Wide Range Of Refractive Indices Research Articles

Correlation among extinction efficiency and other parameters in an aggregate dust model

We study the extinction properties of highly porous Ballistic Cluster-Cluster Aggregate dust aggregates in a wide range of complex refractive indices (, ) and wavelengths ( m). An attempt has been made for the first time to investigate the correlation among extinction efficiency (), composition of dust aggregates (), wavelength of radiation (λ) and size parameter of the monomers (x). If k is fixed at any value between 0.001 and 1.0, increases with increase of n from 1.4 to 2.0. and n are correlated via linear regression when the cluster size is small, whereas the correlation is quadratic at moderate and higher sizes of the cluster. This feature is observed at all wavelengths (ultraviolet to optical to infrared). We also find that the variation of with n is very small when λ is high. When n is fixed at any value between 1.4 and 2.0, it is observed that and k are correlated via a polynomial regression equation (of degree 1, 2, 3 or 4), where the degree of the equation depends on the cluster size, n and λ. The correlation is linear for small size and quadratic/cubic/quartic for moderate and higher sizes. We have also found that and x are correlated via a polynomial regression (of degree 3, 4 or 5) for all values of n. The degree of regression is found to be n and k-dependent. The set of relations obtained from our work can be used to model interstellar extinction for dust aggregates in a wide range of wavelengths and complex refractive indices.

An optical fiber surface plasmon resonance biosensor for wide range detection

An optical fiber surface plasmon resonance biosensor is presented that allows to numerically demonstrate, using transfer matrix method and the finite difference time domain method, the detection range is very wide. Two different structures of graphene photonic crystal multilayer (i.e. sensor I and sensor II) are constructed in the cladding region of single-mode fiber. Graphene is used as the plasma layer instead of the traditional metal. According to the analysis, the properties of graphene can be changed by adjusting the chemical potential µ c . In the spectral region of 1.667| µ c | ћω µ c |, the imaginary part of conductivity σ ″ becomes negative. Thus the weakly bounded low-less TE-SPR is supported by graphene. The results of the numerical simulation show that the relationship between refractive index and resonant wavelength is linear. The sensor I can detect the refractive index range of 1.33–1.4, and the sensitivity is 1942 nm/RIU. The sensor II can detect the refractive index range of 1.41–1.67, and the sensitivity is up to 2315.4 nm/RIU. Therefore, the detection of wide refractive index range of 1.33–1.67 or simultaneous detection of different biological medium concentration is realized by the sensor.

Novel Compact and Low-Cost Ultraweak Fabry–Perot Interferometer as a Highly Sensitive Refractive Index Sensor

A novel compact refractive index (RI) sensor based on an ultra-weak intrinsic fiber Fabry-Perot interferometer (FPI) is proposed and demonstrated, which is simply fabricated by splicing a tiny section of thin-core fiber to a single-mode fiber. Such an FPI exhibits an average RI sensitivity of 240dB/RIU over a wide RI range of 1.3326–1.4305, with a maximum sensitivity of 1110.7dB/RIU at the RI of 1.4305. In addition, the FPI can also achieve the simultaneous measurement of the RI and temperature.

Use of Debye's series to determine the optimal edge-effect terms for computing the extinction efficiencies of spheroids.

The extinction efficiencies of atmospheric particles are essential to determining radiation attenuation and thus are fundamentally related to atmospheric radiative transfer. The extinction efficiencies can also be used to retrieve particle sizes or refractive indices through particle characterization techniques. This study first uses the Debye series to improve the accuracy of high-frequency extinction formulae for spheroids in the context of Complex angular momentum theory by determining an optimal number of edge-effect terms. We show that the optimal edge-effect terms can be accurately obtained by comparing the results from the approximate formula with their counterparts computed from the invariant imbedding Debye series and T-matrix methods. An invariant imbedding T-matrix method is employed for particles with strong absorption, in which case the extinction efficiency is equivalent to two plus the edge-effect efficiency. For weakly absorptive or non-absorptive particles, the T-matrix results contain the interference between the diffraction and higher-order transmitted rays. Therefore, the Debye series was used to compute the edge-effect efficiency by separating the interference from the transmission on the extinction efficiency. We found that the optimal number strongly depends on the refractive index and is relatively insensitive to the particle geometry and size parameter. By building a table of optimal numbers of edge-effect terms, we developed an efficient and accurate extinction simulator that has been fully tested for randomly oriented spheroids with various aspect ratios and a wide range of refractive indices.

