Refractive Index Of Layer Research Articles

Anodized titanium oxide thickness estimation with ellipsometry, reflectance spectra extrema positions and electronic imaging: importance of the interfaces electromagnetic phase-shift

The oxide thickness of anodized titanium samples has been determined through ellipsometry, reflectance spectra extrema positions and electronic imaging. The reflectance spectra extrema position technique is applicable in the case were the oxide layer is thin enough to generate an interference phenomenon inside the oxide layer. When reflected at the air/oxide and oxide/metal interfaces, the electromagnetic field undergoes a phase-shift, which is often neglected in the literature. By comparing the oxide thickness obtained through the different techniques, it is shown that this phase-shift is not negligible for thin oxide layers. The relative error on the oxide thickness is for example of about 50% for a 17 nm thick oxide layer. By studying the discrepancy observed in the literature for the titanium and oxide layer refractive indexes, which is of about 13% in the wavelength range (350–600 nm), the error induced when neglecting the electromagnetic phase-shift is higher than the error induced by the uncertainty on the refractive indexes for oxide thicknesses below about 50 nm.

Multi-channel photonic bandgap engineering in hyperbolic graded index materials embedded one-dimensional photonic crystals

Engineering of multi-channel photonic band gap sensing consequences has been demonstrated in hyperbolic graded index materials embedded one-dimensional (1-D) photonic crystal (PC) in the frequency 150–850 THz region. The multi-channel photonic band gap sensing properties have been investigated by taking into account the reflection and photonic band gap (PBG) spectra of the proposed PC structures. For quarter-wave stacking, we obtain single optical reflection band for band region 646.8 – 434.3 THz with the constituted normal layer refractive index 1.5. Band regions and bandwidths of the single PBG channel can be modulated by changing the refractive index of the constituted normal layer and grading parameter of the hyperbolic graded layer. The number of photonic bands increases with increasing the layer thickness of the GPC structures and leads to work as multi-channel PBG sensors. The operation frequency of the multi-channel PBG sensors can also be tuned by changing the constituted normal layer and grading parameter of the hyperbolic graded layer. These properties lead to design the tunable multi-channel optical sensors/filters engineering. Moreover, the demonstration of the reflection phase shift, group velocity, group delay, and electric field distributions shows the effect of hyperbolic graded index materials on the propagation of light in 1-D PCs. With the engineering of tunable PBGs and structure controllability, hyperbolic graded index materials embedded 1-D PCs provide a promising way to fabricate tunable optical reflectors and multi-channel optical sensors/filters for future optical devices.

Robust, low haze and graded porous anti-reflective glass by Tailoring the Nanostructures

Low haze and anti-reflective glass has high potential applications in automobile and optoelectronic fields. Etching is a novel and effective method to fabricate gradient refractive index anti-reflective layer on glass surface. However, the gradient layer on glass surface prepared by the etching method usually characterizes rough porous structure, and the structural defect results in high haze and low abrasion resistance to restrict its applications. In this paper, a hydrothermal etching method has been explored to prepare anti-reflective glass. It has demonstrated to be a new and facile method successfully to tailor the porous nanostructure of gradient refractive index layer and largely decrease the haze of the glass, by adding complex compound in the etching solution. Compared with the etching solution containing NaOH, adding the complex compound of C6H5Na3O7 in the etching solution has the advantage to overcome the defects. The grain diameter of the graded porous anti-reflective film decreases from ~63 nm to ~18 nm, the etched film thickness increases from 0.44 μm to 1.55 μm, the haze decreases drastically from 23.76% to 1.00%, the reflectivity decreases from 5.88% to 1.08% and the abrasion resistance greatly increases. However, when changing the complex compound from C6H5Na3O7 to Na2HPO4, the haze is 23.44% and has no effect on decreasing haze. Structural characterizations show that the grain size in the porous gradient layer can be easily tailored by changing the ion radius of complex anion in the etching solution, and the optical properties can be controlled. The paper provides new insights into the nanostructures and the preparation approach of anti-reflective glass.

