Broadband Antireflection Coatings Research Articles

A novel teaching-learning based optimization approach for design of broad-band anti-reflection coatings

Abstract The design of thin-film multilayered anti-reflection (AR) coating is quite an intricate task due to highly nonlinear and complex dimensional search space, which includes many local minima. In this paper, a novel teaching-learning based optimization (TLBO) approach is employed to design ultra-low reflective coating over a broad wavelength-band using multilayer thin-film structures for optoelectronic devices. The algorithm is implemented using LabVIEW as a programming tool. Various design specific input parameters such as scanning range of wavelengths, step-size, angle of incidence, number of layers, the name and sequence of coating materials etc. are required to be fed by the user on the graphical user interface. The algorithm minimizes the average reflectivity computed over given wavelength range by tuning the thickness of layers in the multilayer stack. The reliability and evolution of design solution with iterations have been systematically investigated for different learner-sizes. Finally, using the optimized learner size and desired number of iterations, the optimum AR design is obtained in terms of the thickness of each layer for the multilayer AR coating. The effectiveness of the TLBO approach has been compared with that of an established algorithm, i.e. genetic algorithm (GA), by means of Wilcoxon singed ranked test. It is concluded that the TLBO can be a very efficient, simpler and relatively faster approach to address complex optimization problems such as broad-band AR coating designs.

Self-assembled dual-sided hemispherical nano-dimple-structured broadband antireflection coatings

A non-lithography-based approach is developed in this study for assembling monolayer close-packed hemispherical nano-dimple arrays on both sides of a PET film by a scalable Langmuir-Blodgett technology. The resulting gratings greatly suppress specular reflection and therefore enhance specular transmission for a broad range of visible wavelengths, resulting from a gradual change in the effective refractive index at air/PET interface. The experimental results reveal that the antireflection properties of the as-fabricated coatings are affected by the size of the nano-dimples. Moreover, both optical performances of single-sided and dual-sided nano-dimple-structured coatings have been investigated in this study.

High performance broad band antireflective coatings using a facile synthesis of ink-bottle mesoporous MgF2 nanoparticles for solar applications

Ultra-low refractive index thin films suitable for practical antireflective (AR) applications must be highly transparent, economical, and durable against temperature and weather conditions. In this work, we present a high performance broad band antireflective coating using a facile synthesis of Ink-Bottle mesoporous MgF2 nanoparticles. The nanoparticles having high crystalline and dispersible properties were prepared by a deformation-reformation route from coarse commercial MgF2 hydrate powder by Lyothermal synthesis. These nanoparticles, after dispersion in a suitable solvent were used to develop a single layer AR coating by dip-coating technique. We precisely developed coatings tunable to achieve minimum reflection losses between 400 and 1500nm. The AR coating exhibited nearly 100% transmittance within visible range (615–660nm) and an average transmittance of 99% and 97% in the visible (400–800nm) and active solar range (300–1500nm) respectively. Further, use of the AR coating on PV glass led to a net improvement of 6% in efficiency for c-Si solar cells. This work opens a promising approach to improve the device performance of solar cells as well as solar collectors by developing broad band antireflective surfaces using mesoporous nanoparticles.

Design and preparation of broadband antireflective coatings with excellent mechanical properties

Double antireflective coatings with broadband, high-transmittance and scratch-resistance properties were prepared in this research by sol-gel method. The bottom layer was the mixture of silica nanoparticles and linear silicate polymers (SiO2 NPs-LPs) obtained from base/acid two-step catalyzed tetraethyl orthosilicate (TEOS), while the top layer was the mixture of hollow silica nanoparticles and linear silicate polymers (HSiO2 NPs-LPs). By optimizing the refractive index and the thickness of the double coatings, the optical performance of the double coatings was generally consistent with the results of theoretical design. The average transmittance in the wavelength range of 400–1200nm reached 99.1%. Besides, through applying hydroxyl modification on the surfaces of silica particles and hollow spheres, the antireflective coatings exhibited excellent mechanical properties, with its pencil hardness reaching 5H and critical load for scratch being 28.9N. All these good features have made this coating promising in harsh outdoor conditions.

