Ghost hyperbolic surface polaritons in bulk anisotropic crystals.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1-10. 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.

Large-Scale Suspended Graphene Used as a Transparent Substrate for Infrared Spectroscopy.

Due to weak interactions between micrometer-wavelength infrared (IR) light and nanosized samples, a high signal to noise ratio is a prerequisite in order to precisely characterize nanosized samples using IR spectroscopy. Traditional micrometer-thick window substrates, however, have considerable IR absorption which may introduce unavoidable deformations and interruptions to IR spectra of nanoscale samples. A promising alternative is the use of a suspended graphene substrate which has ultrahigh IR transmittance (>97.5%) as well as unique mechanical properties. Here, an effective method is presented for fabrication of suspended graphene over circular holes up to 150 µm in diameter to be utilized as a transparent substrate for IR spectroscopy. It is demonstrated that the suspended graphene has little impact on the measured IR spectra, an advantage which has led to the discovery of several missing vibrational modes of a 20 nm thick PEO film measured on a traditional CaF2 substrate. This can provide a better understanding of molecules' fine structures and status of hanging bands. The unique optical properties of suspended graphene are determined to be superior to those of conventional IR window materials, giving this new substrate great potential as part of a new generation of IR transparent substrates, especially for use in examining nanoscale samples.

Gate-tunable negative refraction of mid-infrared polaritons.

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-MoO3 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.

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.

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.

Ultrasensitive Mid-Infrared Biosensing in Aqueous Solutions with Graphene Plasmons.

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.

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.

Active control of micrometer plasmon propagation in suspended graphene.

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.

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.

Antifouling hydrogel film based on a sandwich array for salivary glucose monitoring.

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.

Low-fouling CNT-PEG-hydrogel coated quartz crystal microbalance sensor for saliva glucose detection.

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.

Ultrasensitive Poly(boric acid) Hydrogel-Coated Quartz Crystal Microbalance Sensor by Using UV Pressing-Assisted Polymerization for Saliva Glucose Monitoring.

Quartz crystal microbalance (QCM) has attracted extensive attention in the field of biological analysis and detection because of its high sensitivity, fast response, real-time measurement, good operability, and low-cost production. However, to detect the trace amounts of small molecules, such as low-concentration saliva glucose under physiological conditions, is still a major challenge. Herein, the surface of a QCM chip was coated with a poly(boric acid)-based hydrogel using UV pressing-assisted polymerization to obtain a simple device for glucose detection. The designed QCM sensor shows a record-low detection limit of glucose (3 mg/L at pH 7.5), which is ∼30 times lower than that of sensors fabricated by conventional surface initiation-spin coating. The outperformance of the poly(boric acid) hydrogel-coated QCM sensor is probably due to the uniform and compact microstructure, as well as the presence of sufficient glucose-binding sites resulting from the hydrogel coating generated by UV pressing-assisted polymerization. This method provides an important solution to detect the trace amounts of small organic molecules or ions and has the potential to push forward the practical applications of QCM sensors.

Structural colouration in the Himalayan monal, hydrophobicity and refractive index modulated sensing.

The Himalayan monal is a bird in the pheasant family, and it is the national bird of Nepal. The bird possesses spectacular iridescent plumage with a range of different metallic colours. Here, we have studied the internal structure of its feathers from different parts of the bird's body and showed that its beautiful colours and iridescence are due to photonic structures present in the internal structure of the feathers. Sharp changes in the reflected brilliance were observed from the feathers upon changing the illumination conditions, such as horizontal and azimuthal angles. The feathers exhibited interesting hydrophobic properties, with the dull-coloured proximal end showing lower hydrophobicity with a contact angle between 90° and 110° compared with the iridescent distal end of a feather exhibiting a contact angle between 115° and 120°, attributed to the change in the internal structure and/or density of the feathers. A quick reversible change in colours of these feathers was observed when they were soaked in water and other liquids, which reversed upon drying. The shift in colour was suggested to be due to the swelling of the keratin layer of barbules that absorbed liquids and as a result modified the refractive index and periodicity of the internal photonic structures. The colour shift response of feathers was different in the case of alcohols and other water-based solutions, suggesting different swelling behaviour of keratin against different liquids; the water-based solution had the more pronounced effect. Such photonic modulation can be utilized in colour selective filters and sensing devices.

The development of an antifouling interpenetrating polymer network hydrogel film for salivary glucose monitoring.

Owing to its rapid response and broad detection range, a phenylboronic acid (PBA)-functionalized hydrogel film-coated quartz crystal microbalance (QCM) sensor is used to non-invasively monitor salivary glucose in diabetic patients. However, nonspecific protein adsorption on the PBA-functionalized hydrogel film can cause dramatic loss of sensitivity and accuracy of the sensor. A traditional zwitterionic polymer surface with ultra-low protein fouling can hinder the interaction of PBA in the hydrogel matrix with glucose molecules owing to its steric hindrance, resulting in poor glucose sensitivity of the sensor. Herein, we developed a novel hydrogel film that enhanced the antifouling properties and sensitivity of the QCM sensor by infiltrating a glucose-sensitive monomer (i.e., PBA) into a zwitterionic polymer brush matrix to form an interpenetrating polymer network (IPN). The IPN hydrogel film could minimize the glucose sensitivity loss since the antifouling polymer distributed in its matrix. Moreover, a stable hydration layer was formed in this film that could prevent water from transporting out of the matrix, thus further improving its antifouling properties and glucose sensitivity. The experimental results confirmed that the IPN hydrogel film possessed excellent resistance to protein fouling by mucin from whole saliva with reductions in adsorption of nearly 88% and could also enhance the glucose sensitivity by nearly 2 fold, compared to the PBA-functionalized hydrogel film. Therefore, the IPN hydrogel film provides improved antifouling properties and sensitivity of the QCM sensor, which paves the way for non-invasive monitoring of low concentrations of glucose in saliva.

