Multilayer Optical Thin Films Research Articles

Optimal design of multilayer optical thin film structure for smart energy saving applications using needle optimization approach

In this work, a novel design of a one dimensional photonic crystal (1D PC) is investigated. The 1DPC structure is composed of alternating layers of tantalum pentoxide (Ta2O5) and silicon dioxide (Sio2). The proposed 1D PC structure is designed to act as short wave pass (SWP) edge filter that selectively passes light of short wavelengths, while the infrared light is blocked. In this study, Essential Macleod software is used to create the optimal design with the computational support of the needle synthesis technique. By varying the incidence angle of the mean polarized light mode, we can determine the features of the optimal SWP edge filter design, which leads to an important application for this filter. It can shed light on the filter’s suitability as a smart energy saving window coating for hot climate regions. The study includes different hot regions in Saudi Arabia such as Mecca, Riyadh, Dammam, Arar and Alaqiq. They were used as case studies in this research. According to the study of the optimal design of SWP edge filter applied in Mecca, Riyadh, Dammam, Arar and Alaqiq provinces, the light transmittance in the visible region is more than 99% during the summer solstice and more than 96% during the winter solstice. The photonic band gab (PBG) is almost constant during the summer solstice without shifting or decreasing in size whereas in the winter solstice, the PBG shifts toward the short wavelengths and decreases in size by increasing the angle of incidence. This allows an amount of solar energy to enter in winter. Riyadh, Dammam, and Arar provinces experienced a significant increase in solar energy during the winter solstice, more than Mecca and Alaqiq provinces.

NEUTRON: Neural particle swarm optimization for material-aware inverse design of structural color

SummaryDesigning optical structures for generating structural colors is challenging because of the complex relationship between the optical structures and the color perceived by human eyes. Machine learning-based approaches have been developed to expedite this design process. However, existing methods solely focus on structural parameters of the optical design, which could lead to suboptimal color generation because of the inability to optimize the selection of materials. To address this issue, an approach known as Neural Particle Swarm Optimization is proposed in this paper. The proposed method achieves high design accuracy and efficiency on two structural color design tasks; the first task is designing environment-friendly alternatives to chrome coatings, and the second task concerns reconstructing pictures with multilayer optical thin films. Several designs that could replace chrome coatings have been discovered; pictures with more than 200,000 pixels and thousands of unique colors can be accurately reconstructed in a few hours.

Multi-layer longitudinally specular optics for distributing diffuse light.

Multi-layer optical thin films can reflect light to guide it along a hollow tube that they internally line. However, reflecting broadband white light for a range of angles requires many precise thin optical layers, which can be too expensive for large-scale applications such as architectural illumination. Here, we present an alternative configuration that requires only one readily achievable aspect of precision-having a substantially constant cross section, perpendicular to the planned longitudinal propagation direction. This aspect conserves a light ray's longitudinal directional component, while the ray's path in the perpendicular direction can be macroscopically chaotic, much like the diffuse reflection characteristic arising from many non-absorbing pigment particles. This macroscopic characteristic is described here as "longitudinally specular" and "transversely diffuse." This new design overcomes two problems that are found with current prism light guides (PLGs), which are a class of hollow light guides that also have a constant cross section. The PLG has two problems: (1) it requires structured surfaces having near-perfect linear prisms that are much larger than the wavelength of light, wasting optical material and increasing absorption; (2) it only works well with light that has been partially collimated, which limits étendue and requires expensive, inefficient input optics. In this paper, a new, to the best of our knowledge, approach, labeled a "multi-layer prism light guide," simultaneously addresses both problems. Although the new structure is more complex than a conventional PLG, it may be simpler to manufacture. One potential application is for guiding sunlight into buildings for illumination purposes, without requiring complex sun-tracking solar collection optics.

Design of multilayer optical thin-films based on light scattering properties and using deep neural networks.

Despite limiting the performance of multilayer optical thin-films, light scattering properties are not as yet controllable by current design methods. These methods usually consider only specular properties: transmittance and reflectance. Among other techniques, design of thin-film components assisted by deep neural networks have seen growing interest over the last few years. This paper presents an implementation of a deep neural network model for light scattering design and proposes an optimization process for complex multilayer thin-film components to comply with expectations on both specular and scattering spectral responses.

