Refractive Index Of Films Research Articles

Nanosilver-Enhanced Far-Field Fluorescence Fluctuations for Super-Resolution Microscopy.

Fluctuation-based super-resolution microscopy enhances image resolution using signal fluctuations, yet the inherent fluctuations of fluorophores limit its spatiotemporal resolution. In this work, we reveal a far-field enhancement (FFE) effect via a nanosilver film that significantly boosts fluorescence fluctuations of fluorophores positioned up to 10 μm away. The FFE effect arises from the interference of scattered light from the nanosilver film and photothermal-induced refractive index changes in the imaging medium, which create periodic auxiliary illumination on the sample. This phenomenon enabled the development of far-field enhanced super-resolution microscopy (FFE-SRM), a technique compatible with commonly used fluorophores. FFE-SRM improves temporal resolution up to 10-fold and enhances spatial resolution by about 2-fold over various SRM methods, including stochastic optical fluctuation imaging, super-resolution radial fluctuation, mean-shift super-resolution, and direct stochastic optical reconstruction microscopy. We demonstrated the potential of FFE-SRM by revealing mitochondrial dynamics in live-cell imaging, advancing super-resolution imaging, and cellular process exploration.

Terahertz laser-fabricated all-polymer hole arrays with high-quality quasi-bound states in the continuum

Abstract Dielectric metasurfaces promise to realize ultrahigh-quality (Q) resonances due to their ultralow material absorption. Most of them are silicon-based metasurfaces, requiring complex fabricated steps and thus suffering high costs. Laser etching processing has simple steps accompanied by low time consumption and exemplary processing efficiency. Here, an all-polymer metasurface based on hole arrays fabricated by laser processing has been proposed and investigated. Such metasurfaces achieve sharp quasi-bound states in the continuum (quasi-BICs) via breaking structural symmetry, form two annular circulation electric fields in different directions, and thus allow strong coupling between holes. Owing to the low refractive index of polymer, the calculated Q-factor reaches 9555 while the diameter discrepancy is 4 μm. Simulated results proved that the Q-factors of quasi-BICs can be further improved by reducing the film thickness and refractive indices of materials, which can be predicted by the fitting equation. Also, the fields in holes can be enhanced by reducing the film refractive index. These results in simulations and experiments provide an alternative method for designing high-Q resonators in terahertz regions.

Mechanistic Analysis of Anodic Oxidation of Gold in KOH (0.1 M) Solution Using the Point Defect Model

The potentiostatic, anodic formation of gold oxide at potentials of 0.55 to 0.80 V versus SHE in aqueous KOH (0.1 M) was studied using an impedance-based Point Defect Model (PDM). The film thickness and refractive indices at each formation potential were estimated using spectroscopic ellipsometry. The thickness of the oxide increases linearly with increasing applied voltage within this range. Mott-Schottky (MS) analysis showed that gold oxide formed in KOH (0.1 M) is an n-type semiconductor, and the dominant defect (Aui3+) density is calculated to be in the order of 1021–1022 (1/cm3). The steady-state current density of the oxide formation was independent of voltage, also in agreement with an n-type oxide. Reasonable agreement between PDM predictions and experimental observations of dominant defect density, steady-state current density, and thickness, demonstrates the value of the PDM in this system.

Optical properties of transition metals complex films derived from hydrazone oxime for optoelectronic devices

Hydrazone incorporating oxime nucleus and its Ni2+, Mn2+, Zn2+ and UO22+ metallic chelates have been prepared and completely characterized by different spectral, theoretical, and analytical techniques including NMR, FT-IR, mass, and EAS spectroscopy. The finding denoted that the Hydrazone ligand bonded as a dibasic tridentate chelator via deprotonated oximato nitrogen, enol carbonyl oxygen and hydrazono azomethine nitrogen atoms resultant in octahedral geometry for Ni2+, Mn2+, Zn2+ and UO22+. Theoretical studies by DFT/B3LYP/LANLDZ together with dipole moment, geometrical optimization, energetic parameters, and energy gap (HOMO–LUMO) were employed to support the geometrical structure of the synthetics. Additionally, these complexes were prepared in the form of thin film onto cleaned glass substrate by spin-coating technique. The values of the optical band gap (Eg) and film index of refraction (n) have been determined by many different methods. It was found that the hydrazone-oxime (OBBH2) film has the highest value of Eg (eV) where the UOBB complex film has the smallest value of Eg (2.56 eV). The latter is very close to the Eg value of ZnSe (2.58 eV). The refractive index changes with wavelengths show normal dispersion which is well discussed in terms of Sellmeier's dispersion model. Also, the nonlinear optical parameters viz. the first (χ(1)) and third (χ(3)) orders of nonlinear optical susceptibilities and the non-linear index of refraction (n2) have been studied. The complex dielectric constant (ε⌣=εr+iεi)), conductivity (σ⌣=σr+iσi), sheet resistance (Rs), and thermal have been investigated over the whole spectral range. According to the results of χ(1), χ(3) and n2 the complex films under study are candidates to be used in optical switching gadgets, optical modulators, and optical signal processing.

