Antireflective Surfaces Research Articles

Erratum to “Analysis of optical and wetting properties of a biomimetic antireflective surface for practical application”

Erratum to “Analysis of optical and wetting properties of a biomimetic antireflective surface for practical application”

Bioinspired Microstructured Polymer Surfaces with Antireflective Properties.

Over the years, different approaches to obtaining antireflective surfaces have been explored, such as using index-matching, interference, or micro- and nanostructures. Structural super black colors are ubiquitous in nature, and biomimicry thus constitutes an interesting way to develop antireflective surfaces. Moth-eye nanostructures, for example, are well known and have been successfully replicated using micro- and nanofabrication. However, other animal species, such as birds of paradise and peacock spiders, have evolved to display larger structures with antireflective features. In peacock spiders, the antireflective properties of their super black patches arise from relatively simple microstructures with lens-like shapes organized in tightly packed hexagonal arrays, which makes them a good candidate for cheap mass replication techniques. In this paper, we present the fabrication and characterization of antireflective microarrays inspired by the peacock spider’s super black structures encountered in nature. Firstly, different microarrays 3D models are generated from a surface equation. Secondly, the arrays are fabricated in a polyacrylate resin by super-resolution 3D printing using two-photon polymerization. Thirdly, the resulting structures are inspected using a scanning electron microscope. Finally, the reflectance and transmittance of the printed structures are characterized at normal incidence with a dedicated optical setup. The bioinspired microlens arrays display excellent antireflective properties, with a measured reflectance as low as 0.042 ± 0.004% for normal incidence, a wavelength of 550 nm, and a collection angle of 14.5°. These values were obtained using a tightly-packed array of slightly pyramidal lenses with a radius of 5 µm and a height of 10 µm.

Toward high efficiency for long-term stable Cesium doped hybrid perovskite solar cells via effective light management strategy

Power conversion efficiency (PCE) and device stability are the two most important concerns for next generation perovskite solar cells (PSCs). Herein, we report an investigation for long-term stable perovskite Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 solar cells toward high efficiency via the light management strategy. To that end, we first experimentally determine complex optical constants of this kind of perovskite thin film by using variable angle spectroscopic ellipsometry measurements. By constructing the proper model, the calculated photocurrent density (Jsc) of 23.2 mA/cm2 of the planar PSC model is only slightly higher than the measured 23.0 mA/cm2 of our champion planar cell with excellent consistency, where the PCE is up to 20.5% with active area of 0.16 cm2, demonstrating a long-term stability capable of retaining 91% of initial PEC for 40 days. It is further found that when the front texture of nanocone array design is applied to the planar cell as a surface antireflection, the Jsc value of 24.8 mA/cm2 with the resulting PCE of 22.1% is projected from the present real planar PSC device with active area of 0.16 cm2.

Arrayed wide-field astronomical camera system for spectroscopic surveys on Extremely Large Telescopes: system architecture, proof-of-concept, and enabling technologies

The next generation Extremely Large Telescopes (ELTs) are promising a profound transformation of humanity’s understanding of the universe by opening our eyes to a myriad of previously unseen astronomical objects across cosmic space time. Some of the key observational contributions to this transformation will, once again, be made through large-scale ultradeep spectroscopic surveys. Such surveys will call for a wide-field system to expand field of view and correct atmospheric dispersion beyond what an uncorrected ELT is canonically capable of. Although the traditional monolithic form of field correctors served our needs very well on 2- to 8-m class telescopes, we recognize challenges around scaling this traditional design to the ELT level, and hence, as detailed here, present a fundamentally different architecture, called the arrayed wide-field astronomical corrector system or AWACS, for the ELTs of two-mirror construction. The AWACS accomplishes field expansion via an array of small units populated over a telescope’s focal surface, compensating for field aberrations and atmospheric dispersion locally but simultaneously. The AWACS units share one common electro-opto-mechanical design, permitting cost-effective high-volume part manufacturing. We detail the architectural features and proof-of-concept on-sky demonstration of the AWACS. In addition, we highlight our recent development results of randomly nano-textured antireflective (AR) surface structures in terms of an immediately viable, super-broadband, high-performance AR solution for not only the AWACS optics, but also broader ranges of electro-optical devices, particularly those subjected to harsh environments such as high-power laser, cryogenic, and space systems. With continuous advances in other relevant fields, the AWACS is uniquely positioned to enable, either by itself or by complementing traditional correctors, wide-field multi-object spectroscopic surveys in the ELT era and beyond.