Determination of Atomic-Layer-Deposited Multilayer Antireflection Coating Parameters Using a Novel X-Ray Reflectivity Approach

Atomic-layer-deposited Al2O3/TiO2/Al2O3 multilayer antireflection stacks used for broadband antireflection coatings were characterized by a novel combination of an X-ray reflectivity layer thickness extraction algorithm and X-ray reflectivity simulations, together with spectroscopic ellipsometry, broadband optical reflectance, glancing incidence X-ray diffraction, transmission electron microscope and energy dispersive X-ray spectroscopy. Layer thicknesses and surface and interface roughnesses or grading with a few angstroms’ precision, and densities with less than one percent uncertainty were assessed. These parameters are key to understanding the role of deposition and processing conditions on the performance of broadband antireflection coatings. Additionally, the effect of air annealing at 400 °Ϲ for 10 min on the structural and optical properties of multilayer Al2O3/TiO2/Al2O3 antireflection coating was revealed with these techniques.

Three-layer tri-wavelength broadband antireflective coatings built from refractive indices controlled silica thin films

A three-layer tri-wavelength broadband antireflective (AR) coating with 100 % transmittance simultaneously at 1064, 532 and 355 nm was designed using TFCalc™ thin film design software. The refractive indices for the bottom, middle and top layers of the designed AR coating are 1.33, 1.21 and 1.10, respectively. To realize the designed AR coating, a template-free sol–gel process was proposed to regulate the refractive index of silica film from 1.09 to 1.44 by simple hybridization. Silica films with refractive index of 1.21–1.44 were prepared by the hybridization of acid-catalyzed silica and base-catalyzed silica, while organically modified silicates films with refractive index of 1.09–1.21 were prepared by the hybridization of tetraethoxysilane, methyltriethoxysilane and hexamethyldisilazane under base-catalyzed condition. These two hybridization methods could conveniently control the porosity, and hence the refractive index of films. Finally, the three-layer tri-wavelength broadband AR coating, which is very important in high-power laser systems, was realized with the transmittance of 99.9, 99.8 and 99.4 % at 1064, 532 and 355 nm, respectively. A three-layer tri-wavelength broadband antireflection coating with nearly 100 % transmittance simultaneously at 1064, 532 and 355 nm is designed by TFCalc™ thin film design software and prepared by a simple sol–gel process. A template-free sol–gel process was proposed to prepare silica films with controlled and ultra-low refractive index.

Millimeter-wave broadband antireflection coatings using laser ablation of subwavelength structures.

We report on the first use of laser ablation to make submillimeter, broadband, antireflection coatings (ARCs) based on subwavelength structures (SWSs) on alumina and sapphire. We used a 515 nm laser to produce pyramid-shaped structures with a pitch of about 320 μm and a total height of near 800 μm. Transmission measurements between 70 and 140 GHz are in agreement with simulations using electromagnetic propagation software. The simulations indicate that SWS-ARCs with the fabricated shape should have a fractional bandwidth response of Δν/νcenter=0.55 centered on 235 GHz for which reflections are below 3%. Extension of the bandwidth to both lower and higher frequencies, between a few tens of gigahertz and a few terahertz, should be straightforward with appropriate adjustment of laser ablation parameters.

Focus on materials, semiconductors, vacuum, and cryogenics

Focus on materials, semiconductors, vacuum, and cryogenics

Design and Deployment of a Multichroic Polarimeter Array on the Atacama Cosmology Telescope

We present the design and the preliminary on sky performance with respect to beams and pass-bands of a multichroic polarimeter array covering the 90 and 146 GHz Cosmic Microwave Background (CMB) bands and its enabling broadband optical system recently deployed on the Atacama Cosmology Telescope (ACT). The constituent pixels are feedhorn-coupled multichroic polarimeters fabricated at NIST. This array is coupled to the ACT telescope via a set of three silicon lenses incorporating novel broad-band metamaterial anti-reflection coatings. This receiver represents the first multichroic detector array deployed for a CMB experiment and paves the way for the extensive use of multichroic detectors and broadband optical systems in the next generation of CMB experiments.

