Spin-orbital angular momentum degeneracy breaking in nanoplasmonic metachain.

The spin and orbital angular momentum (namely SAM and OAM) mode division provides a promising solution to surmount exhausted available degrees of freedom in conventional optical communications. Nevertheless, SAM and OAM are often subjected to the degeneracy of total angular momentum (AM) because they both have integer variables of quantum eigenstates, which inevitably brings about the shortcomings specific to limited signal channels and multiplexing cross talk. Herein, we present a nanoplasmonic metachain that can discriminatively couple any input SAM and OAM components to an extrinsic orbital AM, corresponding to the chirality and topological charge of incident light. Importantly, the unambiguous measurement has a prominent advantage of detecting the arbitrary AM component rather than the total AM. The miniature metadevice offers the possibility of harnessing AM division on chip or in fiber and holds great promise to delve the spin-orbit interactions for topological photonics and quantum cryptography.

Achiral nanoparticle trapping and chiral nanoparticle separating with quasi-BIC metasurface.

Dielectric metasurfaces based on quasi-bound states in the continuum (quasi-BICs) are a promising approach for manipulating light-matter interactions. In this study, we numerically demonstrate the potential of silicon elliptical tetramer dielectric metasurfaces for achirality nanoparticle trapping and chiral nanoparticle separation. We first analyze a symmetric tetramer metasurface, which exhibits dual resonances (P1 and P2) with high electromagnetic field intensity enhancement and a high-quality factor (Q-factor). This metasurface can trap achiral nanoparticles with a maximum optical trapping force of 35 pN for 20 nm particles at an input intensity of 100 mW. We then investigate an asymmetric tetramer metasurface, which can identify and separate enantiomers under the excitation of left-handed circularly polarized (LCP) light. Results show that the chiral optical force can push one enantiomer towards regions of the quasi-BIC system while removing the other. In addition, the proposed asymmetric tetramer metasurface can provide multiple Fano resonances (ranging from R1 to R5) and high trap potential wells of up to 33 kBT. Our results demonstrate that the proposed all-dielectric metasurface has high performance in nanoparticle detection, with potential applications in biology, life science, and applied physics.

Topological photonic crystal biosensor with valley edge modes based on a silicon-on-insulator slab.

In the development of integrated sensing, how to reduce losses and improve robustness has always been one of the key problems to be solved. The topological photonic crystal structure based on the quantum Hall effect has gradually attracted the attention of researchers due to its unique immune defect performance and anti-scattering performance. Here, we have successfully applied the valley photonic crystal structures to topologically manipulate the light within the band gap of 252 THz-317 THz in a silicon-on-insulator platform. We experimentally demonstrated that satisfactory transmission performance can be obtained using the valley-dependent topological edge states below light cone, even if there are structure defects such as lattice missing and lattice mistake near the interface between two kinds VPCs. Based on the features of topological protection, a triangular cavity consisting of three 10×a-length sides is proposed, and the Q factor value reaches 1.83×105 with little influence from defects. Finally, based on drying etching technology, a biosensor with cavity-coupled waveguide structure was prepared, and the RI sensitivity was 1228 nm/RIU.

Highly sensitive nanoparticle sensing based on a hybrid cavity in a freely suspended microfiber.

We report a hybrid cavity structure based on a suspended microfiber with a diameter of 1.7 µm applied to nanoparticle sensing in water. The proposed hybrid cavity is composed of two symmetrical reflectors with a slotted Au layer in the middle. After being characterized by the finite-difference time-domain method, the obtained strong reflection and sufficiently wide band gap provide the potential to realize an ultrasmall mode volume, which can improve sensitivity and lower loss. In addition, an ultrahigh Q/V ratio of 8.2 × 106 (λ/n)-3 and high resonance transmittance of T = 0.53 can be obtained through optimization analysis. After analyzing the trapping force and resonance shift caused by the change in local electric field, it is proven that our proposed cavity exhibits a high sensitivity and offers a convenient and stable method for particle sensing in water.

A Magnetic Field SPR Sensor Based on Temperature Self-Reference.

