Optimal Refractive Index Research Articles

Optical and gas-sensing property enhancement of membranes based on lithium iron phosphate particles with different dispersants

Lithium iron phosphate (LiFePO4) membranes were fabricated by coating LiFePO4 particles dispersed in various dispersants on tin-diffused glass slides and alumina ceramic tubes for xylene gas detection. The influences of dispersants, including polyvinyl alcohol (PVA), sodium dodecylbenzene sulfonate (SDBS), and methyl methacrylate (MMA), on the optical properties and opto-electrical gas-sensing performance of the LiFePO4 particle-based membranes were investigated. The LiFePO4 particle-based membrane, prepared using PVA, exhibited less agglomeration, better transparency, optimum refractive index, and a higher response toward xylene gas at concentrations of 10 ppb to 1000 ppm in a planar optical waveguide sensor. However, in a resistance-type electrochemical sensing system at room temperature, the LiFePO4-PVA membrane exhibited low response even at high xylene concentrations. The membranes sensitivities increased in the order MMA < SDBS < PVA. After dispersing in PVA, the membrane sensitivity toward xylene was increased ~ 100 times, proving the positive effects of PVA on the optical properties and gas-sensing performance of LiFePO4 particles.

Probing the optimal refractive index profile of disordered silicon nanowires for photon management applications

Controlling the light transport using photonic structures is essential for the deterministic control of light trapping and that depends on the nano-scale spatial modulation of refractive index. Here, we study the optimal effective refractive index profile ( n eff ) of vertically-aligned disordered silicon nanowires using the measured spatial-and wavelength-dependent micro-reflectivity values across the nanowire length. A multi-layer model for the nanowire sample is employed to calculate the reflectivity values using the different n eff profiles. The calculated reflectivity values are compared with the measured reflectivity values for different polarization states of light. The results validate that the silicon nanowires with an exponential n eff profile suppress more than 90% reflectivity over a wide angular range for both polarization states of light. Moreover, the nanowires sample with an exponential n eff profile shows a significant enhancement in light trapping and Raman scattering. • Spatial mapping of reflectivity values across the nanowire length using micro-reflectivity setup. • The optimal refractive index profiles of disordered silicon nanowires are estimated which are supported with simulations. • The nanowire sample with an exponential index profile shows enhanced antireflection, light trapping, and Raman scattering. • Our results are useful for applications involving solar cells, photo-detectors, and sensing.

Clarifying and Imaging Candida albicans Biofilms.

The microbial fungus Candida albicans can undergo a change from commensal colonization to virulence that is strongly correlated with its ability to switch from yeast-form growth to hyphal growth. Cells initiating this process become adherent to surfaces as well as to each other, with the resulting development of a biofilm colony. This commonly occurs not only on mucosal tissue surfaces in yeast infections, but also on medical implants such as catheters. It is well known that biofilm cells are resistant to antifungal drugs, and that cells that shed from the biofilm can lead to dangerous systemic infections. Biofilms range from heavily translucent to opaque due to refractive heterogeneity. Therefore, fungal biofilms are difficult to study by optical microscopy. To visualize internal structural, cellular, and subcellular features, we clarify fixed intact biofilms by stepwise solvent exchange to a point of optimal refractive index matching. For C. albicans biofilms, sufficient clarification is attained with methyl salicylate (n = 1.537) to enable confocal microscopy from apex to base in 600 µm biofilms with little attenuation. In this visualization protocol we outline phase contrast refractometry, the growth of laboratory biofilms, fixation, staining, solvent exchange, the setup for confocal fluorescence microscopy, and representative results.

The variations of hydrophilic self-cleaning properties and refractive index dependence in the ZrO2 thin films by Gamma Irradiation

ZrO2 thin film samples were produced by the sol-gel dip coating method. Four different absorbed dose levels (such as ~ 0.4, 0.7, 1.2 and 2.7 Gray-Gy) were applied to ZrO2 thin films. Hence, the absorbed dose of ZrO2 thin film was examined as physical dose quantity representing the mean energy imparted to the thin film per unit mass by gamma radiation. Modification of the grain size was performed sensitively by the application of the absorbed dose to the ZrO2 thin film. Therefore the grain size reached from ~50 nm to 87 nm at the irradiated ZrO2 thin film. The relationship of the grain size, the contact angle, and the refractive index of the irradiated ZrO2 thin film was investigated as being an important technical concern. The irradiation process was performed in a hot cell by using a certified solid gamma ray source with 0.018021 Ci as an alternative technique to minimize the utilization of extra toxicological chemical solution. Antireflection and hydrophilic properties of the irradiated ZrO2 thin film were slightly improved by the modification of the grain size. The details on the optical and structural properties of the ZrO2 thin film were examined to obtain the optimum high refractive index, self-cleaning and anti-reflective properties.

