Antireflective Structures Research Articles

A hierarchical SiPN/CN/MoSx photocathode with low internal resistance and strong light-absorption for solar hydrogen production

The Si/MoSx photocathode is usually fabricated by electrochemical deposition, which high interfacial resistance is usually solved by building a buried junction or inserting a metal layer between Si and MoSx. Both methods, however, involve toxic gas sources or harsh conditions of high pressure. Here, a green and mild route has been designed to insert the N-doped carbon (CN) layer between Si and MoSx by in situ polymerization and post-annealing, which improves the transfer of interfacial carriers and avoids unnecessary absorption of incident light by itself. Meanwhile, combining two-step wet etching and photo-assisted electro-deposition, the SiPN/CN/MoSx photocathode with antireflective structure was obtained.The SiPN/CN/MoSx hybrid photocathode shows excellent performance (0.23 V onset potential and 10 mA·cm−2 photocurrent at 0 V vs RHE in 0.5 MH2SO4 under the condition of simulated sunlight). The design of SiPN/CN/MoSx photocathode will provide a new idea for PEC water splitting.

Sub-wavelength microstructures on lithium triborate surface with high transmittance and laser-induced damage threshold at 1064 nm

This paper presents a sub-wavelength moth-eye-like antireflective structure on lithium triborate (LiB3O5, LBO) surface that can effectively promote the transmittance and maintain high laser-induced damage threshold (LIDT). Fabricated by holographic lithography and reactive ion-beam etching techniques, sub-wavelength periodic arrays can be obtained on the crystal surface. The surface morphology and transmittance were measured with atomic force microscope and spectrophotometer respectively. Single-sided transmittance of the LBO with moth-eye-like structure was confirmed to exceed 98% in the wavelength range of 1028–1300 nm, while is 98.73% at 1064 nm. For 1064 nm laser with the pulse width of 12 ns, the measured 1-on-1 LIDT is 42.57 J/cm2, which is about 2.28 times that of LBO compared with traditional antireflection (AR) coatings.

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.

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.

Fabrication of an Anti-Reflective Microstructure on ZnS by Femtosecond Laser Bessel Beams.

As an important mid-infrared to far-infrared optical window, ZnS is extremely important to improve spectral transmission performance, especially in the military field. However, on account of the Fresnel reflection at the interface between the air and the high-strength substrate, surface optical loss occurs in the ZnS optical window. In this study, the concave antireflective sub-wavelength structures (ASS) on ZnS have been experimentally investigated to obtain high transmittance in the far-infrared spectral range from 6 μm to 10 μm. We proposed a simple method to fabricate microhole array ASS by femtosecond Bessel beam, which further increased the depth of the microholes and suppressed the thermal effects effectively, including the crack and recast layer of the microhole. The influence of different Gaussian and Bessel beam parameters on the microhole morphology were explored, and three ASS structures with different periods were prepared by the optimized Bessel parameters. Ultimately, the average transmittance of the sample with the ASS microhole array period of 2.6 μm increased by 4.1% in the 6 μm to 10 μm waveband, and the transmittance was increased by 5.7% at wavelength of 7.2 μm.

A universal robust bottom-up approach to engineer Greta-oto-inspired anti-reflective structure

The Greta oto butterfly has transparent wings with extraordinary omnidirectional anti-reflection behavior, owing to unusual nanostructures with random height and pitch/space distribution on its wing surface. The beauty and efficacy of such nanostructures are proven designs but difficult to reproduce en masse. Here, we establish a low-cost bottom-up approach by simply stacking monolayer-aligned agitation-assisted deposition of silver nanowire (AgNW) meshes followed by deposition of various overcoats. Such AgNW meshes provide the template to grow nanostructures, imitating those on the Greta oto’s wings that consist of randomly situated nanocones with controllable mean pitches and heights. The resulting nanostructure enhances anti-reflections in both SiO2/AgNW and VO2/AgNW systems, which showed reduced omnidirectional reflectance up to 33% and 70%, respectively. Furthermore, the VO2/AgNW system exhibits enhanced luminous and infrared transmittance without sacrificing solar modulation. Our robust bottom-up approach provides a simpler alternate non-lithographic way to economically produce controlled biomimetic anti-reflective nanostructured coatings.