Design and analysis of omnidirectional solar spectrum reflector using one-dimensional photonic crystal

A one-dimensional photon crystal structure is proposed to design an omnidirectional broadband reflector. The structural parameters are optimized to reflect complete solar spectrum (0.3 to 2.3 μm) for both TE and TM polarized light. The design analysis is carried out using transfer matrix method. The structure is designed using alternate dielectric–dielectric layers of appropriate refractive indices and layer thicknesses. The proposed structure demonstrates more than 98% reflectivity for the complete wavelength range from 0.3 to 2.3 μm along with 73% emission in 8 to 13 μm, thus showing its potential as a dielectric-based reflector in applications such as daytime passive radiative coolers, and as a heliostat in concentrated solar power.

Controlling the null photonic gap and zero refractive index in photonic superlattices with temperature-controllable-refractive-index materials

Temperature-adjustable optical properties of two types of one-dimensional photonic superlattices made from lithium niobate or liquid crystal are theoretically investigated. The control of band structures and density of states is achieved through the temperature with various values of refractive index, layer width, and propagation wavelength. It is presented that a null photonic gap appears at certain temperature points, which are decided by the ratio of layer width and the refractive index. It is shown that the spatial average of the wave vector ⟨ k ⟩ vanishes at a specific temperature as functions of the layer thickness and refractive index. The values of the temperature corresponding to zero-refractive-index (zero- ⟨ n ⟩ ) are demonstrated to be suppressed when the refractive index or the ratio of the layer width is enhanced. Null photonic gap and zero- ⟨ n ⟩ can appear simultaneously at certain wavelengths in the range of visible light, which can be modulated by the temperature. This implies the importance of one-dimensional photonic superlattices made from temperature-controllable-refractive-index materials for many important practical applications.

Estimation of power loss spectra in slightly flattened distorted Bragg fiber waveguide

Dispersion characteristic and fiber loss spectra of a Bragg fiber waveguide having slightly flattened distortion are estimated for sensing applications. The characteristic and optical power loss equations of waveguide are obtained by matching the fields at various boundaries under quarter wave stack conditions. It is found that the optimum power loss is inversely proportional to the ninth power of distortion coefficient. The obtained results show that optimum fiber loss decreases with decrease of refractive index of cladding layer 'b' (low refractive index layer) and increase of refractive index of cladding layer 'a' (high refractive index layer). Also, fiber losses are estimated with increase of the high and low refractive index cladding layer losses. This loss spectrum of fiber can be considered as sensing signal. Since obtained fiber losses in distorted Bragg fiber are higher than standard Bragg fiber therefore, the sensitivity of proposed structure is higher than the standard Bragg fiber. Also, the sensitivity increases with the increase of refractive index of core medium. These results would be helpful to design a novel highly sensitive sensor based on hollow core distorted Bragg fiber.

Lossy Mode Resonance-Based Refractive Index Sensor for Sucrose Concentration Measurement

A detailed analysis of Prism/LiF/ZnO structure based lossy mode resonance sensor is presented for measurement of sucrose concentration. Theoretical and experimental studies using angular interrogation technique has been carried out to achieve high sensitivity for a range of surrounding medium refractive indices (1.33-1.45). Simulation outcomes reveal that a low refractive index layer of LiF may be introduced between substrate and the absorbing film of ZnO to develop a high sensitivity refractive index sensor. Various sensor configurations at different thicknesses of LiF are investigated for both s- and p- polarizations of light at the wavelength of 632.8 nm. The best obtained sensor configurations have been developed experimentally to measure concentration of sucrose solution. The maximum sensitivities of 61.92 °/RIU for s- polarization and 68.80 °/RIU for p-polarization are obtained experimentally, which are found to be in close agreement with the theoretically simulated results.

Influence of wavelength on the bandwidth of W-type plastic-clad silica optical fibers

The bandwidth and steady-state loss of multimode W-type plastic-clad silica optical fibers are investigated by solving the time-dependent power flow equation. The results show how the bandwidth of W-type plastic-clad silica optical fibers can be enhanced by shifting from the red to the infrared wavelength region for different intermediate layer widths and refractive indices of the outer cladding. Such characterization of these fibers is consistent with their manifest effectiveness in reducing modal dispersion and increasing bandwidth.

Numerical analysis of planar multimode optical sensor with active nanolayer

In the paper, numerical studies of the model of an optical sensor, based on interference of modes in a planar one-dimensional step-index configuration, are presented. Calculations are performed using the method of mode field analysis. The structure consists of the single-mode input waveguide, the multimode waveguide that guides only a few modes and the single-mode output waveguide. The structure is covered by a nanometer active sensor layer of a high refractive index, which changes its optical properties in contact with the measured external surrounding. The refractive index variation of an active sensor layer affects the modal properties of the multimode waveguide and the output optical field distribution. By the proper selection of the active layer, the considered configuration can be used for gas detection.