Multilayer optical thin film design with deep Q learning

Multilayer optical film plays a significant role in broad fields of optical application. Due to the nonlinear relationship between the dispersion characteristics of optical materials and the actual performance parameters of optical thin films, it is challenging to optimize optical thin film structure with the traditional models. In this paper, we present an implementation of Deep Q-learning, which suited for the most part for optical thin film. As a set of concrete demonstrations, we optimize solar absorber. The optimal program could optimal this solar absorber in 500 epoch (about 200 steps per-epoch) without any human intervention. Search results perform better than researchers’ manual searches.

71‐3: A New Solution without Quantum Dots for LCD to Achieve More Than 90% BT.2020

In this paper , by using the side‐emitting LED backlight with collimating lens and multilayer optical thin film color filter the LCD were successfully developed with Rosh‐compliant and to achieved more than 90% BT.2020 color gamut .The LED light source can use YAG phosphor or normal nitride RG phosphor without using quantum dots solution.

Numerical Simulation on Optical Characteristics of ZnO/Cu/ZnO Thin Film.

This paper investigates optical characteristics of ZnO/Cu/ZnO multilayer optical thin film by numerical simulation and it is further verified by experiments. The influences of Cu and ZnO thickness on the optical characteristics of ZnO/Cu/ZnO thin films are studied. The results show that the transmittances of ZnO/Cu/ZnO thin film decrease with the increase of thickness of Cu layer but are nonlinear with the ZnO thickness. The maximal light transmission in the visible region is obtained by controlling the thickness of Cu and ZnO films.

Research on quantum efficiency of transmission-mode InGaAs photocathode

To investigate the quantum efficiency of high performance transmission-mode InGaAs photocathode, the quantum efficiency curve is fitted by using the multilayer optical thin film theory and the first principle calculation. A series of the performance parameters are obtained with the relative error less than 5%. It indicates that the thickness of the emission layer is 0.7–0.9μm, the thickness of the window layer is 0.1–0.3μm. Meanwhile the results from the first principle calculation are proved to be reliable as well. In addition, the long-wave response will increase and the highest integral sensitivity will be obtained when the thickness of the emission layer is the optimum value. For the InGaAs photocathode, the back interface recombination velocity mainly leads to the low integral sensitivity, which is caused by the material lattice mismatch.

Effect of passivation layers on characteristics of AlGaInP ridge waveguide laser diodes

We investigate the effect of passivation structure on the optical mode distribution and characteristics of the edge emitting ridge waveguide AlGaInP–GaInP visible laser diodes (LDs). For conventional designs of single-layer Si3N4 or SiO2 passivation, the variation of lateral near-field confinement and the horizontal far-field (FF) divergence can be determined via the modification of dielectric layer thickness. Thin passivation layer suffers from high absorption at the metal interface while thick passivation layer suffers from poor heat dissipation in the ridge waveguide and high scattering loss, resulting in high threshold. We propose a novel design of three-pair Al2O3/Ta2O5 multilayer optical thin films as passivation on the ridge waveguide, which can improve the laser characteristics and the heat dissipation. The measured room-temperature threshold current (Ith) and characteristic temperature (T0) are 44.5 mA and 104.2 K

Dielectric core–shells with enhanced scattering efficiency as back-reflectors in dye sensitized solar cells

Particulate back-reflector films are conventionally used for the improvement of light harvesting in dye solar cells (DSC). The back-reflection of the films is directly related to the scattering efficiency of the individual particles. Inspired by the idea of multilayer optical thin films, here it is demonstrated theoretically and experimentally that putting a SiO2 shell around spherical rutile-TiO2 particles leads to improved light scattering by the particles. These dielectric core–shells not only enhance the overall diffuse reflection of the films, but they also cause a relative improvement in the red and near infrared regions. Back-reflector films of these core–shell particles employed in DSCs result in an enhanced device efficiency of up to 9.52%. Contrary to the general notion, the conventional DSC back-reflector films are far from ideal, i.e. they show less than a 50% reflection in real device conditions. This is due to the limited thickness of the films (a few micron), as well as the low “wet” reflection of the back-reflector films, i.e. the reflection of the scattering particles embedded in a liquid electrolyte. This fact reveals the need for more optimized scattering particles. We suggest dielectric core–shell particles as an alternative.