In this paper, a novel D-shaped photonic crystal fiber sensor for simultaneous measurements of magnetic field and temperature is proposed and characterized. Based on the surface plasmon resonance theory, the D-shaped flat surface coated with a gold layer is in direct contact with magnetic fluid to detect magnetic field, and one of the relatively small air holes near the fiber core is filled with polydimethylsiloxane (PDMS) to sense temperature. The realization of measuring the magnetic field and temperature separately through two channels depends on the fact that the magnetic field only changes the refractive index of the magnetic fluid, but has no effect on the refractive index of PDMS. The refractive index of the magnetic fluid and PDMS can be affected by temperature at the same time. The sensor designed in this work can separate the variations of the magnetic field and temperature simultaneously, therefore solving the cross-sensitivity problem to further improve the magnetic field sensitivity. When the thickness of the gold film is 50 nm and the radius of the filling hole is 0.52 µm, the magnetic field sensitivity and the temperature sensitivity of magnetic field sensor based on temperature self-reference can reach 0.14274 nm/Oe and -0.229 nm/°C, respectively.

Output-Only Damage Detection in Plate-Like Structures Based on Proportional Strain Flexibility Matrix.

For engineering structures, strain flexibility-based approaches have been widely used for structural health monitoring purposes with prominent advantages. However, the applicability and robustness of the method need to be further improved. In this paper, a novel damage index based on differences in uniform load strain field (ULSF) is developed for plate-like structures. When estimating ULSF, the strain flexibility matrix (SFM) based on mass-normalized strain mode shapes (SMSs) is needed. However, the mass-normalized strain mode shapes (SMSs) are complicated and difficult to obtain when the input, i.e., the excitation, is unknown. To address this issue, the proportional strain flexibility matrix (PSFM) and its simplified construction procedure are proposed and integrated into the frames of ULSF, which can be easily obtained when the input is unknown. The identification accuracy of the method under the damage with different locations and degrees is validated by the numerical examples and experimental examples. Both the numerical and experimental results demonstrate that the proposed method provides a reliable tool for output-only damage detection of plate-like structures without estimating the mass-normalized strain mode shapes (SMSs).

Dynamic Plasmon Resonance Tuning for Surface Enhanced Sensing.

We report on fabricating plasmonic nanorod crystals using focused ion beam lithography. We first demonstrate manipulating the profiles of nanorods perpendicularly aligned with the substrate. Then we show accurate control of nanorod outlines can be achieved. We also show that it is feasible to manufacture nanorods obliquely aligned with the substrate. Tunable plasmon resonance can be realized with different tilting angles and geometries. Our approach may find important applications in plasmon-assisted sensing and surface enhanced spectroscopy.

Metamaterial Lensing Devices.

In recent years, the development of metamaterials and metasurfaces has drawn great attention, enabling many important practical applications. Focusing and lensing components are of extreme importance because of their significant potential practical applications in biological imaging, display, and nanolithography fabrication. Metafocusing devices using ultrathin structures (also known as metasurfaces) with superlensing performance are key building blocks for developing integrated optical components with ultrasmall dimensions. In this article, we review the metamaterial superlensing devices working in transmission mode from the perfect lens to two-dimensional metasurfaces and present their working principles. Then we summarize important practical applications of metasurfaces, such as plasmonic lithography, holography, and imaging. Different typical designs and their focusing performance are also discussed in detail.

Metadevices with Potential Practical Applications.

Metamaterials are "new materials" with different superior physical properties, which have generated great interest and become popular in scientific research. Various designs and functional devices using metamaterials have formed a new academic world. The application concept of metamaterial is based on designing diverse physical structures that can break through the limitations of traditional optical materials and composites to achieve extraordinary material functions. Therefore, metadevices have been widely studied by the academic community recently. Using the properties of metamaterials, many functional metadevices have been well investigated and further optimized. In this article, different metamaterial structures with varying functions are reviewed, and their working mechanisms and applications are summarized, which are near-field energy transfer devices, metamaterial mirrors, metamaterial biosensors, and quantum-cascade detectors. The development of metamaterials indicates that new materials will become an important breakthrough point and building blocks for new research domains, and therefore they will trigger more practical and wide applications in the future.

Maskless fabrication of slanted annular aperture arrays.