Large-area surface wettability patterning of metal alloys via a maskless laser-assisted functionalization method

An innovative laser-assisted functionalization (LAF) method is presented in this work to fabricate a superhydrophobic-superhydrophilic patterned surface on a large-area metal alloy with a high process throughput. The LAF method comprises four steps, including two laser processing steps and two surface chemistry modification steps. Two strategies are demonstrated to fabricate extreme wettability patterns on aluminum alloy without using protective masking. Strategy I fabricates contrasting topography and chemistry where superhydrophobic regions are nanostructured with fluorocarbon chemistry, and superhydrophilic regions contain microchannels with nitrile chemistry. Conversely, strategy II fabricates similar microchannel topography for both wettability regions with contrasting chemistry. Surface topography patterning, from low-roughness, isotropic, random nanostructured texture to highly structured texture, is achieved by adjusting the laser parameters during LAF. LAF constitutes the first known laser-based surface engineering method that decouples extreme wettability from surface topography. In addition, anti-reflection surface property, ranging from moderately reduced reflectivity of about 50% to ultralow reflectivity of less than 10%, is achieved using this process. The processing mechanism, surface chemistry, and surface topography are analyzed to demonstrate the simplicity, robustness, and feasibility of the process.

Polymer-coated moth-eye hybrid structure for broadband antireflection in the terahertz region.

A hybrid antireflective structure (ARS) is proposed for enhancing the transmittance of terahertz (THz) waves. This hybrid ARS was made by attaching a polymer-based two-layer coating onto a moth-eye structure on a silicon (Si) substrate. The measured power reflectance of this hybrid ARS remained below 6% in the frequency range of 0.6-2.5 THz, corresponding to the simulated results. The total power reflectance from 0.1 to 2.5 THz was 20% that of the unprocessed Si surface. Besides exhibiting a broadband AR characteristic, this hybrid ARS inherited the cleanable flat surface from the coating structure, which also acts as a protective shield for the moth-eye structures. This high-transmittance, cleanable, flat antireflective surface can not only improve the performance of numerous THz components but also promote the applications of THz waves in daily life.

Zinc-hyperdoped silicon nanocrystalline layers prepared via nanosecond laser melting for broad light absorption

Current techniques for hyperdoping of Si wafer surface by femtosecond laser or ion implantation have encountered several problems for device fabrication, such as severe lattice defects, shallow doping depth, poor electrical activity of impurities, difficult collection of charge carriers, or weak compatibility with present complementary metal–oxidesemiconductor processes. To provide a new way to solve these problems, in this study, Zn-hyperdoped Si layers were prepared on polycrystalline Si substrates by vacuum evaporation and nanosecond (ns) laser melting under different conditions of deposition and laser treatment. As a result, the hyperdoped Si layers of up to micron-level thickness with a broad light absorption (above 90% from the ultraviolet to the visible range and up to 54% for the near-infrared spectrum) and good electrical transport properties (electron mobility of up to 88.5 cm2⋅V−1s−1 and sheet resistance of ~ 100 Ω⋅square−1) were obtained. These hyperdoped Si layers can be flexibly superimposed on the existing Si optoelectronic devices: Such tandem structure avoids the damage of substrate and provides both the layers and substrate properties. In addition, the variation in the microstructural characteristics of the Zn-hyperdoped Si layers consisting of surface and cross-section structures, Zn impurity distribution and concentration, and crystallinity were carefully studied to clarify the relationships between their preparation conditions, microstructures, and physical properties. The results show that a matching between the thickness of Si-Zn deposited layer and the ns-laser irradiation fluence is crucial to achieve a full melting process and to obtain excellent Zn-hyperdoped Si layers. Their near-infrared absorption and electrical transport properties are determined by the combined effects of surface antireflection, Zn-hyperdoping, and nanocrystallization.

Analysis of optical and wetting properties of a biomimetic anti-reflective surface for practical application

Analysis of optical and wetting properties of a biomimetic anti-reflective surface for practical application

Condensation Behavior of Hierarchical Nano/Microstructured Surfaces Inspired by Euphorbia myrsinites.

