Twist-polaritonics provides precise control of light-matter states through the stacking of atomically smooth, anisotropic layers, but has been restricted to the van der Waals (vdW) crystals. Non-vdW crystals, despite their symmetry-broken dielectric responses ideal for exotic polaritons, are challenging to prepare as suitable flakes due to their rigid 3D bonding networks, thus limiting the implementation of deep-subwavelength twist-polaritonics. We established a non-vdW polaritonic platform using ultrathin, single-crystalline β-Ga2O3 nanoflakes synthesized by exploiting its anisotropic bonding hierarchy on the quasi-layered (100)B plane. These flakes exhibited deep-subwavelength polariton confinement beyond λ/20. Moreover, their atomic-scale flatness enabled the assembly of twisted bilayers, in which we observed a topological transition of the polariton dispersion from hyperbolic to elliptical, directly controlled by the twist angle. This work positions β-Ga2O3 as a high-performance nanophotonic platform beyond the vdW family, while proposing that anisotropic bonding hierarchy provides a general strategy to unlock non-vdW twist-polaritonic functionality in a wide range of bulk crystals.

Beta‐phase gallium sesquioxide ( β ‐Ga 2 O 3 ), possessing an ultrawide bandgap and high breakdown voltage, exhibits strong potential for deep‐ultraviolet photodetection and high‐power electronics. However, nanometer‐scale line defects, prevalent in β ‐Ga 2 O 3 growth, degrade device performance by increasing leakage currents and reducing breakdown voltages, thus termed “killer defects”. Critically, the impact of these defects at the atomic scale remains unclear due to limited characterization and a lack of detailed understanding. Here, the observation of novel conductive atomic line defects is reported within β ‐Ga 2 O 3 nanoflakes using near‐field infrared imaging. Combining atomic‐resolution imaging with density functional theory calculations, these defects are identified as interstitial Ga atoms migrating along the c ‐axis. These atomic line defects exhibit a broadband infrared response and quenched cathodoluminescence, indicative of significantly enhanced local conductivity. This elevated conductivity enables subsurface near‐field detection of the defects and remote excitation of phonon polaritons in a hexagonal boron nitride ( h BN) capping layer. These findings underscore the distinct conductivity of atomic‐scale line defects, emphasizing the need for their controlled management during material synthesis and device fabrication, while simultaneously suggesting opportunities for their exploitation in nanophotonic applications.

Beta-phase gallium sesquioxide (β-Ga2O3), possessing an ultrawide bandgap and high breakdown voltage, exhibits strong potential for deep-ultraviolet photodetection and high-power electronics. However, nanometer-scale line defects, prevalent in β-Ga2O3 growth, degrade device performance by increasing leakage currents and reducing breakdown voltages, thus termed "killer defects". Critically, the impact of these defects at the atomic scale remains unclear due to limited characterization and a lack of detailed understanding. Here, the observation of novel conductive atomic line defects is reported within β-Ga2O3 nanoflakes using near-field infrared imaging. Combining atomic-resolution imaging with density functional theory calculations, these defects are identified as interstitial Ga atoms migrating along the c-axis. These atomic line defects exhibit a broadband infrared response and quenched cathodoluminescence, indicative of significantly enhanced local conductivity. This elevated conductivity enables subsurface near-field detection of the defects and remote excitation of phonon polaritons in a hexagonal boron nitride (hBN) capping layer. These findings underscore the distinct conductivity of atomic-scale line defects, emphasizing the need for their controlled management during material synthesis and device fabrication, while simultaneously suggesting opportunities for their exploitation in nanophotonic applications.

Saliva glucose detection using QCM technology has garnered significant attention of non-invasive blood glucose monitoring. Introducing antifouling materials is a pivotal strategy for addressing the issue of indiscriminate protein adsorption in saliva. Nonetheless, equipping surfaces with antifouling properties often comes at the cost of compromising the sensitivity of QCM tests. To tackle this challenge, we engineered an innovative antifouling hydrogel that integrates gold nanoparticles functionalized with L-cysteine (Au-Cys). When immobilized on quartz chips, this hydrogel not only enhanced glucose sensitivity but also significantly reduced nonspecific protein adsorption . The sensitivity of hydrogels without Au-Cys increased from 0.34 Hz/(mg/L) to 0.45 Hz/(mg/L) of hydrogels containing Au-Cys. Additionally, protein adhesion of bovine serum albumin, lysozyme, and mucin was markedly decreased by approximately 25, 10, and 50 times, respectively.