We report maskless fabrication of high-aspect-ratio slanted annular aperture arrays (SAAAs) in gold films using focused ion beam lithography. By tilting the substrate, SAAAs with the desired tilting angle can be fabricated. Our experimental results demonstrate accurate control over aperture size, obliqueness, and reproducibility. We also show that the resulted plasmonic resonances of SAAAs can be effectively tuned via obliqueness control. This versatile approach may enable fabrication of more complicated plasmonic nanostructures. The demonstrated gold SAAAs could also find many potential applications in plasmon-assisted sensing and surface enhanced spectroscopy.

All-optical, polarization-insensitive light tuning properties in silver nanorod arrays covered with photoresponsive liquid crystals.

Active plasmonics has been an interesting and important topic recently. Here we demonstrate the all-optical, polarization-insensitive tunable manipulation of a hybrid system that integrates a silver nanorod array with photoresponsive liquid crystals. The large-area plasmonic nanorod arrays are fabricated by laser interference lithography and ion milling. By covering a layer of photoresponsive liquid crystals, tunable control of plasmon resonance is achieved under an external light pump. The silver nanorod array also enables the homeotropic alignment of the liquid crystals, which makes the all-optical tuning behavior polarization-insensitive. With its advantages of cost-effective fabrication, easy integration, all-optical control, and polarization-insensitivity, the hybrid system could be valuable in many nanophotonic applications.

Incident-angle dependent color tuning from a single plasmonic chip.

We report on a broad color tuning effect covering the visible range from a single plasmonic chip. By simply tilting the orientation of the designed plasmonic chip within a certain range, the photon-plasmon coupling interactions between the incident light and the plasmonic nanostructures on the chip can be finely tuned, resulting in an angle-dependent continuous color filtering effect. The physical mechanism of the device is investigated through the full-wave calculations, which provide important guidance for the design and optimization of the proposed devices. The broad color tuning from the demonstrated single chip will potentially benefit visualization and display technologies, and is particularly useful for the construction of reflection-based spatial light modulators.

Effect of chemical regulation combined with mechanical filtration on deep dewatering and consolidation characteristics of sludge.

To reveal the mechanism underlying deep dewatering of municipal sludge, this paper investigated the sludge characteristics from the perspective of soil mechanics, and analyzed the sludge physical and mechanical properties, stress as well as dewatering behavior during the dewatering process. Before and after cationic polyacrylamide (CPAM) and chitosan (HACC) conditioning, the pressure filtration dewatering time of treated sludge was shorter than that of raw sludge, and the water removal rate was greater than 70% at the 6 MPa pressure. However, with the increase in filter pressing time, the filtration resistance coefficient of sludge increased, and the water pressure in sludge pores rose up, and a longer time was needed for dissipation at the larger pressure, resulting in the slowdown of sludge consolidation. In addition, based on the three-stage Terzaghi Voigt model, when the pressure rose from 2 to 6 MPa, the time from filtration stage to compression stage of raw sludge was shortened, and the second stage played the most important role in the dewatering process. Compared with the raw sludge, the sludge filtration stage was shortened after CPAM or HACC conditioning, and the main dewatering mechanism changed from the second compression stage to the first compression stage, which means the bound water in sludge flocs was transformed into free water. This was also the reason why the dewatering, compression and consolidation rate of the conditioned sludge was faster than that of the raw sludge.

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.

Plasmon-Enhanced Fluorescence Emission of an Electric Dipole Modulated by a Nanoscale Silver Hemisphere.

The spontaneous emission of a fluorophore is altered by the surrounding electromagnetic field. Therefore, the radiation of the fluorophore can be engineered by inter-coupling with the nanoscale plasmons. This work proposes a nanoscale hemisphere structure that enhances the electric field and further modulates its effects on fluorophores by adjusting the radius of the hemisphere. A full-wave simulation is carried out using the finite element method, and the radiation characteristics of the nanoscale hemisphere are studied in detail. Compared with free space, the structure has generated significant enhancement exceeding 30. Through curve fitting, the relationship between the enhanced peak wavelength and the radius of the hemisphere is obtained.

A High Refractive Index Plasmonic Micro-Channel Sensor Based on Photonic Crystal Fiber.