In nature, many extant species exhibit functionalized surface structures during evolution. In particular, wettability affects the functionalization of the surface, and nano/microstructures have been found to enable functions, such as droplet jumping, thereby making self-cleaning, antifog, antibacterial, and antireflection surfaces. Important efforts are underway to understand the surface structure of plant leaves and establish rational design tools for the development of new engineering materials. In this study, we focused on the hierarchical nano/microstructure of the leaves of Euphorbia myrsinites (hereinafter, E. myrsinites), which has a hierarchical shape with microsized papillae, covered with nanosized protruding wax, and observed the condensation behavior on the leaf surface. Si is vertically etched via reactive ion etching (RIE) to artificially mimic the hierarchical nano/microstructures on the leaves of E. myrsinites. We made four types of artificial hierarchical structures, with micropillars having pillar diameters of 5.6 and 16 μm (pillar spacing of 20 and 40 μm, respectively) and heights of 6.5 and 19.5 μm, and nanopillars formed on the surface. The optical observation with a microscope revealed a very high density of condensed droplets on the artificial surface and a stable jumping behavior of droplets of 10 μm or more. Furthermore, in the samples with a micropillar diameter of 5.6 μm and a micropillar height of 19.5 μm, the droplets that had jumped and fallen thereupon bounced off, thereby preventing reattachment. As a result, no droplets of 35 μm or more could exist even after 10 min. In addition, it was clear that a small underlying droplet of less than 10 μm was generated at the bottom of the relatively large secondary droplet existing on the large micropillar of 16 μm, and a frequent coalescence of the droplets occurred. This study revealed the phenomenon of condensation on the surface of plants as well as made it possible to improve the heat exchange process by significantly promoting the heat transfer of condensation using artificial surfaces.

Thickness-controllable synthesis of MOF-derived Ni@N-doped carbon hexagonal nanoflakes with dielectric-magnetic synergy toward wideband electromagnetic wave absorption

Heteroatom nitrogen doping and morphology control have been considered as the effective approaches for improving electromagnetic wave (EMW) absorption ability of carbon-based absorbers. Herein, by controlling the adding dosage of pyridine, thickness controllable MOF-derived Ni@N-doped carbon (Ni@NC) hexagonal nanoflakes presented increased content of doped N atoms and tunable morphology from nanorods-, nanoparticles- to nanoflakes-like. Moreover, it made it possible to adjust the electromagnetic parameters of Ni@NC. The 3D conductive network of carbon framework and magnetic Ni nanoparticles could be obtained from the converted MOF precursor. The controllable dimension and morphology of N-doped carbon skeleton endowed more electronic transportation paths, better anti-reflection surfaces, higher conduction loss and polarization relaxation, while the evenly distributed Ni nanoparticles provided considerable multiple resonances and eddy current. All of these characteristics contributed the electromagnetic wave absorption (EMA) with improved dielectric loss, magnetic loss and impedance matching. The analysis indicated that the sample with 30 wt% of Ni@NC-nf showed the widest bandwidth, which was 6.21 GHz ranging from 11.79 to 18.00 GHz with a thickness of 2.3 mm, while the corresponding radar band was the whole Ku band (12.0–18.0 GHz). Our research provided a valuable approach for fabricating high-efficiency microwave absorption materials by combining the control of morphology and N-doped effect on the absorbers.

Strongly antireflective nano-textured Ge surface by ion-beam induced self-organization

Designing materials surfaces for reducing the reflection of light is imperative towards enhancing the performance of optical and optoelectronic devices. We show that ion implantation under specific experimental conditions is a single step and controllable process to minimize reflection over a large wavelength range by growth of surface nanostructure in a self-organized fashion. For instance, we show a strong reduction in optical reflectance in the ultraviolet–visible-infrared wavelength range (300–3000 nm) for Ge with the evolution of nanoscale self-organized patterns under energetic ion bombardment at oblique angles. As pattern coarsening takes place with increasing ion fluence (for subtle incident angles), the optical reflectance reduces to a significant extent. The evolution of saw-tooth-like structures on Ge surface, at an oblique incidence of 60° to the fluence of 3 × 1018 ions cm−2, leads to a huge reduction in optical reflectance (below 4% over a wide spectral range) and makes it behave effectively like a black-Ge specimen. The huge reduction in the optical reflectance is found to be related to the dimension of the surface features evolved under ion implantation at room temperature. The observed surface morphology gives rise to an effective gradient in refractive index from the top toward the bottom of the patterned surface. Thus, the reduction in refractive index is calculated and correlated with the respective surface morphology. We have also studied change in the wetting property of ion-induced patterned surfaces and correlated it with the reduction in surface free energy due to the presence of oxygen.