The broadband and omnidirectional antireflective performance of perovskite solar cells with curved nanostructures

Various shaped nanostructures have been continually used in the antireflection of organic-inorganic hybrid perovskite solar cells. A full understanding of the shape-dependent light trapping behavior is the basis for finding the excellent broadband and omnidirectional photovoltaic performance. Hence, three-dimensional models of silica sphere, hemisphere, moth-eye and cone nanostructured perovskite solar cells were performed at omnidirectional angle in this work. We proposed electric field intensity per volume as a key parameter, verifying the transition of light trapping modes of several curved nanostructures from normal incidence to oblique incidence. As the incident angle increases, the antireflection effect of four curved structures becomes increasingly crucial, and the differences among them are also greater. Consequently, the moth-eye nanostructured perovskite solar cell has achieved the best photovoltaic performance considering omnidirectional incidence. The short-circuit current density is increased by 8.4% at normal incidence and increased by 36.4% at 60° incidence compared to the planar reference. Our work offers the theoretical analysis for omnidirectional antireflection structure optimization, and provides guidance for the experimental research and production of potential antireflection structures of solar cell.

Reversible embroidered ball-like antireflective structure arrays inspired by leafhopper wings

Highly transparent leafhopper (Thaia rubiginosa) wings are self-decorated with embroidered ball-shaped proteinaceous brochosmoes as distinct anti-predator defenses. The non-sticky brochosomal coating serves as antireflective structures for camouflage in vegetated environments. Inspired by the leafhopper wings, this study reports a new type of reversible antireflection coating enabled by integrating self-assembly methodologies using a shape memory polymer. The resulting embroidered ball-like structure array establishes a refractive index transition on surface, and thereby behaves omnidirectional antireflective characteristics in a broadband visible light region. Interestingly, the highly transparent appearance can be instantly erased and recovered by submerging in common liquids, such as water and ethanol, or by applying contact pressures at ambient conditions. Furthermore, the reversibility and structure-shape effect on the antireflective characteristics are systematically evaluated in this study.

Fabrication of antireflective hydrophobic periodic micro-hole structures on ZnS surface by femtosecond laser direct writing

ZnS has a promising wide range of applications in optical and optoelectronic areas. But the Fresnel reflection on ZnS surface reduces the transmittance of light and causes an obstacle in its applications. Thus, it is crucial to minimize the Fresnel reflection to promote the antireflection property. This paper demonstrates an effective way to fabricate antireflective periodic micro-hole structures on ZnS surface by femtosecond laser direct writing. The effects of laser power, scanning speed on the morphology of the holes are investigated. The fabricated structures exhibit an excellent antireflection property, and achieve more than 30% decrease in reflectance in the near-ultraviolet spectral wavelength range and near-infrared spectral region (200-800 nm), reducing 35% in 280 nm, especially. Besides, the prepared surface demonstrates effective performance concerning hydrophobicity with a contact angle of 107.4°, and the contact angle of unprocessed ZnS is 96.2°. These results show that the femtosecond laser can induce the antireflection and hydrophobic properties on the surface of ZnS, which has a potential application in outdoor optical and optoelectronic devices.

Fiber-end antireflection method for ultra-weak fiber Bragg grating sensing systems

A novel fiber-end antireflection method for reducing the reflection noise in ultra-weak fiber Bragg grating (FBG) sensing systems is proposed in this study. The principle of fiber-end antireflection is analyzed and simulated theoretically, and then an antireflection structure is designed and fabricated. In this method, the fiber end is cut into an inclined surface and covered directly with an ellipsoid photosensitive ultraviolet (UV) glue drop, whose refractive index is similar to that of the fiber core. Under the online monitoring of the fiber-end reflection with the ultra-weak FBG interrogator, the refractive index of the glue drop is adjusted by controlling the UV curing time to achieve optimal reflection suppression. Results show that this method can reduce fiber-end reflection to less than −60 dB and effectively eliminate the influence of fiber-end reflection noise on the last sensor. The proposed method thus provides an excellent solution for the fiber-end processing of ultra-weak FBG sensing systems.