Electromagnetic field enhancement in Bloch surface waves

We present a systematic comparison between guided modes supported by slab waveguides and Bloch Surface Waves (BSWs) propagating at the surface of truncated periodic multilayers. We show that, contrary to common belief, the best surface field enhancement achievable for guided modes in a slab waveguide is comparable to that observed for BSWs. At the same time, we demonstrate that, if one is interested in maximizing the electromagnetic energy density at a generic point of a dielectric planar structure, BSWs are often preferable to modes in which light is confined uniquely by total internal reflection. Since these results are wavelength independent and have been obtained by considering a very wide range of refractive indices of the structure constituent materials, we believe they can prove helpful in the design of future structures for the control and the enhancement of the light-matter interaction.

Wide Range Refractive Index Measurement Using a Multi-Angle Tilted Fiber Bragg Grating

The conventional single-angle tilted fiber Bragg grating (TFBG) can only excite a certain range of cladding modes, limiting it for refractive index (RI) measurement in a wide range. In this letter, we fabricate and demonstrate a multi-angle TFBG, in which five individual TFBGs with tilt angles ranging from 5° to 25° are sequentially inscribed along the core of a single mode fiber within a length of 20 mm. The multi-angle TFBG excites a continuous spectral comb of narrowband-cladding modes over a much wider wavelength range (>170 nm) than a single-angle TFBG, making it suitable for dynamic RI measurement over a wide range (1.15–1.45). We have experimentally measured aqueous solutions with RI ranging from 1.30 to 1.45 using the uncoated (by monitoring the cut-off mode) and gold-coated (by monitoring the surface Plasmon resonance) multi-angle TFBGs, and both methods show linear responses, with RI sensitivities about 500 nm/RIU.

Scattering Properties of Large Irregular Cosmic Dust Particles at Visible Wavelengths

Abstract The effect of internal inhomogeneities and surface roughness on the scattering behavior of large cosmic dust particles is studied by comparing model simulations with laboratory measurements. The present work shows the results of an attempt to model a dust sample measured in the laboratory with simulations performed by a ray-optics model code. We consider this dust sample as a good analogue for interplanetary and interstellar dust as it shares its refractive index with known materials in these media. Several sensitivity tests have been performed for both structural cases (internal inclusions and surface roughness). Three different samples have been selected to mimic inclusion/coating inhomogeneities: two measured scattering matrices of hematite and white clay, and a simulated matrix for water ice. These three matrices are selected to cover a wide range of imaginary refractive indices. The selection of these materials also seeks to study astrophysical environments of interest such as Mars, where hematite and clays have been detected, and comets. Based on the results of the sensitivity tests shown in this work, we perform calculations for a size distribution of a silicate-type host particle model with inclusions and surface roughness to reproduce the experimental measurements of a dust sample. The model fits the measurements quite well, proving that surface roughness and internal structure play a role in the scattering pattern of irregular cosmic dust particles.

Dispersion Enhanced Critically Coupled Ring Resonator for Wide Range Refractive Index Sensing

It is shown that the dispersion in transmission characteristics of a ring resonator designed with silicon-on-insulator waveguides in all-pass configuration can be enhanced significantly by increasing interaction length of the directional coupler. This in turn helps to single-out highly extinct resonance(s) at and around the critically coupled wavelength. Such a device is found to be useful for a wide range of refractive index sensing for the cladding materials/analytes ( ${\text{1.0}} ). As a proof of concept, the sensor devices were fabricated and characterization results are shown to be consistent with theoretical prediction. The fabricated devices have been also used successfully to determine unknown refractive index of a given analyte (Newport F-IMF-150) with an error limit of $\delta n \sim {\text{1.67}} \times {\text{10}}^{-2}$ RIU. Analyzing experimental results, it is shown that the limit of detection can be further reduced ( $\ll {\text{10}^{{-3}}}$ RIU), if the perimeter of the ring is increased without compromising the round-trip waveguide loss. The superiority of such a sensor device lies in its simpler design rule, easier operation, wider range, and nearly accurate detection mechanism.