Broadband and wide-angle hybrid antireflection coatings prepared by combining interference multilayers with subwavelength structures

To reduce the intensity of the Fresnel reflections of optical components, subwavelength structures prepared by reactive ion etching of SiO2 thin films were combined as the outermost layer with a multilayer system made of conventional thin-film materials. A hybrid coating was thus realized, with the nanoscaled structured outermost layer expected to further improve the antireflection properties of common interference stacks. The microscopic and optical spectroscopic analysis of the subwavelength structures revealed that pillar-shaped nanostructures formed during etching exhibit low-refractive-index properties and have a depth-dependent refractive index. To take into account the refractive-index gradient in the coating design, the optical properties of the nanostructures were modeled using the effective-medium approximation. The calculated average effective refractive index turned out to be 1.11 at 500-nm wavelength. A hybrid coating was designed to minimize the residual reflectance in the 400-nm to 900-nm spectral range for BK7 glass substrate. Experimental results demonstrated that the hybrid-coating approach yields a low residual reflectance with very good omnidirectional properties, owing to the properties of the nanostructured surface.

Broadband antireflective superhydrophobic self-cleaning coatings based on novel dendritic porous particles

Novel dendritic porous particles were employed to fabricate broadband antireflective superhydrophobic self-cleaning coatings by facile dip-coating method.

Искусственно созданная дисперсия в полностью диэлектрических градиентных наноструктурах: частотно-избирательные поверхности раздела и туннельные широкополосные просветляющие покрытия

Искусственно созданная дисперсия в полностью диэлектрических градиентных наноструктурах: частотно-избирательные поверхности раздела и туннельные широкополосные просветляющие покрытия

Pulsed laser deposition of refractive-index-graded broadband antireflection coatings for silicon solar cells

This paper reports the design, fabrication and characterization of broadband antireflection coatings for silicon-based solar cells, using a graded-index technique and a novel dual-beam pulsed laser deposition method. A graded refractive index was realized by mixing two different optical materials, silicon and soda-lime glass, in a way that the composition of the mixture changes gradually from silicon (that of the solar cell) to glass (that of the outside protective glass) along the coating thickness direction following pre-designed refractive-index profiles. In this study, two different refractive-index profiles were fabricated, linear and Southwell profiles, on silicon substrates. In order to emulate a protective cover glass for solar-cells, coatings with an additional 400nm-thick outer glass layer were also fabricated. The antireflection coatings showed reflectance of 2.2–5.5% (mostly around 4%) for a wavelength range of 400–1000nm, and for the Southwell profile with a top glass layer, the reflectance was found to be between 2.2% and 4%, which is believed to be close to the theoretically best antireflection performance of a solar cell with a cover glass. Finite-difference time-domain simulations were also performed and the numerical results were in line with the measurement results. Because these coatings are based on silicon and glass and can be fabricated by the one-step pulsed laser deposition procedure, it can simplify the solar-cell structure considerably. The fabricated antireflection coatings may be also used for many other optical applications, such as thin-film optical attenuator and some optical sensors, and the presented pulsed laser deposition method is capable of fabricating a variety of high-performance optical coatings, which cannot be fabricated by other methods.

Broadband antireflection coating for optimized terahertz beam splitters

We investigate the potential of anti-ferromagnetic nanofilms as broadband antireflection coatings in the terahertz frequency range. The anti-ferromagnetic layer is modeled by an analytic wave-impedance matching approach. The experimental results of the transmission and reflection measurements demonstrate the effectiveness of our antireflection coatings. Furthermore, we use anti-ferromagnetic nanofilms as antireflection coating for a terahertz beam splitter. Compared with conventional terahertz beam splitters consisting of an uncoated thick silicon wafer, the coated silicon beam splitter has two advantages: elimination of multiple reflections and improvement of the signal-to-noise ratio for terahertz time-domain spectroscopy in reflection geometry.

