Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy¹, micro-cavity laser technology², optical communications^3-6 as well as sensor technology. The WGM-based sensor applications include those in biology⁷, trace gas detection⁸, and impurity detection in liquids⁹. Mechanical sensors based on microsphere resonators have also been proposed, including those for force^10,11, pressure¹², acceleration¹³ and wall shear stress¹⁴. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies^15,16. A candidate application of this sensor is in the detection of neuronal action potential.The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 μm) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 10⁶). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.     

Subradiant,Superradiant Plasmon Modes and Fano Resonance in a Multilayer Nanocylinder Array Standing on a Thin Metal Film

The optical properties of a novel nanostructure consisting of a hexagonal array of aligned vertically three-layered metal-dielectric-metal nanodisks on a silver film are theoretically studied through the finite-difference time-domain method. The novel nanostructure exhibits three obvious optical transmission bands due to the excitation of subradiant plasmon modes, superradiant plasmon modes, and Fano resonances. Surface plasmon polaritons of the underlying Ag film also play a significant role on these three optical transmission bands via coupling with localized surface plasmons of nanodisk pairs. Moreover, the nanostructure also exhibits a good tunability of optical response by modifying the sizes of cylinders, the thickness of underlying metal film, and the dielectric constant of middle layer. These results demonstrate the nanostructure with great advantages in optical sensors and filters.

     

Highly Sensitive Plasmonic Refractive Index Sensor Based on Dual D-Shaped Photonic Crystal Fiber with Aluminum Nitride-Silver Films

A dual-core and dual D-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor with silver and aluminum nitride (AlN) films is designed. The distribution characteristics of the electromagnetic fields of core and plasmon modes, as well as the sensing properties, are numerically studied by finite element method (FEM). The structure parameters of the designed sensor are optimized by the optical loss spectrum. The results show the resonance wavelength variation of 489 nm for the refractive index (RI) range of 1.36?~?1.42. In addition, a maximum wavelength sensitivity of 13,400 nm/RIU with the corresponding RI resolution of 7.46?×?10^?6 RIU is obtained in the RI range of 1.41?~?1.42. The proposed sensor with the merits of high sensitivity, low cost, and simple structure has a wide application in the fields of RI sensing, such as hazardous gas detection, environmental monitoring, and biochemical analysis.

     

Theoretical Study of a Planar Structure Plasmonic Nanolaser in Visible Regime

The potential of a plasmonic nanolaser using semiconductor gain to compensate the metal loss was investigated theoretically in multilayer planar structure geometry. The propagation constant of surface plasmon (SP) mode, Purcell factor, and modal gain were calculated via transfer matrix method. Near SP resonance, the Purcell factor shows sensitive frequency dependence and exponential decay with distance. The huge Purcell factor leads to an impractical current density about several hundreds kiloampere/square centimeter (kA/cm²). When the spectra peak of optical gain in a semiconductor is shifted about 0.7 eV below SP resonance, the moderate Purcell factor shows a rather broadband enhancement. Net modal gain was achieved at an injected current density of 12.1 kA/cm², comparable to that of conventional photonic laser diodes. The structure is further optimized by inserting a (low permittivity) dielectric spacer between the semiconductor active region and metal.

     

Distributed MEMS Sensors Using Plasmonic Antenna Array Embedded Sagnac Interferometer

A micro Sagnac interferometer is proposed for electron cloud distributed sensors formed by an integrated (micro-electro-mechanical systems) MEMS resonator structure. The Sagnac interferometer consists of four microring probes integrated into a Sagnac loop. Each of the microring probes is embedded with the silver bars to form the plasmonic wave oscillation. The polarized light of 1.50 µm wavelength is input into the interferometer, which is polarized randomly into upstream and downstream directions. The polarization outputs can be controlled by the space–time input at the Sagnac port. Electrons are trapped and oscillated by the whispering gallery modes (WGMs), where the plasmonic antennas are established and applied for wireless fidelity (WiFi) and light fidelity (LiFi) sensing probes, respectively. Four antenna gains are 2.59 dB, 0.93 dB, 1.75 dB, and 1.16 dB, respectively. In manipulation, the sensing probe electron densities are changed by input source power variation. When the electron cloud is excited by the microscopic medium, the change in electron density is obtained and reflected to the required parameters. Such a system is a novel device that can be applied for brain-device interfering with the dual-mode sensing probes. The obtained WGM sensors are 1.35 µm^?2, 0.90 µm^?2, 0.97 µm^?2, and 0.81 µm^?2, respectively. The WGMs behave as a four-point probe for the electron cloud distributed sensors, where the electron cloud sensitivities of 2.31 prads^?1mm³ (electrons)^?1, 2.27 prads^?1mm³ (electrons)^?1, 2.22 prads^?1mm³(electrons)^?1, and 2.38 prads^?1mm³(electrons)^?1 are obtained, respectively.