Efficient All-Optical Plasmonic Modulators with Atomically Thin Van Der Waals Heterostructures.

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.

Photo-induced charge density distribution in metal surfaces and its extraction with apertureless near-field optics.

Electromagnetic (EM) waves impinging on finite metallic structures can induce non-uniform electrical currents and create oscillating charge densities. These local charges govern the important physical processes such as plasmonic behavior or enhanced Raman scattering. Yet the quantitative calculation and probing of the spatial distribution of the charge density still remain challenging at the subwavelength scale. This is especially the case if one considers the boundary effect, where the charge density can become divergent and conventional finite element methods fail to obtain accurate information. With an approach we recently developed, we calculate this charge density for subwavelength structures with and without sharp corners: gold disks and equilateral triangles. We also devise an independent way to extract the surface charge density distributions from experiments using scattering-type scanning near-field optical microscope (s-SNOM). We found that the charge density [Formula: see text] is related to the near field signal S n by [Formula: see text] With no adjustable parameters, the extracted surface charge distribution from the experiments matches well with that from the theoretical prediction, both in magnitude and phase. Our work provides a quantitative study of the surface charge distributions and a systematic and rigorous treatment to extract surface charge distributions at the nanoscale, opening opportunities for mining the near-field data from s-SNOM.

Tunable Modal Birefringence in a Low-Loss Van Der Waals Waveguide.

van der Waals (vdW) crystals are promising candidates for integrated phase retardation applications due to their large optical birefringence. Among the two major types of vdW materials, the hyperbolic vdW crystals are inherently inadequate for optical retardation applications since the supported polaritonic modes are exclusively transverse-magnetic (TM) polarized and relatively lossy. Elliptic vdW crystals, on the other hand, represent a superior choice. For example, molybdenum disulfide (MoS2 ) is a natural uniaxial vdW crystal with extreme elliptic anisotropy in the frequency range of optical communication. Both transverse-electric (TE) polarized ordinary and TM polarized extraordinary waveguide modes can be supported in MoS2 microcrystals with suitable thicknesses. In this work, low-loss transmission of these guided modes is demonstrated with nano-optical imaging at the near-infrared (NIR) wavelength (1530 nm). More importantly, by combining theoretical calculations and NIR nanoimaging, the modal birefringence between the orthogonally polarized TE and TM modes is shown to be tunable in both sign and magnitude via varying the thickness of the MoS2 microcrystal. This tunability represents a unique new opportunity to control the polarization behavior of photons with vdW materials.

High-efficiency modulation of coupling between different polaritons in an in-plane graphene/hexagonal boron nitride heterostructure.

Two-dimensional van der Waals (vdW) materials have a full set of highly confined polariton modes, such as low-loss phonon polaritons and dynamically tunable graphene plasmons, which provide a solution for integrated nanophotonic devices by combining the unique advantages of different polaritons. Highly efficient coupling between these complementary polaritons is key to realize the nanoscale optical integration. However, fluctuations of permittivity or geometry at the abrupt interfaces have been demonstrated as perturbations or scatters of polaritons. Here, in-plane plasmon-phonon polariton coupling in an in-plane graphene/hexagonal boron nitride (BN) heterostructure is studied using a full-wave electromagnetic numerical model. Transmittance between different polaritons is proportional to momentum matching, which can be tuned using the graphene Fermi energy. The transmittance between a graphene plasmon and a BN phonon polariton can be controlled between 0% and 100% within the upper Reststrahlen band of the BN. This is central to many photon devices, such as waveguides, wavefront shapers, filters, modulators and switches. Moreover, we simulate near-field interference patterns in an in-plane heterostructure based on the theoretical dispersion relation of polaritons, enabling scattering scanning near-field optical microscopy a potential experimental method to investigate the coupling between different polaritons. This study provides a theoretical basis for efficient coupling of propagation and modulation between different polaritons in in-plane heterostructures of vdW materials, which could pave a way to design nanoscale multi-functional waveguide devices in integrated photonic systems.

Modern Scattering-Type Scanning Near-Field Optical Microscopy for Advanced Material Research.

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.

Flexible and Electrically Tunable Plasmons in Graphene-Mica Heterostructures.

Flexible plasmonic devices with electrical tunability are of great interest for diverse applications, such as flexible metamaterials, waveguide transformation optics, and wearable sensors. However, the traditional flexible metal-polymer plasmonic structures suffer from a lack of electrical tunability. Here the first flexible, electrically tunable, and strain-independent plasmons based on graphene-mica heterostructures are experimentally demonstrated. The resonance frequency, strength, quality factor, electrical tunability, and lifetime of graphene plasmons exhibit no visible change at bending radius down to 1 mm and after 1000 bending cycles at a radius of 3 mm. The plasmon-enhanced infrared spectroscopy detection of chemicals is also demonstrated to be unaffected in the flexible graphene-mica heterostructures. The results provide the basis for the design of flexible active nanophotonic devices such as plasmonic waveguides, resonators, sensors, and modulators.