Plasmonic sensor based on Fano resonance

Plasmonic sensor based on metallic nanostructures is a promising platform for applications, such as biology, chemistry, materialogy, photonics and bioscience due to their attractive properties. However, the sensitivity of plasmonic sensors is usually limited by broad-spectral features due to large radiative loss of metallic nanostructures in visible region. In this paper, we introduce structural asymmetry to generate the Fano resonance in the metallic nanostructure composed of asymmetric ellipsoidal pair. The distinct Fano-like resonance around wavelength of 681nm possesses sharp peak as narrow as 10.8nm. And the Fano mode exhibits high refractive index sensitivity as large as 299 nm/RIU. Due to the excitation of sharp spectral features, high figure of merit of 27.8 at the Fano resonance is obtained in a wide refractive index range of 1.0~1.1. This device is promising for the applications of high sensitivity microchip sensor.

Sensitivity Enhanced by MoS2–Graphene Hybrid Structure in Guided-Wave Surface Plasmon Resonance Biosensor

Compared with surface plasmon resonance (SPR) biosensor, guided-wave surface plasmon resonance (GWSPR) biosensor has a higher sensitivity. In order to further enhance the sensitivity of the GWSPR biosensor, the MoS2–graphene hybrid structure is proposed to cover on the surface of the thin silicon film. It is demonstrated that the sensitivity of the proposed biosensor configuration can be doubled by using optimized MoS2–graphene hybrid structure. By analyzing the electric field distributions with the proposed GWSPR biosensor, it is shown that the electric field intensity has a dramatic change with a slight change in refractive index of sensing medium, and it also means that the biosensor is sensitive to detect the analytes. Moreover, the proposed GWSPR biosensor has a wide detection range of refractive index of sensing medium, and the sensitivity is increased with the refractive index of sensing medium increasing.

Films Consisting of Innumerable Tapered Nanopillars of Mesoporous Silica for Universal Antireflection Coatings.

Films with a fine structure consisting of innumerable nanopillars of mesoporous silica (MPS) are formed by a reactive ion etching process with a fluorine-containing gas. Each nanopillar has a tapered shape with a uniform height, which effectively suppresses reflection by the formation of an ideal graded refractive index structure. The nanopillars are spontaneously formed under low-pressure conditions, wherein locally deposited Al-F compounds, originating from an alumina plate in the etching chamber, work as a fine etching mask. The high etching rate of the MPS film allows a very high aspect ratio of the nanopillars. The refractive index of the MPS nanopillars can be universally tuned by a controlled incorporation of TiO2 into the mesopores, which results in effective reduction of reflectance on a given substrate. The outstanding antireflection performance is experimentally demonstrated for glass substrates with a wide refractive index range.

Fiber optic refractometric sensors using a semi-ellipsoidal sensing element.

We present theoretical and experimental results for a fiber optic refractometric sensor employing a semi-ellipsoidal sensing element made of polymethyl methacrylate. The double internal reflection of light inside the element provides sensitivity to the refractive index of the external analyte. We demonstrate that the developed sensor, operating at a wavelength of 632 nm, is capable of measurement within a wide range of refractive indices from n=1.00 to n=1.47 with sensitivity over 500 dB/RIU. A comparison of the developed sensor with two more complex refractometric sensors, one based on tapered optical fiber and the other based on suspended-core microstructure optical fiber, is presented.