Simulated Annealing Algorithm and Genetic Algorithm for Optimization Design of Multilayer Optical Thin-Film Coatings

The key parameters for Simulated Annealing Algorithm (SAA) and Genetic Algorithm (GA) were determined by test experiments. With optimal parameters obtained for both algorithms, the multilayer antireflection coatings in the 7.7-12.3μm band and a neutral beam splitter in the visible region were designed. Results showed that, for infrared multilayer antireflection coatings, the evaluation function value of SAA was 18.7% lower than that of GA, and the average transmittance of SAA was 0.4% higher than that of GA. Compared to GA, SAA achieved a higher average transmittance and a more flat transmittance curve. And for the neutral beam splitter, the running time of GA was 0.407s less than that of SAA, whereas the convergence effect of SAA was superior to that of GA. Therefore, the design with SAA has certain advantages for both the infrared multilayer antireflection coating and the beam splitter.

Simulation of reflectivity spectrum for non-absorbing multilayer optical thin films

Reflectivity simulation is an essential tool for the design and optimization of optical thin films. We have developed a reflectivity simulator for non-absorbing dielectric multilayer optical thin films using LabVIEW. The name of the substrate material as well as the material and thickness of each layer of the multilayer stack are fed into the program as input parameters in a pop-up window. The program calculates reflectivity spectrum for the given range of wavelengths using layer thicknesses and dispersion data of refractive indices for the defined stack of dielectric materials. The simulated reflectivity spectra for various combinations of materials in multilayer stacks are presented and compared with the experimental results of the multilayer optical thin films grown by electron-beam evaporation technique.

Multilayer optical thin films for use at terahertz frequencies: method of fabrication

A new method of fabricating multilayer optical coatings used at terahertz frequencies has been developed. Using plasma-enhanced chemical-vapor deposition, a multilayer antireflection coating for germanium optics at terahertz frequencies was fabricated. The coating consists of amorphous silicon and silicon-oxide layers. The transmittance and structure of the coating were experimentally investigated. The transmittance spectrum of the coating on the Ge substrate shows a wideband antireflection behavior in the 40-120 cm(-1) region.

Application of in-situ ellipsometry to the fabrication of multi-layer optical coatings with sub-nanometre accuracy

We report on the use of real-time ellipsometric monitoring in the fabrication of a very demanding multi-layer optical thin film coating for a colour-corrected, near-infra-red (NIR) blocking filter with a 532 nm notch for a laser protection application. The design consists of 79 layers of Ta 2O 5 and SiO 2 of various thicknesses and is extremely sensitive to errors in thickness and deviations of optical properties of the constituent films. The films were deposited by ion-assisted deposition (IAD).

Studies on single layer CeO 2 and SiO 2 films deposited by rotating crucible electron beam evaporation

High index CeO 2 films and low index SiO 2 films are widely used in multi layer optical thin film devices, such as high reflectors, interference filters, anti reflection coatings, etc. Optical properties, such as refractive index, transmittance in the UV, Visible and IR depends upon the deposition conditions, especially the substrate temperature. Single layer films of CeO 2 and SiO 2 have been deposited using a novel rotating crucible electron beam evaporation technique. The refractive indices and UV, VIS and IR transmittance of the films have been measured by varying substrate temperature in the range ambient to 400 °C. The structure of CeO 2 films has also been studied. The refractive index of CeO 2 films at 550 nm increased from 2.00 to 2.41 as the substrate temperature was increased from ambient to 400 °C. The extinction coefficient of these films was negligibly small even at elevated substrate temperatures. CeO 2 films deposited even at ambient temperature were crystalline. The refractive index of SiO 2 films increased marginally from 1.46 to 1.48. UV, Visible and IR transmission characteristics of SiO 2 films indicate that the films are stoichiometric. The density of CeO 2 and SiO 2 films has also been estimated with substrate temperature.

