Omnidirectional Antireflection Coatings Research Articles

Geometric design of antireflective leafhopper brochosomes

In nature, leafhoppers cover their body surfaces with brochosomes as a protective coating. These leafhopper-produced brochosomes are hollow, buckyball-shaped, nanoscopic spheroids with through-holes distributed across their surfaces, representing a class of deployable optical materials that are rare in nature. Despite their discovery in the 1950s, it remains unknown why the sizes of brochosomes and their through-holes consistently fall within the range of hundreds of nanometers across different leafhopper species. Here, we demonstrate that the hierarchical geometries of brochosomes are engineered within a narrow size range with through-hole architecture to significantly reduce light reflection. By utilizing two-photon polymerization three-dimensional printing to fabricate high-fidelity synthetic brochosomes, we investigated the optical form-to-function relationship of brochosomes. Our results show that the diameters of brochosomes are engineered within a specific size range to maximize broadband light scattering, while the secondary through-holes are designed to function as short-wavelength, low-pass filters, further reducing light reflection. These synergistic effects enable brochosomes to achieve a substantial reduction in specular reflection, by up to approximately 80 to 94%, across a broadband wavelength range. Importantly, brochosomes represent a biological example demonstrating short-wavelength, low-pass filter functionality. Furthermore, our results indicate that the geometries of natural brochosomes may have evolved to effectively reduce reflection from ultraviolet to visible light, thereby enabling leafhoppers to evade predators whose vision spectrum encompasses both ultraviolet and visible light. Our findings offer key design insights into a class of deployable bioinspired optical materials with potential applications in omnidirectional antireflection coatings, optical encryption, and multispectral camouflage.

Wide-angle broadband antireflection coatings based on boomerang-like alumina nanostructures in visible region

A novel boomerang-like alumina based antireflective coating with ultra-low reflectance has been produced for light incidence angles form 0 up to 45°. Boomerang-like alumina nanostructures have been fabricated on the BK7 glass substrates by dip-coating and surface modification via hot water treatment. To achieve the lowest residual reflectance, the effect of dip-coating rate and hot-water temperature in the treatment process has been investigated and optimized. To further investigate the boomerang-like alumina nanostructure and extract its graded refractive index profile by fitting the measured reflectance spectrum with the simulated one, a simulation based on the finite-difference time-domain (FDTD) method has been performed. The average reflectance measured at normal incidence for double-sided coated BK7 glass substrates is only 0.3% in the visible spectral region. Considering both sides, the average reflectance of the substrate decreased in the spectral range of 400–700 nm down to 0.4% at incidence angles of 45° by applying the boomerang-like alumina antireflection coatings. The optimized single layer boomerang-like alumina coating on the curved aspheric lens exhibited a low average reflectance of less than 0.14% and an average transmittance of above 99.3% at normal incidence. The presented process is a simple and cost-effective route towards broadband and omnidirectional antireflection coatings, which have promising potential to be applied on substrates having large scales with complex geometric shapes.

Tunable Omnidirectional Antireflection Coatings Inspired by Inclined Irregular Nanostructures on Transparent Blue-Tailed Forest Hawk Dragonfly Wings.

Randomly arranged inclined irregular nanostructure-covered blue-tailed forest hawk dragonfly wings are highly transparent for wide viewing angles. Inspired by the dragonfly wings, monolayer silica colloids are self-assembled on shape memory polymer-coated substrates and utilized as plasma etching masks to pattern disorderly arranged inclined irregular conical structures. The structures build gradual refractive index transitions at various angles of incidences, resulting in omnidirectional antireflection performance over the whole visible wavelength region. In comparison with a bare substrate, the optimized structure-covered substrate presents 10% higher optical transmission at 0° and even 41% higher optical transmission at an angle of incidence of 75°. Importantly, by manipulating the structural configuration of the shape memory polymer-based structures, the corresponding antireflection characteristics can be instantaneously and reversibly eliminated and recovered after drying out of common household liquids or applying contact pressures in ambient environments. The tunable omnidirectional antireflection coatings are prospective candidates for realizing optical modulation, which exhibits an enormous application value in smart windows, intelligent display screens, optical components, and novel optoelectronic devices.