An Optimized Angular Total Internal Reflection Sensor With High Resolution in Vanadium Flow Batteries

An optimized angular total internal reflection (TIR) sensor system without sensing films is proposed to detect the state of charge (SOC) of the vanadium redox flow battery (VFB) in real time. In order to balance the pixel sensitivity and the system noise considering the pixel discreteness of the charge-coupled device (CCD), the distance between the sensor chip and the CCD is adjusted to obtain the optimal refractive index (RI) resolution. The optimal RI resolution is $4.18\times 10^{-6}$ refractive index unit (RIU) at a distance of 257 mm with a dynamic range of 0.074 RIU. An algorithm of N-point average is utilized to further reduce the system noise, and therefore the RI resolution even reaches $8.13 \times 10^{-7}$ RIU at the optimal distance. The whole discharge process of the VFB detected by the optimal TIR sensor demonstrates that during three stages (VO2+ to VO2+, VO2+ to V3+, and V3+ to V2+) of the valence state variation, the RI of the electrolyte is linear with the valence state in each stage. The valence state resolutions of three stages are, respectively, $6.56 \times 10^{-5}$ , $10.04 \times 10^{-5}$ , and $5.35 \times 10^{-5}$ . This experiment verifies that the TIR sensor with high RI resolution can detect the SOC variation rate of the VFB in real time, providing a research tool for studying the kinetics of electrochemical reactions.

Surface plasmon resonance study of the interaction of 4,4′-dianilino-1,1′-binaphthyl-5,5′-disulfonic acid dipotassium salt (bis-ANS) and adenosine triphosphate (ATP) with oligomeric recombinant human lens αA-crystallin

α-Crystallin, an abundant mammalian lens protein made up of two subunits (αA- and αB-crystallin), is involved in the maintenance of the optimal refractive index in the lens. The protein is implicated in the pathophysiology of a large number of retinal diseases including cataract, age-related macular degeneration, diabetic retinopathy, and uveitis. α-Crystallin belongs to the small heat shock protein (sHSP) family, forms large oligomeric structures, and functions as a molecular chaperone appearing very early during embryonic development. To gain mechanistic insight into the structural and functional role of α-crystallin and its alterations in various retinal diseases, it is important to study the interaction chemistry with its known partners. The hydrophobic sites in α-crystallin have been studied extensively using environmentally sensitive fluorescent probes such as 4,4′-dianilino-1,1′-binaphthyl-5,5′-disulfonic acid dipotassium salt (bis-ANS) that interacts with both subunits of α-cystallin in 1:1 stoichiometry at 37 °C and diminishes the chaperone-like activity of the protein. Furthermore, it has been shown that ATP plays a crucial role in the association of α-crystallin with substrate proteins. We use surface plasmon resonance (SPR) to monitor the interactions of immobilized oligomeric recombinant αA subunit of human α-crystallin protein with bis-ANS and ATP. We assess the thermodynamic parameters and kinetics of such interactions at various temperatures. Our results indicate that bis-ANS binds to αA-crystallin with higher affinity when compared with ATP, although both αA-crystallin and αB-crystallin display fast interaction kinetics.

Optimization of angle-pixel resolution for angular plasmonic biosensors

An angular plasmonic sensor based on optimized angle-pixel resolution to detect antibody-antigen binding process is presented. Owing to discrete resonance curves by the CCD pixels, the pixel sensitivity and detection noise of the plasmonic sensors need to be balanced. The angle-pixel resolution is adjusted by changing the distance between the sensor chip and the CCD to obtain the optimal refractive index (RI) resolution for three plasmonic sensors, which are surface plasmon resonance (SPR), long range surface plasmon resonance (LRSPR) and plasmon waveguide resonance (PWR) sensors. Each plasmonic sensor can achieve its optimal RI resolution at the optimal distance, and the LRSPR sensor has the best RI resolution. Attributing to the reference channel and temperature control module in the sensor system, the RI resolution can achieve 4.37 × 10−7 RIU at Normal mode and 1.13 × 10-7 RIU at a slow response Extreme mode. The Protein A-IgG binding reaction experiment indicates that the plasmonic sensor has linear response to IgG concentration in a wide range and the limit of detection (LOD) is 5.45 pM at Extreme mode. The procedure and result of this work are helpful to the optimization of other angular plasmonic sensors.