Femtosecond laser induced simultaneous functional nanomaterial synthesis, in situ deposition and hierarchical LIPSS nanostructuring for tunable antireflectance and iridescence applications

Femtosecond laser induced periodic surface structures (LIPSSs) are excellent biomimetic iridescent antireflective interfaces. In this work, we demonstrate the feasibility to develop tunable iridescent antireflective surfaces via simultaneous synthesis of functional metal-oxide nanomaterials, in situ deposition and hierarchical LIPSSs nanostructuring by means of femtosecond laser ablation (fs-LA) of tungsten (W) and molybdenum (Mo) in air. Adjusting the scanning interval from 1 μm to 20 μm allows the modulation of particle deposition rates on LIPSSs. Diminishing the scan interval enables a higher particle deposition rate, which facilitates the development of better UV-to-MIR ultrabroadband antireflective surfaces with a less pronounced iridescence. Through comparing the reflectance of hierarchical LIPSSs with different densities of loosely/tightly deposited particles, it is found that the deposited WOx and MoOx particle aggregates have high UV-to-MIR ultrabroadband absorbance, especially extraordinary in the MIR range. Loosely deposited particles which self-assembly into macroporous structures outperform tightly deposited particles for ultrabroadband antireflective applications. The presence of loosely deposited MoOx and WOx particle absorbers can cause up to 80 % and 60 % enhancement of antireflectance performances as compared to the tightly particle deposited LIPSSs samples. One stone of “fs-LA technique” with three birds of (particle generation, in situ deposition and LIPSS hierarchical nanostructuring) presented in this work opens up new opportunities to tune the reflectance and iridescence of metallic surfaces.

High‐Efficiency Optical Filters Based on Nanophotonics

Nanophotonic structures are artificial optical substances composed of structures smaller than incident light wavelengths. For example, it can artificially create a unique refractive index that is not found in substances in the natural world. That is, by designing the shape of the nanophotonic structure, an optical filter device that exhibits an optical response on demand can be realized. Also, we can realize applied devices with useful functions that cannot be realized by substances existing in nature, such as antireflective surfaces with 0% reflectance, wavelength‐selective filters with 100% reflectance at specific wavelengths, and filters that shift the resonant wavelength in response to changes in the ambient refractive index. There can be applications to high‐brightness light‐emitting elements, high‐efficiency power generation elements, color filters for full‐color high‐resolution cameras, miniature spectroscopic systems, high‐efficiency biosensors, and so on. In the future, the development of further applications with this technology is widely expected in the fields of information, energy and global environment, and medical care and welfare. This paper describes high‐efficiency optical filters based on nanophotonic technologies. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Nanoimprintable super antireflective matte black surface achieved by hierarchical micro and nano architecture

We report an effective antireflective surface structure fabricated by a sequential process comprising colloidal lithography, maskless plasma etching, and inverted nanoimprinting replication. The hierarchical inverse micro–nano structure is composed of randomly positioned microholes of 3–5 μm in diameter and numerous nanoprotrusions of 60–80 nm diameter located at the bottom surface of the microholes. The inverse micro–nano structure behaves as a high-performance light absorber, exhibiting outstanding optical performances of 0.78% for hemispherical reflectance, and 0.011% for specular reflectance at the incident angle 5° off normal, both on average in the visible range (380–780 nm). Additionally, the optical behavior of the inverse micro–nano structure is numerically investigated with the electric field strength in both frequency and time domains by the finite-element method for Maxwell's equation. As the inverse micro–nano structure exhibits significantly low reflectance and black appearance, it can be applied to antiglare/antireflective surfaces, suitable for the reduction of stray light existing inside of a housing of optical instruments.

Direct Femtosecond Laser Fabrication of Chemically Functionalized Ultra-Black Textures on Silicon for Sensing Applications.

Here, we present the single-step laser-assisted fabrication of anti-reflective hierarchical surface textures on silicon locally functionalized with a photoluminescent (PL) molecular nanolayer. Using femtosecond-laser ablation of commercial crystalline Si wafers placed under a layer of a solution containing rhodamine 6G (R6G) a triethoxysilyl derivative, we fabricated ordered arrays of microconical protrusions with self-organized nanoscale surface morphology. At the same time, the laser-induced temperature increase facilitated surface activation and local binding of the R6G derivative to the as-fabricated nanotextured surface. The produced dual-scale surface textures showed remarkable broadband (visible to near-IR) light-absorbing properties with an averaged reflectivity of around 1%, and the capping molecular nanolayer demonstrated a strongly enhanced PL yield. By performing a pH sensing test using the produced nanotextured substrate, we confirmed the retention of sensory properties of the molecules attached to the surface and validated the potential applicability of the high-performing liquid-assisted laser processing as a key technology for the development of innovative multifunctional sensing devices in which the textured substrate (e.g., ultra-black semiconductor) plays a dual role as a support and PL signal amplifier.