Characterization of SiNx grown at different nitrogen flow and prediction of refractive index using artificial neural networks

SiNx films were grown on silicon substrates by Radio Frequency (RF) magnetron sputtering deposition. The effect of nitrogen flow on the structural and optical properties of the obtained films was investigated using X-ray diffraction, Scanning Electron Microscopy (SEM), UV–Vis–NIR spectrophotometer and spectroscopic ellipsometer, respectively. XRD spectra of the films showed that all films belong to amorphous structure. SEM photographs of SiNx films were analyzed. As a result of the analysis, it was observed that the surfaces of the films had a homogeneous and smooth structure as the nitrogen flow increased. The total and diffuse reflectance spectra of the films were measured and the energy band gaps of the films were determined using the Kubelka-Munk function by using the diffuse reflectance. It was observed that the energy band gap changed as the nitrogen percentage increased. The refractive index of all films was obtained as a function of temperature using a spectroscopic ellipsometer. In the second part of this study, we focused on predicting the temperature dependent refractive indices of the nitrogen flow-dependent films using Artificial Neural Networks (ANN). For the training of the ANN model, wavelength and temperature values from experimental data were used as input and refractive index as output parameters. The simulation and prediction results obtained from this model are compared with the experimental data and interpreted. It is concluded that the ANN approach is suitable for simulating and predicting the temperature dependent refractive index. The models successfully trained with ANN will be especially preferred for predicting the refractive indices of SiNx films, which cannot be measured experimentally, thus providing predictions in non-experimental ranges. In particular, the results obtained by focusing on the ability of the developed artificial neural network (ANN) models to predict the optical properties of SiNx films and their potential to provide information in non-experimental conditions, offer a new approach to quickly and effectively evaluate the optical properties of SiNx films. This approach reveals the importance of artificial intelligence-based methods in materials characterization studies.

Immersion ellipsometry for the uncorrelated determination of ultrathin film thickness and index of refraction: Theory and examples

Immersion ellipsometry can break the well-known correlation between optical constants and thicknesses of ultrathin (<5–10 nm) films, allowing both to be determined. In immersion ellipsometry, ellipsometric data is acquired in air and liquid ambients, and the data sets are combined in the analysis. The contrast in index between the liquid and film adds information to the analysis that breaks the correlation between the film thickness and refractive index that exists for air-only measurements. We describe the theory and practice of immersion ellipsometry. We also discuss the use of multiwavelength immersion ellipsometry to measure the thicknesses and optical constants of two thin films: native oxide on silicon and an alkyl monolayer on that native oxide. The average thicknesses of the native oxide and chloro(dimethyl)octadecylsilane (CDMOS) monolayer were 1.526 ± 0.027 nm and 1.968 ± 0.057 nm, and their average indices of refraction at 633 nm were 1.519 ± 0.005 and 1.471 ± 0.004, respectively. The native oxide and CDMOS monolayer were also characterized with x-ray photoelectron spectroscopy (XPS) and contact angle goniometry. Both the XPS C 1 s peak and the water contact angle increased substantially after monolayer deposition. While immersion ellipsometry has been known for decades, its use has been limited, maybe due to a lack of awareness of the technique and/or the need to immerse the sample surface in a liquid that could be destructive if the sample is not compatible with the liquid. As ultrathin films become widely used in science technology, immersion ellipsometry should increase in importance.