Broadband omnidirectional antireflection coatings inspired by embroidered ball-like structures on leafhoppers

Leafhopper (Thaia rubiginosa) wings exhibit antireflection behaviors for wide viewing angles, resulting from leafhopper-generated brochosomal coatings on the wing surface. The embroidered ball-like brochosomes establish a gradual transition in the refractive index at air/wing interface, leading to the broadband omnidirectional antireflection performance. Inspired by the brochosomal coatings, this study develops a non-photolithography-based approach for assembling embroidered ball-like structure arrays exploiting a modified Langmuir-Blodgett technology. The average visible transmittance of a polymer film can be improved by ca. 8% at normal incidence, and improved by even ca. 24% as the incident angle reaches 75° by introducing the artificial brochosome arrays. The dependence of the structure shape on the antireflection capabilities is also systemically studied in this research.

Reversibly Erasable Broadband Omnidirectional Antireflection Coatings Inspired by Inclined Conical Structures on Blue-Tailed Forest Hawk Dragonfly Wings

Blue-tailed forest hawk dragonfly (Orthetrum triangulare) wings, covered with inclined conical structures, are studying for their high transparency and low reflectance for large viewing angles. However, limited by existing technologies, the exquisite inclined structures are not replicated easily or applied adequately. Here, we combine a shear-induced self-assembly approach and a colloidal lithography technology to create omnidirectional antireflection structures that inspired by dragonfly wings. Non-close-packed colloid crystals are spin-coated and serve as structural templates in a plasma etching procedure to pattern subwavelength inclined conical structures directly on shape memory polymer-coated substrates. The dependence of the antireflection functionality on the shape and inclination of conical structures is systematically investigated in this research. Compared with a featureless substrate, the structure-covered substrate can display a approximately 8% higher average transmittance in the visible wavelength range at normal incidence and even approximately 23% higher average transmittance as the incident angle increases to 75o. Moreover, the reconfigurable structures composed of shape memory polymers can be repeatedly deformed and recovered as a result of external stimuli at ambient conditions, and the corresponding broadband omnidirectional antireflection functionality is therefore reversibly erased and restored.

Dragonfly-Wing-Inspired Inclined Irregular Conical Structures for Broadband Omnidirectional Antireflection Coatings

Blue-tailed forest hawk dragonfly (Orthetrum triangulare) wings exhibit a high transparency at wide viewing angles. The broadband omnidirectional antireflection behaviors of dragonfly wings result ...

Optimization of broadband omnidirectional antireflection coatings for solar cells

Broadband and omnidirectional antireflection coating is generally an effective way to improve solar cell efficiency, because the destructive interference between the reflected and incident light can maximize the light transmission into the absorption layer. In this paper, we report the incident quantum efficiency ηin, not incident energy or power, as the evaluation function by the ant colony algorithm optimization method, which is a swarm-based optimization method. Also, SPCTRL2 is proposed to be incorporated for accurate optimization because the solar irradiance on a receiver plane is dependent on position, season, and time. Cities of Quito, Beijing and Moscow are selected for two- and three-layer antireflective coating optimization over λ = [300, 1100] nm and θ = [0°, 90°]. The ηin increases by 0.26%, 1.37% and 4.24% for the above 3 cities, respectively, compared with that calculated by other rigorous optimization algorithms methods, which is further verified by the effect of position and time dependent solar spectrum on the antireflective coating design.

Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition

We present a new type of nanoporous antireflection (AR) coating based on grass-like alumina with a graded refractive index profile. The grass-like alumina AR coating is fabricated using atomic layer deposition (ALD) of alumina and immersion in heated deionized water. Optical transmittance of 99.5% at 500 nm was achieved with average transmittance of 99.0% in the range of 350-800 nm at normal incidence for double-sided coated glass. Angular spectral transmittance (0-80°) of the double-sided AR coated glass was also measured in the range of 350-800 nm and found to have mean spectral transmittance of 94.0% at 60°, 85.0% at 70°, and 53.1% at 80° angles of incidence, respectively. The grass-like alumina AR coating is suitable for mass production with the presented technique: even hundreds of optical components can be coated in parallel. Furthermore, as an ALD-based technique, the coating can be deposited conformally on surfaces with extreme topography, unlike many spin-coating, physical vapor deposition or glancing angle deposition-based coatings used today.

Optical reflectionless potentials for broadband, omnidirectional antireflection.

Reflectionless potentials (RPs) represent a class of potentials that offer total transmission in the context of one dimensional scattering. An optical realization of RP can exhibit a truly broadband omni-directional antireflection. We report our experimental observations confirming the reflectionless behavior. Stratified media conforming to RP's are designed and fabricated using Al(2)O(3) and TiO(2) heterolayers. They showed < 0.5% reflection over the broad wavelength range of 350 nm to 2500 nm for angles of incidence 0 - 50 degrees. These RPs can be designed on substrates and are thus interesting for optical instrumentation as broadband, omni-directional antireflection coatings.

Design of broadband omnidirectional antireflection coatings using ant colony algorithm.

Optimization method which is based on the ant colony algorithm (ACA) is described to optimize antireflection (AR) coating system with broadband omnidirectional characteristics for silicon solar cells incorporated with the solar spectrum (AM1.5 radiation). It's the first time to use ACA method for optimizing the AR coating system. In this paper, for the wavelength range from 400 nm to 1100 nm, the optimized three-layer AR coating system could provide an average reflectance of 2.98% for incident angles from Raveθ+ to 80° and 6.56% for incident angles from 0° to 90°.

Broadband omnidirectional antireflection coatings for metal-backed solar cells optimized using simulated annealing algorithm incorporated with solar spectrum

Broadband omnidirectional antireflection (AR) coatings for solar cells optimized using simulated annealing (SA) algorithm incorporated with the solar (irradiance) spectrum at Earth's surface (AM1.57 radiation) are described. Material dispersions and reflections from the planar backside metal are considered in the rigorous electromagnetic calculations. Optimized AR coatings for bulk crystalline Si and thin-film CuIn(1-x)GaxSe(2) (CIGS) solar cells as two representative cases are presented and the effect of solar spectrum in the AR coating designs is investigated. In general, the angle-averaged reflectance of a solar-spectrum-incorporated AR design is shown to be smaller and more uniform in the spectral range with relatively stronger solar irradiance. By incorporating the transparent conductive and buffer layers as part of the AR coating in CIGS solar cells (2μm-thick CIGS layer), a single MgF(2) layer could provide an average reflectance of 8.46% for wavelengths ranging from 350 nm to 1200 nm and incident angles from 0° to 80°.

High-performance antireflection coatings utilizing nanoporous layers

Abstract

Porous Polymer Films with Gradient‐Refractive‐Index Structure for Broadband and Omnidirectional Antireflection Coatings

AbstractPorous polymer films that can be employed for broadband and omnidirectional antireflection coatings are successfully shown. These films form a gradient‐refractive‐index structure and are achieved by spin‐coating the solution of a polystyrene‐block‐poly(methyl methacrylate) (PS‐b‐PMMA)/PMMA blend onto an octadecyltrichlorosilane (OTS)‐modified glass substrate. Thus, a gradient distribution of PMMA domains in the vertical direction of the entire microphase‐separated film is obtained. After those PMMA domains are removed, a PS porous structure with an excellent gradient porosity ratio in the vertical direction of the film is formed. Glass substrates coated with such porous polymer film exhibit both broadband and omnidirectional antireflection properties because the refractive index increases gradually from the top to the bottom of the film. An excellent transmittance of &gt;97% for both visible and near‐infrared (NIR) light is achieved in these gradient‐refractive‐index structures. When the incident angle is increased, the total transmittance for three different incident angles is improved dramatically. Meanwhile, the film possesses a color reproduction character in the visible light range.