Triple-layer-coated microspheres for refractive index sensor with internally referenced self-compensated thermal effect

Optical microcavity has an important and promising application in high sensitivity sensing, but thermal drift hinders its practical use. In this study, we propose a triple-layer-coated microsphere resonator, which has a high sensitivity in refractive index sensing with low thermal drift. The refractive indexes of the three layers from the inside to the outside are high, low, and high, respectively. The two high refractive index layers can support their own whispering-gallery modes, called the inner mode (IM) and the outer mode (OM). We study the performance of IM and OM with waveguide coupling in refractive index sensing and temperature sensing. The results show that when the thickness of the middle layer is 550 nm, the refractive index sensitivity of IM and OM will be 0.0168 nm/RIU, 102.56 nm/RIU, and the temperature sensitivity will be –19.57 pm/K and –28.98 pm/K, respectively. The sensing is carried out by monitoring the difference in resonant wavelength between IM and OM and the sensing characteristics are optimized by adjusting the thickness of the middle layer. Further, when <inline-formula><tex-math id="M3">\begin{document}${t_B}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191265_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191265_M3.png"/></alternatives></inline-formula> = 400 nm, the refractive index sensitivity can arrive at 75.219 nm/RIU, the detection limit can reach 2.2 × 10<sup>–4</sup> RIU, and the thermal drift is reduced to 3.17 pm/K, thereby eliminating the effect of thermal drift to a great degree. This study provides the guidance for designing and improving the microsphere refractive index sensors.

Investigation on photonic band gap of a magneto photonic crystal

Magnetic photonic crystal is a subject of extensive research for their remarkable optical and magneto optical responses. In this work a novel one dimensional magneto photonic crystal having a magnetic nanocomposite medium consists of yttrium iron garnet (YIG)and Silver which act as high refractive index layer and air as the low refractive index layer is theoretically designed. Transmission properties of the designed one dimensional photonic crystal is studied using transfer matrix method. The variation of photonic band gap (PBG) have been analysed by varying applied magnetic field, filling factor and number of layers. Appreciable changes in the photonic band gap occurs with porosity.

Ultra-sensitive photonic crystal cancer cells sensor with a high-quality factor

In the present study, we develop a photonic biosensor based on a one-dimensional photonic crystal. Different types of cancer cell detection methods are developed by introducing a defect layer in the photonic crystal using cancer cells. The sensing mechanism of the present cancer sensor is based on varying the refractive index of the analyte, which leads to shift in the transmission or the reflection spectrum. By using the transfer matrix method, the s-polarized light (TE) is calculated, and a significant effect on the resonant defect peak is observed when the angle of the incident light or the defect layer thickness changes. In current work, the resonant defect peak at the optimum conditions is red-shifted from 1538 to 1648nm as the cancer cell layer refractive index changes from 1.350 to 1.401. Therefore, the proposed biosensor is designed with high sensitivity (S=2200nm/RIU) in the range of refractive index from 1.350 (zero cancer cell) to 1.401 (MCF-7 cancer cells). Thus the high Q (quality factor) is enhanced, which in turn makes the detector more sensitive for a slight change in the index.

Understanding the mechanism of Beraha-I type color etching: Determination of the orientation dependent etch rate, layer refractive index and a method for quantifying the angle between surface normal and the 〈100〉, 〈111〉 directions for individual grains

A grey cast iron specimen was investigated by Beraha-I type color etching for the purpose of developing a functional model for the etching kinetics and subsequently a method which would enable the estimation of ferrite grain orientations with respect to specific directions, solely based on optical microscopy. For this, the etching process was observed real-time in a microfluidic cell and a custom image processing software was developed to interpret the color information. Scanning electron microscopy with electron backscatter diffraction (SEM-EBSD) and atomic force microscopy (AFM) were used to determine the grain orientation and the relative thickness of the developed interference layer respectively, which were used for further calculations. It was shown that in the initial phase the etching process experiences a delay, which can be connected to breaking an oxide barrier above the ferrite grains. This delay was found to be influenced by the idle time between sample preparation and the start of etching. However, it was proven that a characteristic cycle etch rate can be defined between the first interference minimum and maximum, which shows correlation with both the 〈100〉 and 〈111〉 directions and which is also independent from the delay during the initiation phase. The exact value and orientation dependence of this cycle etch rate was experimentally determined along with the refractive index of the developed film. Our method was demonstrated to be applicable for the estimation of the angle between the surface normal and the 〈100〉, 〈111〉 directions for individual ferrite grains in the cast iron specimen, with an average absolute error of 3°, by using optical microscopy only. The method is implemented and was made available in the form of a Matlab program.