Investigations on temperature dependent properties of spray deposited tin oxide thin films

The present study reported the temperature dependent properties of tin oxide (SnO2) thin films deposited by using conventional, cost effective and non-vacuum-based spray pyrolysis technique (SPT). The as deposited films were characterized by various characterization techniques such as X ray diffraction (XRD), UV-Vis spectroscopy and Scanning electron microscopy (SEM) to study their structural, optical and morphological properties respectively. XRD analysis revealed tetragonal rutile crystal structure of SnO2 with preferred orientation along (200) plane. The refinement of XRD data was also carried out by Rietveld refinement to obtain the fitting parameters, crystallite size and lattice parameters. UV-Vis study showed considerable change in optical absorbance and transmittance with change in substrate temperature. Effect of substrate temperature on film thickness, refractive index and band gap energy were determined by Swanepoel method and by Tauc’s plot respectively. The Urbach energy was also calculated and were correlated to the defects in SnO2 films. SEM micrograph showed the transformation from granules to nanospheres as a function of substrate temperature. Electrical properties were determined by Hall effect measurements which gives carrier concentration, electrical resistivity and mobility in the order of 1019-1020 /cm3, 10−2-10−3 Ω.cm and 4–23 cm2/Vs respectively.

Metasurface array for single-shot spectroscopic ellipsometry

Spectroscopic ellipsometry is a potent method that is widely adopted for the measurement of thin film thickness and refractive index. Most conventional ellipsometers utilize mechanically rotating polarizers and grating-based spectrometers for spectropolarimetric detection. Here, we demonstrated a compact metasurface array-based spectroscopic ellipsometry system that allows single-shot spectropolarimetric detection and accurate determination of thin film properties without any mechanical movement. The silicon-based metasurface array with a highly anisotropic and diverse spectral response is combined with iterative optimization to reconstruct the full Stokes polarization spectrum of the light reflected by the thin film with high fidelity. Subsequently, the film thickness and refractive index can be determined by fitting the measurement results to a proper material model with high accuracy. Our approach opens up a new pathway towards a compact and robust spectroscopic ellipsometry system for the high throughput measurement of thin film properties.

Azimuthal polarized quasi-bound states in the continuum based on rotational symmetry breaking.

Bound states in the continuum (BICs) allow to obtain an ultrahigh-quality-factor optical cavity. Nevertheless, BICs must be extended in one or more directions, substantially increasing the device footprint. Although super-cavity mode quasi-BICs supported by single nanopillars have been demonstrated recently, their low-quality factor and localized electromagnetic field inside the dielectric nanopillar are insufficient for high-sensitivity refractive index sensing applications. We propose a ring structure rotated by a dielectric sectorial nanostructure, which can achieve a high quality factor by breaking the rotational symmetry of the ring structure with a footprint as small as 3 µm2. As a straightforward application, we demonstrate high performance local refractive index and nanoscale film thickness sensing based on rotational symmetry breaking induced BICs. These BICs reach quality factor and sensitivity of one order of magnitude better than those of conventional super-cavity mode BICs. The proposed method provides insights into the design of compact high quality factor photonic devices, opening up new possibilities for applications in refractive index and nanoscale film thickness sensing.

Optimization design of surface optical characteristics of space solar cells based on transfer matrix method

The antireflection coating (ARC) can improve the photoelectric conversion efficiency of photovoltaic (PV) cells. In this paper, the influence of film thickness and refractive index of single-layer and double-layer ARC on solar light absorption under different spectral conditions is simulated by the transfer matrix method. The optimum values of ARC film thickness and refractive index are obtained. To optimize it at AM 0 (air mass 0) solar irradiance, a 66 nm thick SiN x ARC with a refractive index of 2.0 was used. The PV cell’s maximum power density is 89.87. The maximum power density of the PV cell with double-layer SiN x as ARC is 90.94. This work provides a theoretical basis for the application of ARC in ground PV power generation systems and space solar power systems.

Modeling of the synergistic anti-reflection effect in gradient refractive index films integrated with subwavelength structures for photothermal conversion.