     

Tuning Plasmonic Properties of Truncated Gold Nanospheres by Coating

The influence of TiO₂ coating on resonant properties of gold nanoisland films deposited on silica substrates was studied numerically and in experiments. The model describing plasmonic properties of a metal truncated nanosphere placed on a substrate and covered by a thin dielectric layer has been developed. The model allows calculating a particle polarizability spectrum and, respectively, its surface plasmon resonance (SPR) wavelength for any given cover thickness, particle radius and truncation parameter, and dielectric functions of the particle, the substrate, the coating layer, and the surrounding medium. Dependence of the SPR position calculated for truncated gold nanospheres has coincided with the measured one for the gold nanoisland films covered with titania of different thicknesses. In the experiments, gold films with thickness of 5 nm were deposited on a silica glass substrate, annealed at 500 °C to form nanoislands of 20 nm in diameter, and covered with amorphous titania layers using atomic layer deposition technique. The resulting structures were characterized with scanning electron microscopy and optical absorption spectroscopy. The measured dependence of the SPR position on titania film thickness corresponded to the one calculated for truncated sphere-shaped nanoparticles with the truncation angle of ~50°. We demonstrated the possibility of tuning the SPR position within ~100 nm range by depositing to 30 nm thick titania layer.

     

Broadband Linear-to-Circular Polarization Conversion Realized by the Solid State Plasma Metasurface

In this paper, a solid-state plasma metasurface (SSPM) for linear-to-circular polarization conversion is designed and investigated. The unit cell of such a SSPM consists of three layers from bottom to top, which are the copper coating, the dielectric layer, and the solid-state plasma resonators, respectively. By exciting solid-state plasma resonance units in different regions, we successfully achieved two operating modes (modes I and II). In mode I, the axial ratio band which is less than 3 dB (3 dB AR band) is mainly obtained in 14.34–19.61 GHz (the relative bandwidth is 31.25%). When the proposed SSPM is regulated to mode II, the 3 dB AR band is changed to 8.81–14.34 GHz (the relative bandwidth is 40.90%). The polarization conversion rate, phase difference, AR curves, and surface current diagrams are analyzed to explore the performances of the proposed SSPM. Our design promotes the potential applications of tunable devices.

     

Long-Range Surface Plasmon Resonance–Based Sensitivity Study on D-shaped Ag-MgF2-Coated Models with Analyte Variations

In this report, a novel D-shaped long-range surface plasmon resonance (LRSPR) fiber base sensor has been introduced. The demonstration of proposed sensor involves two D-shaped silver-coated models to study the sensitivity responses. The entire study with the constructed models is based on a single-mode fiber. The models are multilayered consisting of metal, dielectric, and analyte as separate layers. Silver (Ag) and magnesium fluoride (MgF2) strips are used as metal and dielectric layers respectively. The constituency of analyte as an interface excellently standardized the models for sensitivity detection. In this report, a large range of analyte refractive indices (RI) which varies from 1.33 to 1.38 is appraised for the proposed models to characterize the sensitivity. The entire context is encompassed by the wavelength region from 450 to 850 nm with an interval of 20 nm. Sensitivities in this report are measured based on the analyte position from the core and metal for both models. For each of the two models, the analyte is placed as the top layer. RIs of the applied metal (Ag) are measured using the Drude-Lorentz formula. The simulated sensitivities for model-1 and model-2 vary from 6.3?×?10³ nm/RIU to 8.7?×?10³ nm/RIU.