High temperature annealing effects on spectral, microstructural and laser damage resistance properties of sputtered HfO2 and HfO2-SiO2 mixture-based UV mirrors

Hafnium oxide is widely used as high refractive index material for making high power laser coatings for UV spectral range. Thermal annealing is one of the most efficient post-processing techniques for improving optical coating stress, its absorption, and resistance to laser radiation. In this article we investigate high temperature annealing effects on optical, microstructural, surface and laser radiation resistance properties of pure hafnia and two different hafnia-silica mixture-based sputtered dielectric mirrors for 266 nm wavelength. Three distinct annealing temperatures were selected for different coatings, leading to amorphous and polycrystalline phases of high refractive index layers. Annealing increased reflectance of all mirrors. Annealing at high temperatures that triggered crystallization considerably increased LIDT of all coatings. In addition, substantial increase in transmittance at shorter wavelength range was registered. Our results suggest that high temperature annealing above crystallization onset of HfO2 and HfO2-SiO2 mixture based dielectric mirrors has a great potential of increasing their damage resistance for UV laser pulses.

Ultra-broad band antireflection coating at mid wave infrared for high efficient germanium optics

This work mainly demonstrates the development process of an ultra-broad band antireflection (AR) coating that maximizes the optical performance of germanium optical elements. A multilayer stack exhibiting high efficient AR performance on Ge consists of thin multi-layers of aluminum oxide (Al2O3) and germanium (Ge) named as low and high refractive index layer materials. A three-layer design with a very tight thickness tolerance (3%) was optimized by Optilayer software. Al2O3 and Ge layers were deposited by a plasma assisted e-beam evaporation system. Ultra-high efficient multilayer AR coating on Ge has a base reflectance 0.005% at 3550 nm and an average reflectance of 0.256% over the entire mid wave infrared spectrum. Multilayer AR coating on Ge offers lower base and average reflectance than the works reported before. Moreover, AR coating on Ge offers a cost effective process cycle due to its number of layers and its thickness for providing ultra-broadband and high efficient optical performance for mid wave infrared electro-optical applications.

About nature of the effective surface charge on InAs crystals transformation during anode oxide layer growing

Dynamics of changes in fluorine atoms distribution through grown anodic oxide layer thickness and the effective surface charge on InAs crystals under such layers has been studied. Anodic oxidation was performed in alkaline electrolyte with fluorochemical additive component in galvanostatic mode at anode current densities 0.05 or 0.5 mA·cm−2. The layers thickness in boundes 32—51 nm varied by electrodes final voltage setting in range 15—25 V. The layer thickness and refractive index was measured by ellipsometric method, and distribution of fluorine atoms through thickness — by photoelectron−spectroscopy method, combined with ion etching. At the same time, based on grown layers there were produced MIS structures, and from calculation of theirs capacitance−voltage characteristics are determined effective surface charge and surface states density, corresponding to different layer thicknesses.Main results are reduced to the facts during layers growing despite of anodizing current density comes their sealing, the profile of fluorine atoms distribution shifts towards InAs, positive effective surface charge gradually decreases from 3.6 · 1011 to 2.0 · 1011 cm−2 at surface states density in (6—7) · 1011 eV·cm−2 range for all cases. Based on comparison of these data and theoretical concepts of MIS structure charge construction, there was made a conclusion about gradual built−in charge distancing from the border with InAs in the process of growing anodic oxide layer, which explains observed effective surface charge decrease during layer thickness increasing. This results indicates that the layer growth rate exceeds the built−in charge displacement rate towards InAs.

Massive Enhancement of Optical Transmission across a Thin Metal Film via Wave Vector Matching in Grating-Coupled Surface Plasmon Resonance.