Photothermal materials generally suffer from challenges such as low photothermal conversion efficiency and inefficient full-spectrum utilization of solar energy. This paper proposes gradient refractive index transparent ceramics (GRITCs) integrated with subwavelength nanostructure arrays and simulates the synergistic anti-reflection effect by an admittance recursive model. An innovative subwavelength structure, possessing a superior light-trapping capability, is initially crafted based on this model. Subsequently, various intelligent optimization algorithms including genetic algorithm, particle swarm optimization, and simulated annealing are employed to optimize the structure of gradient refractive index films respectively. Finally, the photothermal conversion efficiencies of devices based on different photothermal materials are calculated. The simulations and finite-difference time-domain calculations demonstrate that the three-layer GRITCs integrated with an optimal SNA exhibit outstanding full-spectrum and omnidirectional anti-reflection performance. The solar transmittance of the devices can exceed 97% for light wavelengths ranging from 300 to 2500 nm over the full angle of incidence. Our results reveal that the synergistic anti-reflection effect in the SNAs and GRITCs can enhance the photothermal conversion efficiency by more than 20%.

Plasmon enhancement of photosensitivity of Ag–chalcogenide glass thin film structures

In this paper, we present the results of studying the features of plasmon- enhanced photostimulated diffusion of silver into thin films of chalcogenide glasses (ChG), in particular, As 2 S 3 and GeSe 2 . To ensure excitation of surface plasmon-polaritons (SPPs) at the interface between silver and ChG films, silver diffraction gratings with periods of 899 and 694 nm were used as substrates. The samples were exposed to the p-polarized radiation of a He-Ne laser (λ = 632.8 nm). The radiation of the same laser, attenuated by two orders of magnitude, was used to detect SPP, which enabled to study the kinetics of photostimulated processes in the thin-layer structure of Ag–ChG. It has been established that in the initial period of exposure, the SPP electromagnetic field significantly enhances the photostimulated flux of silver ions in ChG (by 2-3 times). The photodissolution kinetics of Ag in ChG is defined by the features of the granular structure of the investigated thin chalcogenide films. For the GeSe 2 film with the effective thickness 8 nm, the kinetics of the film refractive index increase caused by silver photodoping is well approximated by a logarithmic dependence. For the Ag–As 2 S 3 structure (the effective thickness of the As 2 S 3 film is 14.8 nm), this kinetics is closer to the linear one; moreover, for photodoping without SPP excitation, the kinetics is somewhat superlinear, while with plasmon excitation, it is sublinear. The main physical mechanism responsible for the acceleration of the process of photostimulated diffusion in the structure under study appears to be an accelerated generation of electron-hole pairs, which takes place in the ChG layer near the interface with the metal, where the SPP electromagnetic field strength is maximum, and/or plasmon- assisted hot carrier generation due to plasmon scattering on the surface of the metal film and subsequent internal photoemission of electrons from silver into chalcogenide.

Preparation of gradient refractive index films on glass surface and its anti-reflection properties

This study presents the creation of a gradient refractive index film on the surface of aluminosilicate glass using a two-step process involving chemical etching and subsequent multilayer sol-gel coating with hollow silica nanospheres. The primary objective was to achieve effective anti-reflection properties. Various analytical techniques such as FESEM, TEM, UV-Vis-NIR spectrophotometry, ellipsometer, and FT-IR spectrometry were employed to investigate the impact of glass surface morphology and refractive index on optical characteristics. The results revealed the successful formation of a porous structure on the glass surface through chemical etching. Subsequently, this porous structure was uniformly filled and layered with hollow silica spheres of different sizes, resulting in a stepwise gradient refractive index coating. Notably, the pore size of the hollow spherical silica colloidal particles progressively increased from 0 nm to 23 nm, corresponding to distinct film thicknesses and refractive indices. This stepwise modulation contributed to the establishment of a gradient refractive index within the coating. By transitioning the refractive index of the film layer from that of glass to air, the film acted as a matching layer, thereby achieving broadband anti-reflection. The efficiency of this anti-reflection was demonstrated by an increase in transmittance from an initial value of 91.8–95.32% following the two-step etching process. With the subsequent SiO2 coating, the transmittance continued to improve, ultimately reaching a high value of 97.47%. This improvement in transmittance was attributed to the gradual enhancement of the film layer.

Wearable Fiber SPR Respiration Sensor Based on a LiBr-Doped Silk Fibroin Film.