     

Effect of dielectric constant of medium on protonation equilibria of ethylenediamine

Abstract

The effect of dielectric constant of medium on protonation equilibria has been studied by determining protonation constants of ethylenediamine pH metrically in various concentrations (0–60%v/v) of acetoni-trile– and ethylene glycol–water mixtures, at an ionic strength of 0.16mol L^?1 and at 303.0 K. MINIQUAD75 computer program has been used for the calculation of protonation constants. Linear and non-linear variations of step-wise protonation constants with reciprocal of dielectric constant of the solvent mixtures have been attributed to the dominance of the electrostatic and non-electrostatic forces, respectively. The trend is explained on the basis of solute–solute and solute–solvent interactions, solvation, proton transfer processes and dielectric constants of the media.     

Growth dynamics and the proximate biochemical composition and fatty acid profile of the heterotrophically grown diatom <Emphasis Type="Italic">Cyclotella cryptica</Emphasis>

To investigate the nutritional value of the diatom Cyclotella cryptica as an alternative feed for aquaculture, its heterotrophic growth characteristics were studied. First, the proximate biochemical composition and fatty acid profiles were studied under a controlled heterotrophic growth condition. The approximate total ash, carbohydrate, lipid, and protein content were 245 mg g⁻¹ (dry weight), 360 mg g⁻¹, 165 mg g⁻¹ and 260 mg g⁻¹, respectively. Polyunsaturated fatty acids accounted for 24.5, 31.3, 45.1 and 17.3% of the total lipids in the phospholipid, sterol, free fatty acid and triglyceride classes. Secondly, the effect of aeration and agitation rates on the specific growth rate of C. cryptica under heterotrophic conditions was studied. The maximum specific growth rate was not significantly affected (P > 0.05) by the rate of agitation within the range of 100 to 160 rpm, but it was significantly affected (P > 0.05) by the rate of aeration. Optimal growth occurred when the aeration rate was within the range of 0.44 to 1.07 v/v/min. Viability measurements throughout the growth period showed that the C. cryptica cells remained viable in spite of the varied cultivation conditions. Hydrodynamic forces are an important parameter within biological systems, and optimisation is crucial for the successful scale-up of microalgal cultivation systems. Whilst the investigation was preliminary in nature, the information gained in this study will be useful for the continual development of an alternative and cost-effective feed for bivalve spat rations.     

Bioreactor type affects the accumulation of phenolic acids and flavonoids in microshoot cultures of Schisandra chinensis (Turcz.) Baill.

Microshoots of the East Asian medicinal plant species Schisandra chinensis (Chinese magnolia vine) were grown in bioreactors characterized by different construction and cultivation mode. The tested systems included two continuous immersion systems—a cone-type bioreactor (CNB) and a cylindric tube bioreactor (CTB), a nutrient sprinkle bioreactor (NSB), and two temporary immersion systems (TIS)—RITA® and Plantform. Microshoots were grown for 30 and 60 days in the MS medium enriched with 1 mg l^?1 NAA and 3 mg l^?1 BA. The accumulation of two groups of phenolic compounds: phenolic acids and flavonoids in the bioreactor-grown S. chinensis biomass, was evaluated for the first time. In the microshoot extracts, seven phenolic acids: chlorogenic, gallic, p–hydroxybenzoic, protocatechuic, syringic, salicylic and vanillic, and three flavonoids: kaempferol, quercitrin and rutoside, were identified. The highest total amount of phenolic acids (46.68 mg 100 g^?1 DW) was recorded in the biomass maintained in the CNB for 30 days. The highest total content of flavonoids (29.02 mg 100 g^?1 DW) was found in the microshoots maintained in the NSB for 30 days. The predominant metabolites in all the tested systems were: gallic acid (up to 10.01 mg 100 g^?1 DW), protocatechuic acid (maximal concentration 16.30 mg 100 g^?1 DW), and quercitrin (highest content 21.00 mg 100 g^?1 DW).