Enhancing Performance of GaAs Photodiodes via Monolithic Integration of Self‐Formed Graphene Quantum Dots and Antireflection Surface Texturing

III–V semiconductor‐based photodiodes with graphene incorporated have been studied in recent years due to the attractive optoelectronic properties of graphene, including optical transparency and enhanced photoresponsivity. The photoresponsivity can be further improved by converting the semiconductor surface into a 3D antireflection (AR) structure. However, difficulties in transferring graphene on top of structured surfaces degrade the interfacial quality and limit their photoresponsivity. Herein, a high‐performance GaAs photodiode structure with self‐embedded graphene quantum dot (GQD) and simultaneously formed periodic AR 3D surface texturing is reported, all produced by a one‐step wet etching process in a solution of hydrogen fluoride (HF) and potassium permanganate (KMnO4) using graphene as a transparent mask. Compared with the planar counterpart without graphene, the photodiodes demonstrated here show an enhancement of photocurrent by 22 times, photoresponsivity by 25 times, and normalized photocurrent to dark current ratio by approximately two orders of magnitude. The improved photoresponsivity of 9.31 mA W−1 is attributed to the increased absorption from AR texturing and the enhanced heterointerface carrier transfer from GQDs to GaAs. This simple, clean yet effective method enables the monolithic incorporation of graphene and graphene‐derived materials on 3D semiconductor structures for applications across a wide range of wavelengths.

Optical properties of stepped-cone silicon nanostructures fabricated by nanosphere mask and RIE method

ABSTRACT Highly ordered silicon nanostructures were fabricated using the nanosphere mask and reactive ion etching method. A self-assembled SiO2 nanospheres monolayer obtained by spin-coating method was used as an etching mask for pattern transfer onto the Si substrate. The shape and height of these nanostructures were precisely controlled by the etching time. The optical properties of as-prepared nanostructure were investigated by experiments and simulation. Experimental results showed that the average reflectance in wavelength range of 400–1000 nm decreased from 33.6% to 4.6%. Simulation results were in good agreement with experimental results. Accordingly, this novel method is therefore considered to be an easy approach to fabricate different nanostructures for broadband antireflective surfaces for solar cells.

FABRICATING AND TESTING BLACK SILICON SOLAR CELLS

Black Si (b-Si) is a needle-like surface of crystalline Si. The optical radiation incident on it is almost completely absorbed due to multi-reflections from the side surfaces of the needles, which makes b-Si attractive for solar cell applications as antireflection surfaces. Currently, b-Si solar cells are fabricated based on the sequences used for traditional (pyramidal textured) solar cells, namely first, an antireflection surface is formed, and then high-temperature treatments such as oxidation and diffusion are carried out. However, in this case, due to large surface area, the diffusion of dopants within b-Si is much more efficient than in a planar, without texture, wafer. It leads to high doping concentrations and non-uniformity in the dopant. In addition, during the diffusion process, a surface layer of glass is grown, and subsequently removed by wet etching, which leads to noticeable changes in the morphology and structure of b-Si. As a result, after thermal and chemical treatments, the optical characteristics of the b-Si are retrogressed. A new fabrication sequence of b-Si solar cells has been proposed and tested, according to which, the b-Si with preferable height of needles is formed after the main chemical and thermal treatments of the technological process. It has been shown that all performance parameters (open-circuit voltage, short current, fill factor and conversion efficiency) of the new b-Si solar cells are higher than those of the conventional solar cells. Joint use of b-Si and pyramidal textured surfaces can get a more tangible effect.

Formation of porous silicon on N-type Si (1 0 0) and Si (1 1 1) substrates by electrochemical anodization method

Abstract Porous silicon (PSi) was formed on n-type Si (1 0 0) and Si (1 1 1) substrates via the electrochemical anodization method. The samples were anodized in HF: ethanol solution (1:1) and flowed with a varied current density of 10, 20, 30, 40, and 50 mA/cm2 for 30 min. The reflectance, the pores size, and the pores depth of PSi were characterized by UV–Vis spectroscopy, SEM instrument, and Dektak Profilometer, respectively. The higher current density results in lower reflectance of PSi on both Si (1 0 0) and Si (1 1 1) substrates, while SEM images and Dektak tests confirmed that the pore depth and the pore size of PSi increased. It is concluded that PSi could be applied to solar cell devices as an anti-reflective surface. PSi reflectance on Si (1 0 0) is lower than on the Si (1 1 1) substrate. It suggested that in DSSC, PSi on Si (1 0 0) could be considered to have better anti-reflective properties.

激光加工减反射表面研究进展

激光加工减反射表面研究进展