A new concave shaped high refractive index plasmonic sensor with a micro-channel is proposed in this work, which comprises an analyte channel in the core hole. The sensor is elaborately designed to reduce the interference effect from the metal coating. Furthermore, the impact of the proposed structure on the sensitivity is also investigated by engineering the geometric parameters using the finite element method. We select gold as the plasmonic material in this theoretical study because it is widely used to fabricate plasmonic and metamaterial devices due to its chemical stability and compatibility. According to wavelength interrogation technique, simulations results show that this sensor can obtain maximal wavelength sensitivity of 10,050 nm/refractive index unit. In view of the excellent indicators of this device, it has important development potential in chemical and biological research fields.

Super-Resolution Imaging with Graphene.

Super-resolution optical imaging is a consistent research hotspot for promoting studies in nanotechnology and biotechnology due to its capability of overcoming the diffraction limit, which is an intrinsic obstacle in pursuing higher resolution for conventional microscopy techniques. In the past few decades, a great number of techniques in this research domain have been theoretically proposed and experimentally demonstrated. Graphene, a special two-dimensional material, has become the most meritorious candidate and attracted incredible attention in high-resolution imaging domain due to its distinctive properties. In this article, the working principle of graphene-assisted imaging devices is summarized, and recent advances of super-resolution optical imaging based on graphene are reviewed for both near-field and far-field applications.

Sensitivity-Enhanced SPR Sensor Based on Graphene and Subwavelength Silver Gratings.

A novel surface plasmon resonance (SPR) sensor with graphene and subwavelength gratings is proposed to improve the sensing performance. A series of numerical analyses were performed to investigate the effect of structural parameters on the sensing performance, such as minimum reflectance at resonance (MRR), full width at half maximum (FWHM), and resonance angle. The results indicated that near-zero MRR (2.9 × 10-6) and narrow FWHM (about 3.5 deg) could be obtained by optimizing the geometrical parameters. Moreover, the influence of the number of graphene layers on sensitivity was also studied. The maximum sensitivity of the designed sensor could reach 192 deg/refractive index unit (RIU), which is a great enhancement compared to the silver-only SPR sensor. In addition, ethylene glycol solutions with different refractive indices were detected. The results showed that the sensitivity of the sensor could reach 220.67 deg/RIU, and the proposed sensor had excellent linearity between the resonance angle and refractive index, enabling extensive potential practical sensing applications.

Assessment of microvasculature flow state with a high speed all-optic dual-modal system of optical coherence tomography and photoacoustic imaging.

We propose a high speed all-optic dual-modal system that combines spectral domain optical coherence tomography (SDOCT) and photoacoustic imaging (PAI) to evaluate microvasculature flow states. A homodyne interferometer was used to remotely detect the surface vibration caused by photoacoustic (PA) waves. The PA excitation, PA probing and SDOCT probing beams share the same X-Y galvanometer scanner to perform fast two-dimensional scanning. In addition, we introduced multi-excitation, dual-channel acquisition and sensitivity compensation to improve the imaging speed of the PAI sub-system. The total time for imaging a sample with 256 × 256 pixels is less than 1 minute. The performance of the proposed system was verified by in vivo imaging of the vascular system in a mouse pinna with normal and then blocked blood circulations. The experimental results indicate that the proposed system is capable of revealing different blood flow states (static and moving) and is useful for the study of diseases related to functional blood supply.

Reagent-free simultaneous determination of glucose and cholesterol in whole blood by FTIR-ATR.

Reagent-free determination of multiple analytes is an active and promising field of research in clinical analysis. In this work, the determination of glucose and cholesterol in whole blood using Fourier transform infrared (FTIR) spectroscopy equipped with an attenuated total reflectance (ATR) accessory was performed. A comprehensive sample selection rule in multi space based on SPXY was proposed, termed C-SPXY. The core idea is to make full use of different derivative spectra space to construct the calibration set which preserves the more effective information. On this basis, a partial least squares (PLS) regression fusion modeling method was also presented aiming at improving prediction accuracy of glucose and cholesterol concentration in whole blood samples. Compared with other methods based on single spectra space, the proposed fusion model based on multi spectra space C-SPXY method provides smaller RMSEP values. Experimental results demonstrate that the proposed method and model provides superior predictive power and holds a good application prospect in the field of clinical analysis.