. As a practical demonstration, we have created perovskite anisotropic nanowaveguides and shown a significant impact of anisotropy on high-order guiding modes. These findings pave the way for halide perovskites as a next-generation platform for tunable anisotropic photonics.

Light People: Professor Cheng-Wei Qiu

EDITORIAL: "Mind-controlled information transmission" is one of human's ultimate fantasies for the development of science and technology. On June 11, 2022, eLight Journal simultaneously published two research papers on brainwave-based metasurfaces, and proposed a new concept of metasurface controlled by mind for the first time in the world. This major scientific breakthrough rushed up to the hot search list of public media. It was read more than ten million times, and the related video has been played 600,000 times, which aroused the audience's strong interest and discussion, and really made a significant leap toward "mind control". We are greatly honored to invite Prof. Cheng-Wei Qiu from National University of Singapore, the corresponding author of both above-mentioned articles and the co-Editor-in-Chief of eLight Journal, to have an exclusive interview. Prof. Cheng-Wei Qiu showed us the profound interpretation of his research works and shared his unique insights on journal development and talent cultivation.

Calibrate the non-orthogonal error of AFM with two-dimensional self-traceable grating

The calibration of the non-orthogonal error in nanoscale measurements is of paramount importance for analytical measuring instruments. Particularly, the calibration of non-orthogonal errors in atomic force microscopy (AFM) is essential for the traceable measurements of novel materials and two-dimensional (2D) crystals. The 2D self-traceable grating with a theoretical non-orthogonal angle of less than 0.0027° and an expanded uncertainty of 0.003° (k = 2) are measured by the Metrological Large Range Scanning Probe Microscope (Met. LR-SPM). In this study, we characterized the local and overall non-orthogonal error in AFM scans and proposed a protocol to tune the optimal scanning parameters of AFM minimizing the non-orthogonal error. We presented the method for accurately calibrating a commercial AFM system for non-orthogonal by establishing a detailed uncertainty budget and errors analysis. Our results verified the important advantages of the 2D self-traceable grating in calibrating precision instruments.

A Review of Scalable Hexagonal Boron Nitride (h-BN) Synthesis for Present and Future Applications

Hexagonal boron nitride (h-BN) is a layered inorganic synthetic crystal exhibiting high temperature stability and high thermal conductivity. As a ceramic material it has been widely used for thermal management, heat shielding, lubrication, and as a filler material for structural composites. Recent scientific advances in isolating atomically thin monolayers from layered van der Waals crystals to study their unique properties has propelled research interest in mono/few layered h-BN as a wide bandgap insulating support for nanoscale electronics, tunnel barriers, communications, neutron detectors, optics, sensing, novel separations, quantum emission from defects, among others. Realizing these futuristic applications hinges on scalable cost-effective high-quality h-BN synthesis. Here, the authors review scalable approaches of high-quality mono/multilayer h-BN synthesis, discuss the challenges and opportunities for each method, and contextualize their relevance to emerging applications. Maintaining a stoichiometric balance B:N = 1 as the atoms incorporate into the growing layered crystal and maintaining stacking order between layers during multi-layer synthesis emerge as some of the main challenges for h-BN synthesis and the development of processes to address these aspects can inform and guide the synthesis of other layered materials with more than one constituent element. Finally, the authors contextualize h-BN synthesis efforts along with quality requirements for emerging applications via a technological roadmap.

Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal

Various optical crystals possess permittivity components of opposite signs along different principal directions in the mid-infrared regime, exhibiting exotic anisotropic phonon resonances. Such materials with hyperbolic polaritons-hybrid light-matter quasiparticles with open isofrequency contours-feature large-momenta optical modes and wave confinement that make them promising for nanophotonic on-chip technologies. So far, hyperbolic polaritons have been observed and characterized in crystals with high symmetry including hexagonal (boron nitride), trigonal (calcite) and orthorhombic (α-MoO₃ or α-V₂O₅) crystals, where they obey certain propagation patterns. However, lower-symmetry materials such as monoclinic crystals were recently demonstrated to offer richer opportunities for polaritonic phenomena. Here, using scanning near-field optical microscopy, we report the direct real-space nanoscale imaging of symmetry-broken hyperbolic phonon polaritons in monoclinic CdWO₄ crystals, and showcase inherently asymmetric polariton excitation and propagation associated with the nanoscale shear phenomena. We also introduce a quantitative theoretical model to describe these polaritons that leads to schemes to enhance crystal asymmetry via the damping loss of phonon modes. Ultimately, our findings show that polaritonic nanophotonics is attainable using natural materials with low symmetry, favouring a versatile and general way to manipulate light at the nanoscale.