Optimized flexible cover films for improved conversion efficiency in thin film flexible solar cells

Thin film solar cell technologies are being developed for lower cost and flexible applications. For such technologies, it is desirable to have inexpensive, flexible cover strips. In this paper, we demonstrate that transparent silicone cover glass adhesive can be doped with TiO2 nanoparticles to achieve an optimal refractive index and maximize the performance of the cell. Cells covered with the film doped with nanoparticles at the optimal concentration demonstrated a ∼1% increase in photocurrent over the plain (undoped) film. In addition, fused silica beads can be incorporated into the flexible cover slip to realize a built-in pseudomorphic glass diffuser layer as well. This additional degree of freedom in engineering flexible solar cell covers allows maximal performance from a given cell for minimal increased cost.

Optimized Dielectric Film for Maximum Sensitivity of Nearly Guided‐Wave Surface Plasmon Resonance Sensor

In this paper, the dielectric film of the nearly guided‐wave surface plasmon resonance (NGWSPR) sensor is optimized to achieve the maximum sensitivity. By optimizing the thickness of dielectric (Si, AlAs, TiO2, and Si3N4) film in a NGWSPR sensor, the optimized sensitivity of the NGWSPR sensor can be obtained. The optimized sensitivity of the NGWSPR sensor with AlAs film is 5.43, 8.47, and 24.41% higher than that of the NGWSPR sensor with Si, TiO2, or Si3N4 film respectively. In addition, by analyzing the electric field intensity of NGWSPR sensors, the origin of highest sensitivity achieved by the NGWSPR sensor with AlAs film is investigated. Furthermore, by comparing the optimized sensitivity of the NGWSPR sensor with different refractive index of dielectric film, the existence of optimal refractive index of dielectric film for the maximum sensitivity is confirmed. Besides, the effects of refractive index of prism on the optimal dielectric refractive index and the maximum sensitivity are also studied.

One-dimensional ordinary–slow extraordinary–Bernstein mode conversion in the electron cyclotron range of frequencies

The ordinary–slow extraordinary–Bernstein (O-SX-B) mode conversion in the electron cyclotron range of frequencies (ECRF) is revisited in slab geometry. The analytical formula of the O-SX conversion efficiency by Mjølhus is upgraded to include the magnetic field gradient, and the analytical expression of the SX-B conversion efficiency by Ram and Schultz is generalized for the case of oblique injection. Therefore, the conversion efficiency and optimal parallel refractive index for the whole O-SX-B conversion are obtained analytically and a shift of optimal parallel refractive index due to SX-FX loss is found. Full wave calculations are also presented to be compared with the analytical results.

A Lookup Table-Based Method for Estimating Sea Surface Hemispherical Broadband Emissivity Values (8–13.5 μm)

Sea surface hemispherical broadband emissivity (BBE, 8–13.5 μm) is a vital parameter for calculating surface radiation budgets. Such data are currently unavailable. This paper proposes a lookup table-based method for retrieving sea surface hemispherical BBE values. The physicallybased sea surface emissivity model of Wu and Smith, together with the optimal refractive index, were used to generate hemispherical BBE values under wind speeds ranging from zero to 50 m/s. A lookup table of hemispherical BBE values as a function of wind speed was established and used to retrieve sea surface hemispherical BBE values under foam-free conditions. The accuracy of the estimates of hemispherical BBE was 0.003, given a wind speed of zero. The foam effect was explicitly considered. After incorporating the foam effect, hemispherical BBE was expressed as a linear function of the hemispherical BBE values of sea water and foam, weighted by the fraction of foam coverage. With this method, we have produced an hourly sea surface hemispherical BBE product with a resolution of 10 km and global coverage that covers the period from 2003 to 2005, using wind speed data from Modern-Era Retrospective analysis for Research and Applications (MERRA)-2.