Negative refraction provides a platform to manipulate mid-infrared and terahertz radiation for molecular sensing and thermal emission applications. However, its implementation based on metamaterials and plasmonic media presents challenges with optical losses, limited spatial confinement, and lack of active tunability in this spectral range. We demonstrate gate-tunable negative refraction at mid-infrared frequencies using hybrid topological polaritons in van der Waals heterostructures. Specifically, we visualize wide-angle negatively refracted polaritons in α-MoO 3 films partially decorated with graphene, undergoing reversible planar nanoscale focusing. Our atomically thick heterostructures weaken scattering losses at the interface while enabling an actively tunable transition of normal to negative refraction through electrical gating. We propose polaritonic negative refraction as a promising platform for infrared applications such as electrically tunable super-resolution imaging, nanoscale thermal manipulation, enhanced molecular sensing, and on-chip optical circuitry.

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.

A polarization splitter-rotator (PSR) based on an asymmetric directional coupler (ADC) with molybdenum disulfide (MoS 2 ) waveguides is proposed. Owing to the high in-plane refractive index and the extreme birefringence of MoS 2 , an ultra-small cross-polarization coupling length of 6.0 $\mu$m can be achieved. At the working wavelength of 1.55 $\mu$m, this ADC-type PSR exhibits a high TM 0 -to-TE 0 polarization conversion efficiency of 96.97% and a low conversion loss of 0.38 dB. Additionally, the TE 0 -to-TE 0 insertion loss and cross-talk are remarkably low, measuring at 0.006 dB and -31.53dB, respectively. We verify through simulations that the extremely high birefringence of MoS 2 is a necessity for achieving simple and ultra-compact ADC-PSR with straight waveguide geometries. This exceptional impact of extreme birefringence of MoS 2 indicates a unique and promising prospect for developing high-performance nanophotonic components based on the van der Waals on insulator (vdW-OI) configuration.

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.

Due to the two-dimensional character of graphene, the plasmons sustained by this material have been invariably studied in supported samples so far. The substrate provides stability for graphene but often causes undesired interactions (such as dielectric losses, phonon hybridization, and impurity scattering) that compromise the quality and limit the intrinsic flexibility of graphene plasmons. Here, we demonstrate the visualization of plasmons in suspended graphene at room temperature, exhibiting high-quality factor Q ~33 and long propagation length > 3 μm. We introduce the graphene suspension height as an effective plasmonic tuning knob that enables in situ change of the dielectric environment and substantially modulates the plasmon wavelength, propagation length, and group velocity. Such active control of micrometer plasmon propagation facilitates near-unity-order modulation of nanoscale energy flow that serves as a plasmonic switch with an on-off ratio above 14. The suspended graphene plasmons possess long propagation length, high tunability, and controllable energy transmission simultaneously, opening up broad horizons for application in nano-photonic devices.

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.

Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering‐type scanning near‐field optical microscopy (s‐SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5–3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm −1 ), and temporal (<10 fs) resolution. The history of s‐SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research.

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.

Identifying nanoscale biomolecules in aqueous solutions by Fourier transform infrared spectroscopy (FTIR) provides an in situ and noninvasive method for exploring the structure, reactions, and transport of biologically active molecules. However, this remains a challenge due to the strong and broad IR absorption of water which overwhelms the respective vibrational fingerprints of the biomolecules. In this work, a tunable IR transparent microfluidic system with graphene plasmons is exploited to identify ≈2 nm-thick proteins in physiological conditions. The acquired in situ tunability makes it possible to eliminate the IR absorption of water outside the graphene plasmonic hotspots by background subtraction. Most importantly, the ultrahigh confinement of graphene plasmons (confined to ≈15 nm) permits the implementation of nanoscale sensitivity. Then, the deuterium effects on monolayer proteins are characterized within an aqueous solution. The tunable graphene-plasmon-enhanced FTIR technology provides a novel platform for studying biological processes in an aqueous solution at the nanoscale.

Surface phonon polaritons (SPhPs) in van der Waals (vdW) materials are of great interest in fundamental and applied research fields. Probing the characteristics of vdW SPhPs at cryogenic temperatures is an essential task for their implementation in low-temperature physics. However, the most commonly used characterization technique of vdW SPhPs—scattering-type scanning near-field optical microscopy (s-SNOM) operating in a tapping mode (an intermittent-contact mode)—can be problematic at low temperatures because the sample being tested may become brittle and fragile. Therefore, high fracture toughness is desired for the samples under intermittent-contact s-SNOM scanning at low temperatures. In this work, by taking α-phase molybdenum trioxide (α-MoO3) as an example, we first confirm the potential surface deterioration induced by tip-sample interactions at low temperatures. Then, we propose to use few-layer hexagonal boron nitride as a mechanically tough yet optically passive cladding layer to enhance the surface stability of α-MoO3. Finally, we demonstrate the validity of our surface reinforcement strategy by probing the previously unexplored temperature dependence of SPhPs within the third Reststrahlen band of α-MoO3. Our method allows a sustained operation of tapping mode s-SNOM at cryogenic temperatures with negligible effect on intrinsic properties of SPhPs.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale 1 – 10 . So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell’s equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