     

Speciation of binary complexes of Co(II), Ni(II) and Cu(II) with L-aspartic acid in propylene glycol–water mixtures

Abstract

Speciation of binary complexes of Co(II), Ni(II) and Cu(II) with L-aspartic acid in (0-60% v/v) propylene glycol-water mixtures was studied pH metrically at 303.0±0.1 K and at an ionic strength of 0.16 mol L^-1. The binary species refined were ML, ML₂, ML₂H₂, ML₂H₃ and ML₂H₄. The stabilities of the complexes followed the Irving-Williams order i.e.Co(II) <Ni(II) < Cu(II). The linear variation of stability constants as a function of dielectric constant of the medium indicated the dominance of electrostatic forces over non-electrostatic forces. Some species were stabilised due to electrostatic interactions and some were destabilised due to the decreased dielectric constant. The order of ingredients influencing the magnitudes of stability constants due to incorporation of errors in their concentrations was alkali > acid > ligand > metal. Equilibria for the formation of binary complexes were proposed based on the forms of the ligand and their existence at different pH values.     

Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime

We calculate the forces of single-beam gradient radiation pressure laser traps, also called “optical tweezers,” on micron-sized dielectric spheres in the ray optics regime. This serves as a simple model system for describing laser trapping and manipulation of living cells and organelles within cells. The gradient and scattering forces are defined for beams of complex shape in the ray-optics limit. Forces are calculated over the entire cross-section of the sphere using TEM₀₀ and TEM₀₁^* mode input intensity profiles and spheres of varying index of refraction. Strong uniform traps are possible with force variations less than a factor of 2 over the sphere cross-section. For a laser power of 10 mW and a relative index of refraction of 1.2 we compute trapping forces as high as ~ 1.2 × 10^-6 dynes in the weakest (backward) direction of the gradient trap. It is shown that good trapping requires high convergence beams from a high numerical aperture objective. A comparison is given of traps made using bright field or differential interference contrast optics and phase contrast optics.     

Optics of the butterfly eye

The afocal apposition optics of butterfly eyes was examined from both a geometrical optics and a wave optics point of view. We used several different species of butterfly but put special emphasis on a common Australian nymphalid,Heteronympha merope. From the anatomy of the retina, the optics of isolated components of the eye and the ophthalmoscopy of the intact living eye we derived the following.

Influence of dielectric constant on protonation equilibria of maleic acid and L-asparagine in acetonitrile water-mixtures

Abstract

The protonation constants of maleic acid and L-asparagine have been studied pH-metrically in various concentrations (0–50% v/v) of acetonitrile–water mixtures maintaining an ionic strength of 0.16 mol L^-1 at 30⁰C. The protonation constants have been calculated using the computer program MINIQUAD75 and are selected based on statistical parameters. Linear variation of step-wise protonation constants (log K) with the reciprocal of the dielectric constant of the solvent mixture has been attributed to the dominance of the electrostatic forces.     

Partial nitritation at elevated loading rates: design curves and biofilm characteristics

There is a need to develop low operational intensity, cost-effective, and small-footprint systems to treat wastewater. Partial nitritation has been studied using a variety of control strategies, however, a gap in passive operation is evident. This research investigates the use of elevated loading rates as a strategy for achieving low operational intensity partial nitritation in a moving bed biofilm reactor (MBBR) system. The effects of loading rates on nitrification kinetics and biofilm characteristics were determined at elevated, steady dissolved oxygen concentrations between 5.5 and 7.0 mg O₂/L and ambient temperatures between 19 and 21 °C. Four elevated loading rates (3, 4, 5 and 6.5 g NH₄⁺-N/m² days) were tested with a distinct shift in kinetics being observed towards nitritation at elevated loadings. Complete partial nitritation (100% nitrite production) was achieved at 6.5 g NH₄⁺-N/m² days, likely due to thick biofilm (572 µm) and elevated NH₄⁺-N load, which resulted in suppression of nitrite oxidation.