English

In this study, we fabricated broadband anti-reflection coatings using a method based on layer-by-layer self assembly of positively charged layer double hydroxide (LDH) nanosheets and negatively charged silica nanoparticles via electrostatic interaction. Scanning electron microscopy and atomic force microscopy were used to observe the morphology, structure, and surface topography of LDH/SiO 2 multilayer coatings. The anti-reflection properties of the coatings were investigated by UV visible spectrophotometry. Glass substrates covered with the LDH/SiO 2 multilayer coatings exhibited broadband anti-reflection properties. The obtained [LDH(0.4 g/L)/SiO 2 (25 nm)] 8 , [LDH(0.4 g/L)/SiO 2 (50 nm)] 10 , and [LDH(0.8 g/L)/SiO 2 (25 nm)] 6 coatings exhibited the best broadband anti-reflection properties among the as-prepared LDH/SiO 2 multilayer coatings with different deposition cycles. Transmission levels of 97% were achieved in these optimal systems. Moreover, a maximum transmittance of 98% was achieved at a wavelength of 550 nm in the [LDH(0.4 g/L)/SiO 2 (25nm)] 8 system and at 700 nm in the [LDH(0.8 g/L)/SiO 2 (25nm)] 6 system. Different packing patterns of the two oppositely charged nanomaterials (dense packing of LDH nanosheets and loose stacking of silica nanoparticles) and the moderate textured surface of the coatings contributed to the enhanced light transmission and reduced wavelength dependence in the UV visible spectral range.

Broadband antireflection coatings for optical lenses

Broadband antireflection coatings for optical lenses

Random Au–Ag Composite Nanoparticles as Broadband Antireflection Coating for Si Surface

Random Au–Ag Composite Nanoparticles as Broadband Antireflection Coating for Si Surface

Novel-type nanostructured SiO2 antireflection coatings and their application in Cu(In,Ga)Se2 solar cells

Broadband antireflection coatings (ARCs) are a considerable way to sufficiently harvest more solar light into the solar cells due to dramatically reducing the reflection losses at the surface of substrate. Novel structures consisting of continuous SiO2 film and monodispersed SiO2 nanospeheres (NSs) array are proposed to fabricate ARCs according to mimicking the moth-eye-like nanostructure with a periodic sub-wavelength protuberance. Three distinct architectures of coatings based on SiO2 film and SiO2 NSs were successfully obtained by sol–gel dip-coating and electrostatic self-assembly technique, and the structural, morphological and optical properties of the coatings were investigated. Antireflection functionality of the coatings is achieved over the solar spectrum, and the solar transmittance of the substrate in visible region is increased by 7.02% on average with maximum of 8.21% by introducing coating of SiO2 NSs/SiO2 film. The power conversion efficiency of Cu(In,Ga)Se2 solar cells is clearly increased from 11.66% to 12.59% by using with as-prepared ARCs. The novel-type nanostructured ARCs are promising materials in both optical and photovoltaic applications.

Artificial dispersion of all-dielectric gradient nanostructures: Frequency-selective interfaces and tunneling-assisted broadband antireflection coatings

The non-locality of optical properties of gradient dielectric nanofilms, stipulated by smooth spatial distributions of refractive index, is shown to create the peculiar plasma-like dispersion of non-polar dielectric films, determined by the shapes and sizes of these distributions. Gradient all-dielectric nanostructures, characterized by the artificial heterogeneity-induced nonlocal dispersion and providing the broadband antireflection tunneling regime of energy transport in the visible and infrared ranges, are designed and tested. The wave energy flow in these structures is supported due to interference of evanescent and antievanescent modes formed by the non-Fresnel reflections of these modes on the discontinuities of gradient of refractive index on the boundaries of adjacent nanofilms. The transmittance spectra of these structures in the visible and infrared ranges, characterized by strong dispersion nearby the red edge of visible range, almost constant high transmittance in the near infrared range and weak dependence of tunneling energy flow upon the multilayer structure thickness, are calculated; the experimental verifications of these effects are presented. The perspectives to use the tunneling of light in gradient media for reconsideration of Hartman paradox are shown. Potential of periodical gradient all-dielectric nanostructures for optimized design of optical dispersive elements and broadband antireflection coatings for the visible and IR spectral range, respectively, is discussed.

Gradient index antireflection coatings on glass containing plasma-etched organic layers

The plasma etching process was originally developed to produce antireflective (AR) nanostructures on polymer substrates. A common vacuum coating system equipped with a plasma source was used. The technique is now applied to deposit and etch organic films on glass. The resulting organic nanostructured layers exhibit a low effective refractive index that can be tuned down to approximately 1.1. Broadband AR coatings were developed by combining inorganic materials and organic nanostructured layers in such a way that the effective index decreases in a stepwise manner or gradually from the index of the substrate to that of the ambient medium. They exhibit AR properties from 400 nm to 1200 nm at normal and oblique light incidence.