A glucose biosensor prepared using interpenetrating polymer network (IPN) hydrogel as a sensing material is the subject of growing interest due to its fast response and high sensitivity. However, the IPN hydrogel circumvents the traditional antifouling strategy, which often requires thick antifouling coating that can result in poor glucose sensitivity owing to its energetic physical barrier (greater than 43 nm); thus a complex, time-consuming and high-cost salivary preprocessing is needed to remove protein contaminants before salivary glucose detection using the IPN hydrogel. This limits its practical application in trace salivary glucose-level monitoring. Herein, a new hydrogel film based on a sandwich array (HFSA) with a weak physical barrier, which exhibits superior antifouling and sensitivity in salivary glucose detection is reported. HFSA relies on the formation of the sandwich structure containing substrate-grafted, surface-grafted zwitterionic polymer brushes (pSBMA) and phenylboronic acid (PBA)-functionalized hydrogel. The synergistic effect originating from pSBMA brushes on the surface of HFSA and inside the HFSA matrix provides a suitable physical barrier (∼28 nm) and a robust hydration layer for HFSA, which can enhance its sensitivity and antifouling. The results show that HFSA reduce the adsorption of nonspecific protein in 10% saliva by nearly 90% and enhanced the glucose sensitivity by 130%, compared to the IPN hydrogel film. These results demonstrate that HFSA exhibits significant potential as an antifouling and sensitive glucose probe for QCM sensors in non-invasive salivary glucose monitoring.

Saliva glucose detection based on a quartz crystal microbalance (QCM) sensor has emerged as a promising tool and a non-invasive diagnostic technique for diabetes. However, the low glucose concentration and strong protein interference in the saliva hinder the QCM sensors from practical applications. In this study, we present a robust and simple anti-fouling CNT-PEG-hydrogel film-coated QCM sensor for the detection of saliva glucose with high sensitivity. The CNT-PEG-hydrogel film consists of two layers; the bottom base PBA-hydrogel film is designed to recognize the glucose while the top CNT-PEG layer is used to restrict protein adsorption and improve the biocompatibility. Our results show that this CNT-PEG-hydrogel film exhibited a 10-fold enhancement on the detection limit compared to the PBA-hydrogel. Meanwhile, the adsorption of proteins on the surface of the CNT-PEG-hydrogel film, including bovine serum albumin (BSA), mucin (MUC), and fibrinogen (FIB), were reduced by 99.1%, 77.8%, and 83.7%, respectively. The CNT-PEG-hydrogel film could detect the typical saliva glucose level (0-50 mg L-1) in 10% saliva with a good responsivity. To sum up, this new tool with low-fouling film featuring high stability, specificity, and selectivity holds great potential for non-invasive monitoring of saliva glucose in human physiological levels.

All-optical modulators are attracting significant attention due to their intrinsic perspective on high-speed, low-loss, and broadband performance, which are promising to replace their electrical counterparts for future information communication technology. However, high-power consumption and large footprint remain obstacles for the prevailing nonlinear optical methods due to the weak photon-photon interaction. Here, efficient all-optical mid-infrared plasmonic waveguide and free-space modulators in atomically thin graphene-MoS2 heterostructures based on the ultrafast and efficient doping of graphene with the photogenerated carrier in the monolayer MoS2 are reported. Plasmonic modulation of 44 cm-1 is demonstrated by an LED with light intensity down to 0.15 mW cm-2 , which is four orders of magnitude smaller than the prevailing graphene nonlinear all-optical modulators (≈103 mW cm-2 ). The ultrafast carrier transfer and recombination time of photogenerated carriers in the heterostructure may achieve ultrafast modulation of the graphene plasmon. The demonstration of the efficient all-optical mid-infrared plasmonic modulators, with chip-scale integrability and deep-sub wavelength light field confinement derived from the van der Waals heterostructures, may be an important step toward on-chip all-optical devices.

The development of electronics and photonics is entering a new era of ultrahigh speed sensing, data processing, and telecommunication. The carrier frequencies of the next‐generation electronic devices inevitably extend beyond radio frequencies, marching toward the nominally photonics‐dominated territories, e.g., terahertz and beyond. As a result, electronic and photonic techniques naturally merge and seek common ground. At the forefront of this technical trend is the field of polaritonics, where polaritons are half‐light, half‐matter quasiparticles that carry the properties of both “bare” photons and “bare” dipole‐carrying excitations. The Janus‐faced nature of polaritons renders the unique capability of operando control using photoexcitation or applied electric field. Here, state‐of‐the‐art ultrafast polaritonic phenomena probed by scattering‐type scanning near‐field optical microscope ( s ‐SNOM) techniques is reviewed. The ultrafast dynamical control and loss‐reduction of the polariton propagation are discussed with special emphasis on the creation and probing of the tip or edge induced plasmon– and phonon–polaritons in low‐dimensional systems. The detailed technical aspects of s ‐SNOM and its possible future development are also presented.