Biomimetic Ultra‐Black Sponge Derived from Loofah and Co‐MOF for Long‐Term Solar‐Powered Vapor Generation and Desalination

Designing ultra‐black materials with strong and broadband light absorption and efficient photothermal conversion properties is of great importance in many fields, including camouflage, catalysis, thermal therapy, and solar energy conversion. The West African Gaboon viper (Bitis rhinoceros), a master of camouflage, features black spots on its dorsal scales. The crest‐ridge structure in its black scales has been proven to be a typical antireflection structure. Herein, an artificial biomimetic ultra‐black sponge (BUBS) that imitates the crest‐ridge structure in the black scales of B. rhinoceros is successfully fabricated through growth of vertically aligned carbon nanosheet arrays derived from cobalt‐based metal–organic framework (Co‐MOF) on a loofah sponge. The vertically aligned carbon nanosheet arrays result in enhanced broadband light absorption (up to 97.08% on average) and photothermal conversion (equilibrium temperature of 80.7 °C under 1 sun illumination) of BUBS. Benefiting from its efficient light absorption and photothermal conversion, the BUBS is incorporated into solar‐powered vapor generation and desalination devices. The BUBS shows a higher water evaporation rate and efficiency of 1.93 kg m−2 h−1 and 98.5%, respectively, compared with other state‐of‐the‐art devices under 1 sun illumination. Furthermore, the BUBS also shows continuous and high water evaporation rates and excellent salt‐rejection in various brines, ensuring its potential application in solar‐powered desalination.

Improvement in THz light extraction efficiencies with antireflection subwavelength gratings on a silicon prism

An antireflection (AR) structure that incorporates a subwavelength grating (SWG) is a promising candidate for suppressing the Fresnel reflection of a silicon prism used as a component of an injection-seeded THz wave parametric generator (is-TPG) to improve the efficiency in extracting THz waves. Here, a two-dimensional binary AR-SWG with a 20 μm period is designed and then numerically and experimentally realized. The measured transmittance of the AR-SWG is discovered to be greater than 80% from 0.5 to 4.0 THz and to exceed 90% from 1.25 to 2.83 THz. Moreover, the AR-SWG is tightly attached to the Si prism of the is-TPG by direct-bonding technology. Compared to the is-TPG without AR-SWGs, the output increases by 1.3–1.5 times from 1.09 to 2.52 THz. The improvement in THz light extraction efficiency is thus successfully demonstrated.

Antireflective and Superhydrophilic Structure on Graphite Written by Femtosecond Laser.

Antireflection and superhydrophilicity performance are desirable for improving the properties of electronic devices. Here, we experimentally provide a strategy of femtosecond laser preparation to create micro-nanostructures on the graphite surface in an air environment. The modified graphite surface is covered with abundant micro-nano structures, and its average reflectance is measured to be 2.7% in the ultraviolet, visible and near-infrared regions (250 to 2250 nm). The wettability transformation of the surface from hydrophilicity to superhydrophilicity is realized. Besides, graphene oxide (GO) and graphene are proved to be formed on the sample surface. This micro-nanostructuring method, which demonstrates features of high efficiency, high controllability, and hazardous substances zero discharge, exhibits the application for functional surface.

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.