Topologically reconfigurable magnetic polaritons

Hyperbolic polaritons in extremely anisotropic materials have attracted intensive attention due to their exotic optical features. Recent advances in optical materials reveal unprecedented dispersion engineering of polaritons, resulting in twistronics for photons, canalized phonon polaritons, shear polaritons, and tunable topological polaritons. However, the on-demand reconfigurability of polaritons, especially with magnetic anisotropic dispersions, is restricted by weak natural magnetic anisotropy and hence remains largely unexplored. Here, we show how origami fused with artificial magnetism unveils a versatile pathway to topologically reconfigure magnetic polaritons. We experimentally demonstrate that the three-dimensional origami deformation allows to reconfigure hyperbolic or elliptic topology of polariton dispersion and modulate group velocity. With group velocity transitioning from positive to negative directions, we further report reconfigurable origami polariton circuitry in which the polariton propagation and phase distribution can be tailored. Our findings provide alternative perspectives on on-chip polaritonics, with potential applications in energy transfer, sensing, and information transport.

Two-dimensional natural hyperbolic materials: from polaritons modulation to applications

Natural hyperbolic materials (HMs) in two dimensions (2D) have an extraordinarily high anisotropy and a hyperbolic dispersion relation. Some of them can even sustain hyperbolic polaritons with great directional propagation and light compression to deeply sub-wavelength scales due to their inherent anisotropy. Herein, the anisotropic optical features of 2D natural HMs are reviewed. Four hyperbolic polaritons (i.e., phonon polaritons, plasmon polaritons, exciton polaritons, and shear polaritons) as well as their generation mechanism are discussed in detail. The natural merits of 2D HMs hold promise for practical quantum photonic applications such as valley quantum interference, mid-infrared polarizers, spontaneous emission enhancement, near-field thermal radiation, and a new generation of optoelectronic components, among others. The conclusion of these analyses outlines existing issues and potential interesting directions for 2D natural HMs. These findings could spur more interest in anisotropic 2D atomic crystals in the future, as well as the quick generation of natural HMs for new applications.

Infrared plasmons propagate through a hyperbolic nodal metal

Metals are canonical plasmonic media at infrared and optical wavelengths, allowing one to guide and manipulate light at the nanoscale. A special form of optical waveguiding is afforded by highly anisotropic crystals revealing the opposite signs of the dielectric functions along orthogonal directions. These media are classified as hyperbolic and include crystalline insulators, semiconductors, and artificial metamaterials. Layered anisotropic metals are also anticipated to support hyperbolic waveguiding. However, this behavior remains elusive, primarily because interband losses arrest the propagation of infrared modes. Here, we report on the observation of propagating hyperbolic waves in a prototypical layered nodal-line semimetal ZrSiSe. The observed waveguiding originates from polaritonic hybridization between near-infrared light and nodal-line plasmons. Unique nodal electronic structures simultaneously suppress interband loss and boost the plasmonic response, ultimately enabling the propagation of infrared modes through the bulk of the crystal.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Spin angular momentum and nonreciprocity of ghost surface polariton in antiferromagnets

We investigated the spin angular momentum (SAM) and nonreciprocity of ghost surface polariton (GSP) at the surface of an antiferromagnet (AF) in the normal geometry, where the AF easy axis and external field (H₀) both are normal to the AF surface. We found that the dispersion equation is invariant when the inversions of wavevector and external magnetic field, k→-k and H₀→-H₀, are taken. However, its polarization and SAM are nonreciprocal. The SAM is vertical to the propagation direction of GSP, and consists of two components. We analytically found that the in-plane component is locked to H₀, or it is changed in sign due to the inversion of H₀. The out-plane one is locked to k since it is changed in sign as the inversion of k is taken. Either component contains an electric part and a magnetic part. Above the AF surface, the two electric parts form the left-handed triplet with the wavevector k, but the two magnetic parts form the right-handed triplet with k. In the AF, the chirality of the SAM changes with the distance from the surface. The SAM is very large on or near the surface and it may be very interesting for the manipulation of micron and nano particles on the AF surface. These are obviously different from the relevant features of conventional surface polaritons. The SAM also is field-tunable.

Near-Isotropic Polariton Heat Transport along a Polar Anisotropic Nanofilm

The heat transport of surface phonon-polaritons propagating along a polar uniaxial anisotropic nanofilm is studied for different orientations of its optical axis, film thicknesses, and temperatures. For an hBN nanofilm, it is shown that i) the propagation of polaritons can be described in terms of even and odd modes that generalize the transverse magnetic and transverse electrical ones that typically appear in isotropic films. ii) The frequency spectrum of polaritons can efficiently be tuned with the angle between the film optical axis and their propagation direction. iii) The polariton thermal conductivity takes higher values for a thinner or hotter nanofilm. iv) The even and odd modes have a remarkable contribution to the total polariton thermal conductivity, which takes a value higher than 5.6 Wm⁻¹K⁻¹ for a 25-nm-thick nanofilm at 500 K. The obtained results thus uncover some key features of the propagation and heat transport of polaritons in uniaxial nanofilms.