In Situ Surface Assembly Derived Ultralow Refractive Index MgF2–SiO2 Hybrid Film for Tri‐Layer Broadband Antireflective Coating

For the broadband antireflective (AR) coating working in specific wavelengths, the precise control over refractive index and film thickness of each layer is critical to guarantee the peak position locating at the targeted wavelengths. In this paper, a simple sol–gel procedure without post‐treatment is used to prepare MgF2–SiO2 hybrid coating with tunable refractive index for the fabrication of tri‐layer tri‐wavelength broadband AR coating. The MgF2–SiO2 coating with ultralow refractive index of 1.12 is realized through in situ surface assembly of negatively charged silanol groups on vesicle‐like MgF2 particles to obtain porous crosslinking structure. The electrostatic attraction between the negatively charged silanol groups and positively charged MgF2 colloidal particles plays a key role. Furthermore, the MgF2–SiO2 coating with a wide range of refractive index between 1.20 and 1.44 is achieved by adjusting the addition amount of acid‐catalyzed silica sol into the vesicle‐like MgF2 sol. According to the theoretical design, a tri‐layer broadband AR coating with refractive index model of 1.12/1.26/1.36 is prepared, which can enhance the transmittance of quartz substrate to near 100% simultaneously at 351, 527, and 1053 nm.

Optimizing plasmonic nanoantennas via coordinated multiple coupling.

Plasmonic nanoantennas, which can efficiently convert light from free space into sub-wavelength scale with the local field enhancement, are fundamental building blocks for nanophotonic systems. Predominant design methods, which exploit a single type of near- or far-field coupling in pairs or arrays of plasmonic nanostructures, have limited the tunability of spectral response and the local field enhancement. To overcome this limit, we are developing a general strategy towards exploiting the coordinated effects of multiple coupling. Using Au bowtie nanoantenna arrays with metal-insulator-metal configuration as examples, we numerically demonstrate that coordinated design and implementation of various optical coupling effects leads to both the increased tunability in the spectral response and the significantly enhanced electromagnetic field. Furthermore, we design and analyze a refractive index sensor with an ultra-high figure-of-merit (254), a high signal-to-noise ratio and a wide working range of refractive indices, and a narrow-band near-infrared plasmonic absorber with 100% absorption efficiency, high quality factor of up to 114 and a wide range of tunable wavelength from 800 nm to 1,500 nm. The plasmonic nanoantennas that exploit coordinated multiple coupling will benefit a broad range of applications, including label-free bio-chemical detection, reflective filter, optical trapping, hot-electron generation, and heat-assisted magnetic recording.

Quantification of silicone degradation for LED packages using finite element analysis

Light emitting diodes (LEDs) have been widely used for illumination because of their electrical efficiency and reliability. Their long lumen maintenance is not easy to be evaluated when it is compared with their efficiency because the reliability test is a time consuming task. LEDs are required to undergo many reliability tests and sometimes create failures induced by high thermo-mechanical stresses. One of the major failure modes is the lens or encapsulant crack. Therefore, the lens and encapsulant materials are required to contain high temperature stability as well as high optical properties. One of the materials for LEDs is polydimethylsiloxane (PDMS) which can have a wide range of refractive indices from 1.40 to 1.57. In this investigation, silicone degradation was quantitatively evaluated using finite element analysis. The finite element simulation showed that thermal–mechanical stress in volume of interest (VOI). As PDMS has viscoelastic properties, strain rates induced by the viscosity effect were focused and computed. Furthermore, the correlation between the strain rates and lumen depreciation was discovered, which leads to a lumen depreciation model based on the strain rates. Using the model, the tendency of the lumen maintenance for LED packages could be estimated using numerical analysis without time-consuming tests.

Characterization of zinc oxide coated optical fiber long period gratings with improved refractive index sensing properties

A fiber-optic refractive index (RI) sensor based on a long period fiber grating (LPFG) coated with a zinc oxide (ZnO) thin film was fabricated and characterized. A method to overcoat the LPFG's with a homogeneous ZnO thin films was developed.Characterization of ZnO thin films, deposited simultaneously on silicon (Si) planar substrates, was performed using Scanning Electron Microscope, Energy Dispersive X-ray Spectroscopy and X-ray Photoelectron Spectroscopy. The LPFGs with ZnO coatings from 29 to 145nm of thickness were characterized and compared in terms of the wavelength shift and the intensity of the attenuation bands changing the surrounding refractive index (SRI) from 1.300 to 1.600. An average wavelength sensitivity of ∼7162nm/RIU was achieved in the RI range from 1.440 to 1.456 and more than 12,000nm/RIU at 1.440 RI. Using a ZnO film thickness of 116nm and in the RI region between 1.320 and 1.360 the average sensitivity of ∼806nm/RIU was measured for a 145nm thick film. Working as an intensity sensing device, the 87nm coated LPFG shows a linear sensitivity of 216.4dB/RIU in a wide range of RI from 1.340 to 1.420.