Influence of deposition parameters on optical properties of <inline-formula><math display="inline" overflow="scroll"><msub><mi>TiO</mi><mrow><mn>2</mn></mrow></msub></math></inline-formula> films

Titanium dioxide is extensively used as high index material for multilayer optical thin film device applications operating in the visible and near infrared region. The performance (high reflectance/transmittance and laser-induced damage) of the device is decided by the absorption in the films. The refractive index and extinction coefficient of TiO2 films is strongly influenced by the deposition parameters and stoichiometry of the evaporation material. In the present investigation, TiO2 films have been deposited by reactive electron beam evaporation of TiO, Ti2O3, and TiO2 in a neutral and ionized oxygen atmosphere. A Heitmann-type discharge source has been fabricated in the laboratory and used to ionize oxygen. Deposition parameters such as oxygen partial pressure (5 X 10(-5) to 5 X 10(-4) torr), rate of deposition (60 to 210 Angstrom min(-1)), and substrate temperature (25 to 250degreesC) were varied during the preparation of the films. The optical constants of TiO2 films were estimated from the spectrophotometer data. In-situ optical monitoring of TiO2 films with suboxides as the starting material showed the presence of considerable absorption in the films deposited in neutral oxygen, even under favorable deposition conditions. Postdeposition heating was necessary to reduce the absorption in the films. TiO2 films with minimum absorption have been made using TiO2 starting materials. Absorption-free films have also been obtained using ionized oxygen with the starting materials, even at higher substrate temperatures. The observed variation in optical properties has been explained on the basis of mismatches between the film growth and rate of oxidation.

Modified needle method with simultaneous thickness and refractive-index refinement for the synthesis of inhomogeneous and multilayer optical thin films

Useful features of an efficient thin-film synthesis technique based on the refinement of inhomogeneous films are adapted to the needle method. As in the former approach, all the layer refractive indices and thicknesses are refined simultaneously, and special attention is given to computation speed and accuracy. In particular, key parameters are calculated from analytical formulas, namely, the gradients used in the optimization routines and the optimum needle positions and refractive indices. Inhomogeneous as well as multilayer solutions are possible. The efficiency of the modified approach is demonstrated by the solution of a complex nonpolarizing, wide-angle antireflection coating problem. The results are compared with those obtained in the past by the conventional needle method and by refinement of inhomogeneous films.

Plasma deposition of optical films and coatings: A review

Plasma enhanced chemical vapor deposition (PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin film systems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasma systems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others.

Measurement of the Effect of Temperature on Stress Distribution and Deformation in Multilayer Optical Thin Film Structures

AbstractThis study investigates methods of predicting the deformation and stress distribution in multilayer optical thin film structures. The thin film layers include materials of various thermal expansion coefficients, elastic moduli, and melting temperatures. Each layer is deposited at a different temperature, causing complex thermal and deposition stresses throughout the structure. In addition, since the deposition temperatures of some of the layers are high (>600°C), stress relaxation and plastic flow may occur in materials with low melting temperatures. A combination of theoretical predictions and experimental measurements is used to measure and quantify the deformation caused by residual and thermal stresses in the films as well as any plastic deformation that may have occurred. Results from a model using multilayer plate bending theory to determine the elastic deformation of the device due to thermal stresses are reported. These predictions, as well as a more common method of predicting film stress and curvature, are compared to experimentally measured curvature changes as a function of temperature in the samples. However, when plastic deformation begins to occur at high temperatures, the residual stress and degree of deformation are no longer predictable based on elastic theory alone, and have to be measured experimentally. Plastic deformation in the substrate is discussed as a cause of a high observed curvature following sample heating.

Investigation of acousto-optic tunable filter parameters for fast spectrometer application

Ideally an acousto-optic tunable filter is capable for selecting a single optical wavelength from a broadband polychromatic light beam simply by its electronic tuning. In practice, however, both the wavelength resolution and the operating wavelength range of the filter are limited by its electric and optical properties. An experimental configuration of a computer-controlled, fast acousto-optic spectrometer, based on an acousto-optic tunable filter operating in the 400- to 800-nm visible range with an average resolution of 10 nm is constructed and studied. The spectrometer has been operated in broadband optical measurements, such as determination of transmission spectra of complex, multilayer optical thin films and also real-time, in situ monitoring of evaporation processes. During the experiments, an anomalous behavior of the filter was observed that seriously limited the performance of the filter in the blue region. Some requirements necessary for spectrometer application are summarized and theoretical and experimental results are given illustrating the existence and also the elimination of this anomalous behavior.