Respiration is essential for supporting human body functions. However, a biocompatible fiber respiration sensor has rarely been discussed. In this study, we propose a wearable fiber surface plasmon resonance (SPR) respiration sensor using a LiBr-doped silk fibroin (SF) film. The SPR sensor monitors respiration by responding to airway humidity variation during inhalation and exhalation. We fabricated the SPR respiration sensor by depositing the core of a plastic-clad optical fiber with a gold film and an SF-LiBr composite film. The SF-LiBr composite film can absorb water through the interaction between water molecules and hydrogen bonds linking fibroin chains. Thus, humidity variation can change the SF-LiBr composite film's refractive index (RI), altering the phase-matching condition of the surface plasmon polaritons and shifting the SPR spectral dip. In experiments, we test the effect of the LiBr doping ratio on humidity response and confirm that the SF-22.1 wt % LiBr sensor has balanced performances. The SF-22.1 wt % LiBr sensor has a broad sensing range of 35-99% relative humidity (RH), a reasonable overall sensitivity of -6.5 nm/% RH, a fast response time of 135 ms, a quick recovery time of 150 ms, good reversibility, and good repeatability, which is capable of tracking different respiration states and patterns. Finally, we encapsulate this sensor in a conventional nasal oxygen cannula for wearable respiration monitoring, proving that the sensor is suitable for high-sensitivity, real-time, and accurate respiration monitoring.

Structure and Optical Characteristics of DLC Films

SummaryUsing a RF‐Plasma Enhanced Chemical Vapor (RF‐PECVD) Depositions method, employing hydrogen and methane gas, Diamond like carbon (DLC) films were fabricated on glass and silicon substrates. The impact of varying CH4 to H2 ratios on the resulting film's structure, optical characteristics, and mechanical properties was investigated. The deposition process occurred at methane flow rates spanning 5 to 40 sccm. The films' surface morphology, and roughness were analyzed through Atomic Force Microscopy. Moreover, the Vickers hardness tests was employed to determine the hardness of produced films. The optical behavior of the DLC thin films was explored utilizing UV‐visible spectrometry and ellipsometry. A thorough investigation was conducted to understand how the CH4 flow rate influences layer growth, morphology, topography, and how these factors relate to the films' transmittance, reflectance, refractive index, and optical band gap.

Improved electrical carrier dynamics and UV detection performance of Zn1-xWxO nanostructured thin films

Current work reports development of Zn1-xWxO nanostructured thin films with Tungsten (W) doping level of 0.0, 1.0, 2.5 and 5.0 wt.% onto glass substrates via successive ionic layer adsorption and reaction (SILAR) method. Films demonstrates improved carrier dynamics and UV photodetection in correlation with W doping level and their suitability in advanced optoelectronics devices. The specimen morphological and structural properties is examined via field emission scanning electron spectroscopy (FESEM) and X-ray diffraction (XRD). In order to understand the electronic structure and carrier dynamics, Raman, UV–Vis, photoluminescence (PL) and time resolve photoluminescence (TRPL) Spectroscopy were employed. Films growth reveals crystallization in hexagonal wurtzite structure and preferred orientation along (002) plane (c-axis). In addition, no phase was observed in diffractograms due to W doping in ZnO matrix. An important effect of W incorporation in ZnO matrix was observed on the grain size and morphology as can be seen in FESEM images. Films optical parameters such as band gap, absorption index and refractive index of films are obtained from absorbance, transmittance and reflectance spectra in UV–Vis-NIR region. The PL spectra of films reveals quenching effect after adding W doping level. The TRPL spectra of films shows that the carrier decay time have been improved with increase in W doping concentrations. On the other hand, films UV detection performance are examined, which reveals significant improvement in response-recovery time, responsivity, external quantum efficiency and detectivity.

Swanepoel method for estimating the film thickness and complex index of refraction by using only the lower envelope: Special case

Over the last few decades, a large number of research papers on the optical properties of thin films have been published, and significant efforts have been made to supply a mathematical formulation that can describe the transmittance (T(λ)) and reflectance (R(λ)) of various optical systems. Swanepoel suggested a straightforward analysis using the upper (TM) and lower (Tm) envelopes of measured transmittance. He suggested a good correlation between the index of refraction (n), Tm, and absorbance (x) in the form of (Tm=16n2sx/(A+Bx+Cx2) where s, A, B, and C parameters are given in the text). He solved this equation to determine the value of the refractive index (see eq. (9) in Ref.1). Here comes the question why we do not resolve the equation to determine the value of the absorption coefficient?In this investigation the author tries to estimate both the film refractive index (n), absorbance (x) based only on the values of Tm and without any dependence on the film thickness (t). Also, a new analysis to find the film thickness precisely is based on the interference fringes main equation (2 nt = mλ).