Tunable plasmonic resonances based on elliptical annular aperture arrays on conducting substrates for advanced biosensing

Introducing a conducting metal layer and the structural asymmetry to elliptical annular aperture arrays, multiple plasmonic coupled-resonant modes are generated under normal incidence in the visible light range. The electromagnetic fields can be strongly enhanced at resonant modes in this device, which increases the interaction volume of the detected analyte and optical fields; therefore, multiple plamonic coupled modes exhibit higher refractive index sensitivity than as large as 610nm/RIU. The distinct Fano-like resonance around a wavelength of 681nm originates from the interference between bonding dipolar and the quadrupolar modes. Due to the excitation of sharp spectral features as narrow as 7nm, high figure of merits of 94 at the Fano-like dip is obtained in a wide refractive index range of 1.33-1.40. Furthermore, to generate strong Fano-like resonance, the geometric shape of ellipse is selected, which is a good geometric shape candidate compared to the circle shape. This device is promising for biosensing applications with high sensitivity and low limit of detection.

Tunable Plasmonic Resonances in the Hexagonal Nanoarrays of Annular Aperture for Biosensing

In this paper, we demonstrate a nanostructure sensor based on hexagonal arrays of annular aperture operating in the near-infrared wavelength range. The strong coupling interaction between propagating surface plasmons (PSP) mode and localized surface plasmons (LSP) mode in the designed structure generates two sharp spectral features under normal incidence. The mode coupling strongly enhances the electromagnetic fields and increases the interaction volume of the analyte and optical field. A high refractive index sensitivity of 623 nm/RIU is demonstrated in a wide refractive index range of 1.33 to 1.40. Due to the excitation of sharp spectral feature, as narrow as 7 nm, high figure of merits of 93 was obtained in the refractive index range, which is nearly 10 times larger than that from hole arrays and disk arrays. Furthermore, sharp spectral feature in the designed structure provides more error margin for structure parameters, which is advantageous for experimental realization of systems without requiring challenging fabrication resolution. The sensor is promising for biosensing applications with high sensitivity and low limit of detection.

Sol-Gel Material-Enabled Electro-Optic Polymer Modulators.

Sol-gels are an important material class, as they provide easy modification of material properties, good processability and are easy to synthesize. In general, an electro-optic (EO) modulator transforms an electrical signal into an optical signal. The incoming electrical signal is most commonly information encoded in a voltage change. This voltage change is then transformed into either a phase change or an intensity change in the light signal. The less voltage needed to drive the modulator and the lower the optical loss, the higher the link gain and, therefore, the better the performance of the modulator. In this review, we will show how sol-gels can be used to enhance the performance of electro-optic modulators by allowing for designs with low optical loss, increased poling efficiency and manipulation of the electric field used for driving the modulator. The optical loss is influenced by the propagation loss in the device, as well as the losses occurring during fiber coupling in and out of the device. In both cases, the use of sol-gel materials can be beneficial due to the wide range of available refractive indices and low optical attenuation. The influence of material properties and synthesis conditions on the device performance will be discussed.

A simple and wide-range refractive index measuring approach by using a sub-micron grating

This paper presents the design and simulation results of a high-precision low-cost refractometer that demonstrates the main advantage of a wide measurement range (1 ≤ n ≤ 2). The proposed design is based on the diffractive properties of sub-micron gratings and Snell's Law. The precision and uncertainty factors of the proposed system were tested and analyzed, revealing that the proposed refractometer demonstrates a wide measurement range with sensitivity of 10−4.