Enhancement of the refractive index of sputtered zinc oxide thin films through doping with Fe2O3

Certain optical applications, such as antireflective coatings on solar cells, require transparent films with high refractive indices (>2). Zinc oxide (ZnO) is a transparent semiconductor with exceptional optical properties. However, its refractive index in the transparent dispersionless region of the visible spectrum is lower than 2. In this study, we enhanced the refractive index of sputtered ZnO thin films through doping with iron oxide (Fe2O3), where ZnO targets were doped by 0.5–2.0 wt% of Fe2O3. The films were polycrystalline with smooth surfaces. Differential transmittance spectra were employed to derive the optical band gaps of the films, which showed a minor variation by ±0.03 eV due to doping. The refractive index was extracted from the transmittance spectra using a Cauchy equation and was further fitted by a single-oscillator model. The optimum refractive index was obtained for the films prepared from the target doped with 1.5 wt% of Fe2O3. Enhancement of the refractive index was accompanied by a reduction of optical absorption in the doped films.

Mode hybridization and conversion in silicon-on-insulator nanowires with angled sidewalls.

The mode property and light propagation in a tapered silicon-on-insulator (SOI) nanowire with angled sidewalls is analyzed. Mode hybridization is observed and mode conversion between the TM fundamental mode and higher-order TE modes happens when light propagates in a waveguide taper which is used very often in the design of photonic integrated devices. This mode conversion ratio is possible to be very high (even close to 100%) when the taper is long enough to be adiabatic, which might be useful for some applications of multimode photonics. When the mode conversion is undesired to avoid any excess loss as well as crosstalk for photonic integrated circuits, one can depress the mode conversion by compensating the vertical asymmetry in the way of reducing the sidewall angle or introducing an optimal refractive index for the upper-cladding. It is also possible to eliminate the undesired mode conversion almost and improve the desired mode conversion greatly by introducing an abrupt junction connecting two sections with different widths to jump over the mode hybridization region.

Dielectric spheres with maximum forward scattering and zero backscattering: a search for their material composition

Nanoparticles exhibiting zero backscattering but a large scattering cross section in the forward direction should play a key role as light diffracting elements in photonic devices like solar cells. Using Mie theory we address lossless dielectric spheres that were recently reported to possess a magnetodielectric response to the illuminating wave, and analyze their scattering cross section together with their zero-backwards scattering conditions. We show that there is an optimum particle refractive index (m = 2.47), which yields maximum forward scattering without backwards scattering of light.

Efficient passivation of solar cells by silicon nitride

In this study we investigated the deposition parameters of a PECVD silicon nitride (SiN) film leading to an efficient passivation of multicrystalline silicon solar cells. The parameters investigated were the temperature, the power and the refractive index through the gas process flow rate ratio SiH4/NH3.Using a symmetrical structure SiN/Si/SiN and Quasi-Steady-State Photo-Conductance (QSSPC) characterization we found that 380 °C and 4500 W are the optimal temperature and optimal power, respectively. The optimal refractive index was determined using a method which encompasses optical and electrical properties of SiN films deposited on multicrystalline silicon solar cells. This method, based on the calculation of the short circuit current densities, revealed that the optimal refractive index is 1.9.

Effects of para-fluorine substituent of polystyrene on gradient-index fiber-optic properties

To study the effects of fluorine substituent of polystyrene (PSt) on gradient-index fiber-optic properties, a poly(para-fluorostyrene) (P(p-FSt))-based graded-index plastic optical fiber (GI POF) is fabricated, and its properties are compared with those of a PSt-based GI POF. The para-fluorine substitution positively affects the glass transition temperature (Tg) of the core, wavelength dispersion of the optimum refractive index profile, bandwidth, and attenuation. The core Tg of the P(p-FSt)-based GI POF is 88°C, which is higher than that of the PSt-based GI POF by 9°C when both fibers have an identical numerical aperture (NA=0.2). The optimum refractive index profile coefficient for the P(p-FSt)-based GI POF varies from 2.2 to 2.1 in the 600–800nm range, whereas that for the PSt-based GI POF varies from 2.6 to 2.3 in the same wavelength region. The bandwidth of the P(p-FSt)-based GI POF is intrinsically higher than that of PSt-based GI POF. Moreover, the fiber attenuation of the P(p-FSt)-based GI POF was significantly smaller than that of the PSt-based GI POF over the source wavelength range. Our study demonstrates that P(p-FSt) has favorable properties as a GI POF base material.