We demonstrate how distinct surface plasmon resonance modes on opposite sides of a metal-coated grating can be coupled across the metal film. This coupling occurs by matching the resonance conditions on each side of the grating by tuning the refractive index directly adjacent to the metal film. In the first example, we deposited a high refractive index layer of tin oxide on top of the grating to red-shift the front side surface plasmon until it coupled with the backside surface plasmon across a semitransparent ∼45 nm thin silver grating. By shifting the resonance condition of the nearby surface plasmon, this high refractive index coating creates an effective matching of wave vectors across the metal film, allowing them to couple and enhance the optical response. A massive increase in the magnitude of enhanced transmission is observed, increasing from a 6-fold transmission enhancement through a bare silver grating to a near 100-fold enhancement after deposition of a tin oxide layer of appropriate thickness (∼310 nm). This optical transmission enhancement is then probed through computational modeling and by experiments with liquids of various refractive index values. The matched system shows an increased amplitude sensitivity with respect to refractive index changes and a waveguide like behavior within the tin oxide film. As an alternative configuration, we also demonstrate coupling the front and back-side plasmon modes by using a lower refractive index substrate in order to blue-shift the back-side surface plasmon. Coupling between the two plasmon modes is then demonstrated by introducing aqueous solutions of various refractive index values. Under the proper conditions, this matched system also shows a substantial enhancement in transmission. This technique of wave vector matching provides a route to substantially increasing the plasmon enhanced optical transmission through metal gratings, which has potential application in improved plasmonic sensing, spectroscopy, and plasmon-based optical devices.

Nanowire Length, Density, and Crystalline Quality Retrieved from a Single Optical Spectrum.

We propose spectral domain attenuated reflectometry (SDAR) for fast characterization of nanomaterial growth. The method is demonstrated here for zinc oxide (ZnO) nanowires (NWs) which are grown vertically in random forest fashion showing that it is not limited to well-ordered NWs. We show how SDAR can provide, on the basis of a single measured spectrum, simultaneous information on nanowire length, nanowire density (through nanowire/air filling ratio), and crystalline quality (through band gap). The robustness of the proposed method is assessed first through comparison with information obtained from SEM and XRD taken as reference. In SDAR, the process for fast extraction of NW thickness and filling ratio values makes use of the interference pattern contrast and the spectral periodicity in the reflection response which involve a best fit of the measured spectra with simple theoretical modeling based on the effective medium approach, achieved with a mean square error down to 0.1%. The results also suggest the existence of either 2 or 3 layers of different effective refractive index, hence providing insight on possible growth mechanisms.

Study of aging effects on optical properties and residual stress of HfO2 thin film

Hafnium dioxide (HfO2) thin films are generally used as high refractive index layers in optical coating devices. Aging effects on optical and mechanical properties of the films are very crucial for long-standing performance and stability of the devices, therefore knowledge of the aging effects is very important. In the present work, HfO2 thin film has been prepared using electron beam evaporation. The optical properties and residual stress of the film have been investigated in six months interval over a period of one-year in ambient condition to probe its aging behaviour. The measured refractive index increases with aging and its change is Δn = 0.011 at wavelength of 550 nm. The dispersion parameters such as εL, N/m*, εs, and Ed increase with aging, while E0 decreases. The measured film residual stress is tensile in nature. The residual stress decreases with aging and the change in stress in the film is around 78% in a year. A simple model has been utilized to establish and explain a correlation between the measured stress and the film porosity which varies with aging time. The results suggest that the change in the residual stress is significant in contrast to a very small change in the refractive index with aging.

Surface plasmon resonance biosensor based on graphene and grating excitation.

A surface plasmon resonance biosensor based on a graphene-decorated grating excitation structure is proposed in this paper. The biosensor consists of a three-layer structure, including a graphene layer, a grating layer, and a high refractive index layer. The material of the grating layer is silica. The graphene is physically deposited on the grating ridges. An incident light with transverse magnetic polarization is used to excite surface plasmons in the mid-infrared spectral region, which is highly localized at both ends of the graphene layer. The property of the sensor is improved by the high refractive index dielectric layer, which enhances the absorption of incident light and increases the depth of the spectra. The finite-difference time-domain method is used to simulate the property of the sensor. The structure of the sensor could be optimized by changing the structural parameters and comparing the simulation results. The effective refractive index (RI) on the surface and the wavelength of the reflective resonance absorption peak will be changed when the surface of graphene adsorbs the surrounding analyte. The results show that the relationship between the analyte RI and the resonance wavelength is linear. The measurement range of analyte RI is 1-1.8, and the sensitivity is 2780nm/RIU.