Effect of heavy ions irradiation on LiTaO3 crystal

In this work, congruent LiTaO3 crystals are irradiated by heavy ions Fe, Xe, and Bi respectively at fluence range of 1 × 1012 ~ 2 × 1015 cm−2. The effects of heavy ions irradiation are studied by using optical transmission spectroscopy, Rutherford backscattering, optical microscopy, X-ray diffraction.Heavy ion irradiation presents a unique effect in transmission spectra which is quite opposite to the previous reported results of light ions irradiation. The latter could often cause a significant optical absorption in visible light region. The former, however, not only induce no optical absorption, but can enhance transmittance over a broad spectral range (340–2500 nm). It is found that the discrepancy may originate from different damage process during ions irradiation. Light ion irradiation can create numerous of simple defects (like oxygen vacancies) serving as color centers which can cause an optical absorption. However, direct amorphization dominates the damage process induced by heavy ions, during which it hardly produces simple defects. Moreover, due to direct heavy ions bombardment, special textures are formed on the surfaces of the samples which serve as antireflection structures and enhance optical transmissions.

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.

3D InGaN nanowire arrays on oblique pyramid-textured Si (311) for light trapping and solar water splitting enhancement

We study Ⅲ-nitride nanowire (NW) anti-reflection structures employed as photoelectrocatalyst for solar water splitting. We found that 1D vertical InGaN NW arrays tilted by 73° exhibit maximized photocurrent. Therefore, we grow 3D InGaN NW arrays on the facets of oblique pyramid-textured Si (311) (OPSi/InGaN) by plasma-assisted molecular beam epitaxy, exhibiting facets tilted by 73°. In addition, using finite difference time domain simulations we find the crucial impact of the asymmetry of the oblique pyramid InGaN NW arrays on the light trapping. Compared with InGaN NWs grown on a planar Si substrate (Si/InGaN), the OPSi/InGaN photoanode exhibits ~500% enhancement of the photocurrent due to various light trapping effects attributed to: 1D vertical NW arrays, their tilt, 3D arrangement and asymmetry. Decorated with NiOOH co-catalyst, the OPSi/InGaN photoanode exhibits photocurrent densities in the mA cm−2 range. The present research provides a rational design for the fabrication of nanostructured photoelectrocatalyst with enhanced light absorption.

Optimization of Antireflective Layers of Silicon Solar Cells: Comparative Studies of the Efficiency Between Single and Double Layer at the Reference Wavelength

The deposition of an antireflection coating (ARC) on the front side of the solar cells allows a significant reduction of the losses by reflection. It thus allows an increase in the efficiency of the cells. Various materials are used as an antireflection layer. For our studies, we focused on the deposition of some materials as an antireflection layer on the solar cell such as SiO₂, Si₃N₄, TiO₂, Al₂O₃, MgF₂, and studied the efficiency of the latter. A theoretical study of antireflection layers has shown that a single antireflection layer does not have as low a reflectivity as a double antireflection layer over a large wavelength range. Thus, our interest was focused on double and multiple antireflection layers. The influence of parameters such as the thickness of the layer (s) as well as the associated refractive indexes on the optical properties of the antireflective structure has been studied. It was found that there are optimal thicknesses and refractive indices for which the reflectivity of the antireflective system is almost zero over a wider or shorter range of wavelengths. The same phenomena are noted in the study of the external quantum efficiency of the solar cell with these materials.

All solution-processed silver nanowires composite silica nanospheres antireflection structure with synergetic optoelectronic performance

The silver nanowires/SNSs AR composite TCFs have demonstrated the synergetic effect on optoelectronic performance via a facile solution method, reaching sheet resistance of 49.43 Ω sq−1 dropped by 8.66% and transmittance of 99.84% increased by 6.94%.

Thermo-mechanical dynamics of nanoimprinting anti-reflective structures onto small-core mid-IR chalcogenide fibers [Invited

Thermal nanoimprinting is a fast and versatile method for transferring the anti-reflective properties of subwavelength nanostructures onto the surface of highly reflective substrates, such as chalcogenide glass optical fiber end faces. In this paper, the technique is explored experimentally on a range of different types of commercial and custom-drawn optical fibers to evaluate the influence of geometric design, core/cladding material, and thermo-mechanical properties. Up to 32.4% increased transmission and 88.3% total transmission are demonstrated in the 2–4.3 µm band using a mid-infrared (IR) supercontinuum laser.