•

Polariton thermal conductivity of a uniaxial anisotropic nanofilm

•

Polariton heat transport driven by the optical axis of a polar anisotropic nanofilm

•

Higher thermal conductivities for thinner or hotter nanofilms

•

Near-isotropic thermal response of a strongly anisotropic nanofilm

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.

Enabling Active Nanotechnologies by Phase Transition: From Electronics, Photonics to Thermotics

Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.

Long-wavelength infrared selective emitter for thermal infrared camouflage under a hot environment

Thermal infrared camouflage as a kind of counter-surveillance technique has attracted much attention owing to the rapid development of infrared surveillance technology. Various artificial optical structures have been developed for infrared camouflage applications under cold ambient environment (low thermal radiation), but the realization of infrared camouflage under a hot environment (high thermal radiation) is also highly desirable and has been rarely reported. Here, a lithography-free, ultra-thin, high performance long-wavelength infrared (LWIR) selective emitter for thermal infrared camouflage in a high radiation environment is proposed and experimentally demonstrated. Experimental results show that our designed selective emitter exhibits average emissivity higher than 90% over the LWIR range from 8 to 14 µm and low emissivity less than 35% outside this window. Numerical simulations were performed to optimize the geometrical structures and reveal that such a selective emission effect is attributed to the combination of multiple hybrid plasmonic resonances. LWIR thermal images show that the selective emitter can perfectly blend into the high radiation backgrounds. Furthermore, it is found that the sample displays angle-independent emission properties, indicating that our emitter offers great potential for application in evading large-angle detection.

Unique surface polaritons and their transitions in metamaterials

We investigated surface polaritons in a metamaterial composed of polar-crystal layers and antiferromagnetic layers. In a specific geometry, two surface polaritons were predicted, which are a unique ghost surface polariton (GSP) and surface hybrid-polarization polariton (SHP). The two surface polaritons occupy different segments of one smooth dispersion curve and are magnetically tunable. An external magnetic field along the antiferromagnetic easy axis can bring about the switch or transition between the two surface polaritons and meanwhile performs the necessary condition for the existence of two surface polaritons. In the metamaterial, either surface polariton consists of two branch waves. The branch waves of the GSP are coherent and have the same amplitude and different phases, but those of the SHP are not coherent and have different amplitudes and phases. The main characteristic of the GSP is that its fields oscillate and attenuate with the distance away from the metamaterial surface and exhibit interferent fringes on the plane normal to the surface.

Giant optical anisotropy of WS2flakes in the visible region characterized by Au substrate assisted near-field optical microscopy

Transition metal dichalcogenides (TMD) have attracted considerable attention in the field of photonic integrated circuits due to their giant optical anisotropy. However, on account of their inherent loss in the visible region and the difficulty of measuring high refractive index materials, near-field characterizations of the optical anisotropy of TMD in the visible region have inherent experimental difficulties. In this work, we present a systematical characterization of the optical anisotropy in tungsten disulfide (WS₂) flakes by using scattering-type scanning near-field optical microscopy (s-SNOM) excited at 671 nm. Transverse-electric and transverse-magnetic (TM) waveguide modes can be excited in WS₂flakes with suitable thickness, respectively. With the assistance of the Au substrate, the contrast of the near-field fringes is enhanced in comparison with the SiO₂substrate. By combining waveguide mode near-field imaging and theoretical calculations, the in-plane and out-of-plane refractive indexes of WS₂are determined to be 4.96 and 3.01, respectively, indicating a high birefringence value up to 1.95. This work offers experimental evidence for the potential application of WS₂in optoelectronic integrated circuits in the visible region.

Vibrational Strong Coupling between Surface Phonon Polaritons and Organic Molecules via Single Quartz Micropillars

Vibrational strong coupling (VSC), the strong coupling between optical resonances and the dipolar absorption of molecular vibrations at mid-infrared frequencies, holds the great potential for the development of ultrasensitive infrared spectroscopy, the modification of chemical properties of molecules, and the control of chemical reactions. In the realm of ultracompact VSC, there is a need to scale down the size of mid-infrared optical resonators and to elevate their optical field strength. Herein, by using single quartz micropillars as mid-infrared optical resonators, the strong coupling is demonstrated between surface phonon polariton (SPhP) resonances and molecular vibrations from far-field observation. The single quartz micropillars support sharp SPhP resonances with an ultrasmall mode volume, which strongly couples with the molecular vibrations of 4-nitrobenzyl alcohol (C₇ H₇ NO₃ ) molecules featuring pronounced mode splitting and anticrossing dispersion. The coupling strength depends on the molecular concentration and reaches the strong coupling regime with only 7300 molecules. The findings pave the way for promoting the VSC sensitivity, miniaturing the VSC devices, and will boost the development of ultracompact mid-infrared spectroscopy and chemical reaction control devices.

Optical axis-driven modulation of near-field radiative heat transfer between two calcite parallel structures

Recently, the increasing research on the anisotropic optical axis (OA) has provided a novel way to control light. However, this method is rarely applied to modulate the near-field radiative heat...