Direct measurement of the extinction coefficient by differential transmittance.

A new procedure to measure the extinction coefficient k of film materials that are relatively transparent is presented. This procedure does not require the use of an optical-constant model or the knowledge of extra physical properties of the material, such as the specific heat capacity. It involves preparing a sample with two areas, at least one of them coated with the film, whereas the other may remain uncoated or may be coated with a different thickness of the same material. The differential transmittance between the two sample areas is shown to be proportional to k of the film material in the following measurement conditions: the incident light is p polarized and it impinges at the film material Brewster angle. The differential transmittance is obtained with a single measurement by making the light beam or the sample to oscillate with respect to one another and by using a lock-in amplifier; for normalization purposes, the transmittance in one of the sample areas is also measured. The proportionality factor between the normalized differential transmittance and k only involves the wavelength, the film thickness, and the Brewster angle. The knowledge of the film Brewster angle requires that the film refractive index (n) is measured beforehand; this can be performed with standard procedures, such as ellipsometry, since such techniques are efficient at measuring n of a transparent material, but are inefficient at measuring a small k. The procedure is exemplified with the calculation of k in the far ultraviolet of AlF3 films deposited by evaporation. The dependence of the uncertainty of k obtained with this procedure is analyzed in terms of the uncertainty of the film n, of wavelength, and of the degree of polarization of the incident beam. The selection of a substrate with similar n to the film material is also discussed. The uncertainties involved with the present procedure were analyzed for a specific example and an uncertainty of 2 × 10-5 in k calculation is considered feasible.

Investigating the effect of Te on the structural and physical properties of CdSe films for optoelectronic applications

Thermal evaporation was used to fabricate (CdSe)1−x(Te)x (0 at% ≤x ≤ 0.2 at%) films on ultra-clean substrates made of glass. The films in our study were deposited at room temperature in an evacuated chamber of approximately 3 × 105 mbar with a deposition rate of 2.8–2.3 Å. The thin films initially had a polycrystalline structure with a cubic crystal system, transitioning to an amorphous structure at 0.1 % Te content, before returning to a polycrystalline structure with a hexagonal crystal system at higher Te content. The microstructural parameters evaluated included dislocation density) δ (, lattice strain (ε), lattice constant (a), and average crystallite size (Davg). Scanning electron microscopy indicated that the grain size of the samples was around 10–60 nm. Swanepoel’s relation was applied to determine the film thickness (d) and refractive index (n) of the samples at specific points on the wavelength spectrum. Furthermore, the refractive index (n), and extinction coefficient (k) were calculated as functions of wavelength using Cauchy’s relation. The optical band gap (Eg) showed a slightly decreased, and the Urbach energy (Eu) increased with the Te ratio. Based on the Fourier transform infrared absorption spectrum, the CdSe vibrational mode was found to exist between 400 cm−1 and 800 cm−1. Additionally, the Raman spectrum of CdSe thin films was presented to enhance our understanding of the compounds vibrational characteristics. By carefully selecting the ideal ratio of transition metal dopants, our study opens the door to enhancing the physical properties of CdSe for use in optoelectronic devices.

Elastic and thermo-elastic characterizations of thin resin films using colored picosecond acoustics and spectroscopic ellipsometry

Colored Picosecond Acoustics (CPA) and Spectroscopic Ellipsometry (SE) are combined to measure elastic and thermoelastic properties of polymer thin-film resins deposited on 300 mm wafers. Film thickness and refractive index are measured using SE. Sound velocity and thickness are measured using CPA from the refractive index. Comparing the two thicknesses allows checking consistency between both approaches. The same combination is then applied at various temperatures from 19° to 180°C. As the sample is heated, both thickness and sound velocity change. By monitoring these contributions separately, the Temperature Coefficient on sound Velocity (TCV) and the Coefficient on Thermal Expansion are deduced. The protocol is applied to five industrial samples made of different thin-film resins currently used by microelectronic industry. Young’s modulus varies from resin to resin by up to 20%. TCV is large on each resin and varies from one resin to another up to 57%.