     

SPR Label-Free Biosensor with Oxide-Metal-Oxide-Coated D-Typed Optical Fiber: a Theoretical Study

In this article, a surface plasmon resonance (SPR) biosensor based on D-typed optical fiber coated by Al₂O₃/Ag/Al₂O₃ film is investigated numerically. Resonance in near infrared with an optimized architecture is achieved. Refractive index sensitivity of 6558 nm/RIU (refractive index unit) and detection limit of 1.5 × 10⁻⁶ RIU, corresponding to 0.4357 nm/μM and detection limit of 23 nM in BSA (bovine serum albumin) concentration sensing, are obtained. The analysis of the performance of the sensor in gaseous sensing indicates that this proposed SPR sensor is much suitable for label-free biosensing in aqueous media.

     

Bidirectional Edge Asymmetric Light Transmission in Metal/Dielectric Device Based on Asymmetric Diffraction

Asymmetric light transmission (ALT) or optical diode-like nanodevices have attracted many research interests in recent years for its rosy potential application in all optical computing and information systems. In this work, we propose and numerically demonstrate a bidirectional edge asymmetric light transmission (BE-ALT) device, which is composed by the easy-processing metal/dielectric cylinders arranged periodically on glass substrate. The ALT effect in the proposed BE-ALT device shows a saltation at one critical wavelength, i.e., the asymmetric subtraction owns different signs for the wavelength larger and smaller than the critical wavelength. The asymmetric subtraction designed in this work changes dramatically from − 60% to + 80% at around 600 nm, which can be effectively manipulated by applying different structure parameters. The underlying physical mechanism has been investigated systematically, including the asymmetric diffraction effect, localized surface plasmonic resonance (LSPR), and the waveguide mode (WGM). Our designed BE-ALT device provides a new choice for the practical applications of ALT effect.

     

Evaluating the Potential of an Antibody Against Recombinant OmpW Antigen in Detection of Vibrio cholerae by Surface Plasmon Resonance (SPR) Biosensor

Surface plasmon resonance (SPR), a highly sensitive and label-free optical biosensing technique, is a powerful tool for studying biomolecular interactions. An immunosensor for rapid, sensitive, and selective detection of Vibrio cholerae on the basis of SPR is reported. Recombinant OmpW antigen (a bacterial outer-membrane protein) of V. cholerae was expressed and purified and raising of polyclonal rabbit anti-OmpW was done. Antibodies were immobilized on a sensor surface and interactions between OmpW protein and the whole cell of V. cholerae with immobilized antibodies were studied in different experiments. The aim of this study was to evaluate the potential of anti-OmpW in detection of V. cholerae by developing an immunosensor based on SPR. The results showed high affinity interaction between OmpW and anti-OmpW (K _D = 2.4 ± 0.07 × 10⁻⁹ M) and SPR signals had a linear relationship with the number of V. cholerae ranging from 1 × 10² to 1 × 10⁷ cells/mL with limit of detection of 50 cells/mL. The specificity of the developed immunoassay was examined using some non-V. cholerae bacteria which did not produce any significant responses. This method is rapid, sensitive, and specific to target V. cholerae with a total analysis time of less than 60 min.

     

Study of Design Tunable Optical Sensor and Monochromatic Filter of the One-Dimensional Periodic Structure of TiO2/MgF2 with Defect Layer of Liquid Crystal (LC) Sandwiched with Two Silver Layers

In this paper, we have inspected the optical characteristics of one-dimensional periodic structure (1DPS) of TiO₂ and MgF₂ dielectric materials with defect layer of liquid crystal (LC) sandwiched with two silver layers, i.e., (TiO₂|MgF₂)³|Ag|LC|Ag|(TiO₂/MgF₂)³ using transfer matrix method (TMM). The optical tunable properties of considered periodic structures investigated at different incident angles and temperatures for TE and TM modes. Our study shows that absorption peak of 1DPS varies with incident angle and temperature. The defect layer (Ag-LC-Ag), sandwiched LC within two metallic (Ag) layers, exhibits the surface plasmon waves at the metal LC interfaces. The effect of surface plasmon waves can be better understand through the optical sensing property of such defect periodic structure. The detailed study concludes that such a type of one-dimensional periodic structure (1DPS) may be useful to design a tunable sensor and monochromatic filter.