Reduction of scattering using thin all-dielectric shells designed by stochastic optimizer

Adaptive differential evolution method has been used for optimization of all-dielectric multilayer coatings in order to reduce total scattering from spherical targets. The optimal refractive index profiles have been found for various sizes of targets and thicknesses of coatings. Few profile types that appear to be optimal for various geometrical parameters have been identified. Scattering of the target with diameter of 0.75 λ has been reduced by 85% using 0.16 λ thick coating formed by isotropic dielectric materials. For larger targets, scattering reduction becomes smaller, but it still reaches 50% for targets with the diameter of 1.5 λ. The obtained designs provide a route to implement cloaking without the use of magnetic and anisotropic metamaterials.

Excellent c-Si surface passivation by low-temperature atomic layer deposited titanium oxide

In this work, we demonstrate that thermal atomic layer deposited (ALD) titanium oxide (TiOx) films are able to provide a—up to now unprecedented—level of surface passivation on undiffused low-resistivity crystalline silicon (c-Si). The surface passivation provided by the ALD TiOx films is activated by a post-deposition anneal and subsequent light soaking treatment. Ultralow effective surface recombination velocities down to 2.8 cm/s and 8.3 cm/s, respectively, are achieved on n-type and p-type float-zone c-Si wafers. Detailed analysis confirms that the TiOx films are nearly stoichiometric, have no significant level of contaminants, and are of amorphous nature. The passivation is found to be stable after storage in the dark for eight months. These results demonstrate that TiOx films are also capable of providing excellent passivation of undiffused c-Si surfaces on a comparable level to thermal silicon oxide, silicon nitride, and aluminum oxide. In addition, it is well known that TiOx has an optimal refractive index of 2.4 in the visible range for glass encapsulated solar cells, as well as a low extinction coefficient. Thus, the results presented in this work could facilitate the re-emergence of TiOx in the field of high-efficiency silicon wafer solar cells.

Determination of the suitable refractive index of solar cells silicon nitride

In silicon solar cells studies, the optimal refractive index of plasma- enhanced chemical vapor deposited silicon nitride films is usually determined by an electrical characterization. This technique is done by minority carrier lifetime or surface recombination velocity measurement. We developed in this study a method which encompasses electrical and optical film properties. This method is based on the calculation of the short circuit current densities of multicrystalline silicon solar cells. The optimal refractive index is determined by the maximum short circuit current density.Films with the following refractive indices were studied: 1.9, 2.0, 2.1 and 2.4. The thicknesses are those of an optimal anti-reflection coating (one-quarter wavelength).The optical characterization of these films deposited on multicrystalline silicon wafers and on corning glass gave a minimal weighted reflection for refractive index of 2.0 and a maximum transmission for refractive index of 1.9, respectively.The QSSPC characterization revealed that the film refractive index of 1.9 performed the best passivation quality. Internal quantum efficiencies of simulated multicrystalline silicon solar cells coated with these films were determined by PC1d program simulation.Short-circuit current densities calculated using these experimental and simulated data revealed that the optimal refractive index is 1.9.

Optimization of the Refractive Index of a Gap Material Used for the 4-layer DOI Detector

We have developed a 4-layer depth-of-interaction (DOI) detector which consists of four layers of scintillation crystal arrays and a position sensitive photomultiplier tube (PS-PMT). To control the behavior of scintillation light in each DOI crystal array, some reflectors between crystals are removed so that all crystal responses in the four layers are expressed in one two-dimensional (2D) position histogram by implementing an Anger-type calculation of the PS-PMT signals. Since the method utilizes spread of scintillation light through the boundary of the crystals with no reflectors, positioning performance in the 2D position histogram depends on the crystal dimensions, the crystal surface finish, and gap materials. In this work, we propose an adjustment method for crystal identification performance of the 4-layer DOI detector by selecting the optimal refractive index for the gap materials between the crystals in which the reflectors are removed. We fabricated single-layer detectors and 4-layer detectors using LYSO scintillators and evaluated the crystal identification performance, while varying the refractive index of the gap materials (by using different optical cements). As a result, we found the 2D position histogram changes as a function of the refractive index of the optical cement between crystals and the optimal refractive indices can be determined for the detector using the LYSO crystals. We concluded that the crystal response of the 2D position histograms can be adjusted and crystal identification performance can be optimized by changing the refractive index of the gap materials.