Solution-processed omnidirectional antireflection coatings on amorphous silicon solar cells

Solution-processed spherical surface textures are demonstrated on commercial amorphous silicon solar cells. The texture is formed with a monolayer of silica microspheres by convective coating, followed by a spin-on-glass film. It is found that the spherical texture reduces the reflectivity of the cells in the wavelength regime of 400–1200 nm, thus broad spectrum. It is also found that the spherical texture improves the efficiency of the cells at various incident angles from surface normal to at least 60°. The improvement in efficiency increases at larger incident angles to as high as 12% relative improvement, demonstrating its omnidirectionality. Current-voltage characteristics show that the efficiency improvement with coating is largely due to the increased short circuit current, while the open-circuit voltage remains the same. Therefore the efficiency improvement is attributed to more light coupled into the cells. This omnidirectional surface texture offers an attractive solution for antireflection in both polycrystalline silicon and thin-film solar cells.

Broad‐band and Omnidirectional Antireflection Coatings Based on Semiconductor Nanorods

A broad-band and omnidirectional antireflection coating consisting of semiconductor nanowires is fabricated. The reflection is reduced due to a graded refractive index, which is achieved by a certain nanorod-length distribution of cylindrical and conically shaped rods. An increased transmission and, accordingly, a reduced reflection are demonstrated for the visible and near-infrared parts of the spectrum.

Design of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials by genetic algorithm

Designs of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials are optimized using a genetic algorithm. Co-sputtered and low-refractive-index materials allow the fine-tuning of refractive index, which is required to achieve optimum anti-reflection characteristics. The algorithm minimizes reflection over a wide range of wavelengths and incident angles, and includes material dispersion. Designs of antireflection coatings for silicon-based image sensors and solar cells, as well as triple-junction GaInP/GaAs/Ge solar cells are presented, and are shown to have significant performance advantages over conventional coatings. Nano-porous low-refractive-index layers are found to comprise generally half of the layers in an optimized antireflection coating, which underscores the importance of nano-porous layers for high-performance broadband and omnidirectional antireflection coatings.

Microstructured surface design for omnidirectional antireflection coatings on solar cells

We recently demonstrated a new process for the formation of partially spherical structures as an omnidirectional antireflection coating (omni-AR). In this paper, we report the simulation results of the angular and spectral dependences of the total reflectivity on various microstructured surfaces based on the rigorous coupled-wave analysis. Close to zero reflection can be achieved in these microstructured surfaces over an extended spectral region for large ranges of light incident angles. The impact of feature size, density, shape, and refractive index has all been investigated. The experimental results agree reasonably well with the theoretical work. Such an omni-AR structure offers an attractive solution to current crystalline silicon solar cells, as well as future thin film, quantum dot, and organic solar cells.

Omni-Directional Anti-Reflection Coatings From Solutions

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A new approach for broad-band omnidirectional antireflection coatings

It is shown that broad-band antireflection coatings with extra large angular range can be designed based on the concept of reflectionless potentials. Numerical calculations for inhomogeneous films with or without substrate demonstrate the above capabilities for both TE and TM polarizations. The design possibilities are infinite and the underlying concept does not rely on standard use of quarter wave plates. Suitable inhomogeneous layers on both sides of a lossless thin dielectric film can thus render it invisible.

Toward perfect antireflection coatings. 2. Theory.

Recently we performed a numerical investigation of antireflection coatings that reduce significantly the reflection over a wide range of wavelengths and angles of incidence, and we proposed some experiments to demonstrate their feasibility. We provide a theoretical description of omnidirectional antireflection coatings that are effective over a wide range of wavelengths.