Antireflective Coatings: Conventional Stacking Layers and Ultrathin Plasmonic Metasurfaces, A Mini-Review

NIR-ViS-UV broadband absorption in ultrathin electrochemically-grown, graded index nanoporous platinum films

Nanoporous platinum broadband absorber has attracted interest in thermosensorics and IR photodetection due to its unique properties. In this work we report the physical mechanism underlying broadband absorption in electrochemically-grown, nanoporous Pt films by analyzing NIR-ViS-UV spectral ellipsometry data of nanoporous Pt films in dependence on the Pt film thickness (27, 35, 38, 48 nm). For the two thinner films a single layer model with a graded optical index Pt surface layer was used. For the two thicker films a two-layer optical model with a constant optical index Pt substrate layer and a graded optical index Pt surface layer was used. The graded optical index of the Pt surface layer reduces surface reflectivity and the constant optical index Pt substrate layer supports multiple reflections in the Pt film. Finally, we relate the thickness dependent optical index with the nanostructure of the nanoporous Pt film, which can be controlled in the electrochemical growth process. We observed that while in the transverse plane the multilayer exhibits graded refractive index, in the top horizontal planes the multilayer assembly exhibits discontinuous refractive index values due to the distribution of platinum crystal islands in air, which allows a metamaterial behavior of the whole system.

Synthesis of Mesoporous Silica Sol with Low Refractive Properties for Increasing Transmittance

Currently, coating with anti-reflective materials is an attractive approach to improve the quality of screen-based displays. In this study, mesoporous silica particles were systematically synthesized as a function of surfactant (i.e., CTAC-cetyltrimethylammonium chloride) concentration to serve as main coating fillers possessing low refractive indices. Precisely changing the amount of the CTAC surfactant, silica sol with an average diameter of 50 nm exhibits distinctively different specific surface areas, pore size, and pore volume. Prior to the preparation of final coating solutions containing these silica particle fillers, the percentage of solid content was optimized on a glass slide. The use of 50 wt% solid content exhibited the highest transmittance of light. Among various content levels of silica sol, the use of 3.5 wt% of silica particles in the solid content displayed the highest transmittance (i.e., best anti-reflectiveness). Under the almost identical coating layers prepared with the fixed amount of silica particles possessing different surface areas, pore size, and pore volume, it appears that the largest pore volume played the most important role in improving the anti-reflective properties. Experimentally understanding the key feature of low-refractive filler materials under the optimized conditions could provide a clear view to develop highly effective anti-reflective materials for various display applications.

Enhanced Durability and Antireflective Performance of Ag-Based Transparent Conductors Achieved via Controlled N-Doping

Ag-based transparent conductors (TCs) are often proposed as an alternative to ITO coatings. However, while their performance has been widely demonstrated, their environmental durability is frequently overlooked or addressed with the use of highly specific encapsulating layers. In this work, the durability and antireflective performance of Ag-based TCs are simultaneously enhanced. To do so, a transfer matrix modeling approach is used to determine the general requirements for high performance antireflective properties as a function of Ag thickness and dielectric refractive indices, offering more widely applicable insight into stack optimization. Coating durability is investigated as a function of the Ag microstructure, which is modified by altering the N2 concentration used for doping of the Ag layer and the selection of the seed layer. Increasing N2 concentration during Ag deposition was found to decrease grain size and durability of Ag coatings deposited on Si3N4 whereas all coatings on ZnO(Al) showed higher stability. Significantly higher durability is found when specifically combining intermediate N2 concentrations in the sputtering gas mixture (Ag(N):5%, compared to 0% and 50%) and a ZnO(Al) seed layer, and a mechanism accounting for this increased durability is proposed. The addition of NiCrNx protective coatings increases the system durability without altering these trends. These findings are combined to fabricate a highly performant Ag-based TC (TV = 89.2%, RVFS = 0.23%, 21.4 Ω), which shows minimal property changes following corrosion testing by immersion in a heated and highly concentrated aqueous NaCl solution (200 g/L, 50 °C).

Mechanically-Durable Antireflective Subwavelength Nanoholes on Glass Surfaces Using Lithography-Free Fabrication

Traditional multilayer antireflection (AR) surfaces are of significant importance for numerous applications, such as laser optics, camera lenses, and eyeglasses. Recently, technological advances in the fabrication of biomimetic AR surfaces capable of delivering broadband omnidirectional high transparency combined with self-cleaning properties have opened an alternative route toward realization of multifunctional surfaces which would be beneficial for touchscreen displays or solar harvesting devices. However, achieving the desired surface properties often requires sophisticated lithography fabrication methods consisting of multiple steps. In the present work, we show the design and implementation of mechanically robust AR surfaces fabricated by a lithography-free process using thermally dewetted silver as an etching mask. Both-sided nanohole (NH) surfaces exhibit transmittance above 99% in the visible or the near-infrared ranges combined with improved angular response at an angle of incidence of up to θi = 60°. Additionally, the NHs demonstrate excellent mechanical resilience against repeated abrasion with cheesecloth due to favorable redistribution of the shearing mechanical forces, making them a viable option for touchscreen display applications.

Enhancing Silicon Solar Cell Performance Using a Thin-Film-like Aluminum Nanoparticle Surface Layer

Solar cells play an increasing role in global electricity production, and it is critical to maximize their conversion efficiency to ensure the highest possible production. The number of photons entering the absorbing layer of the solar cell plays an important role in achieving a high conversion efficiency. Metal nanoparticles supporting localized surface plasmon resonances (LSPRs) have for years been suggested for increasing light in-coupling for solar cell applications. However, most studies have focused on materials exhibiting strong LSPRs, which often come with the drawback of considerable light absorption within the solar spectrum, limiting their applications and widespread use. Recently, aluminum (Al) nanoparticles have gained increasing interest due to their tuneable LSPRs in the ultraviolet and visible regions of the spectrum. In this study, we present an ideal configuration for maximizing light in-coupling into a standard textured crystalline silicon (c-Si) solar cell by determining the optimal Al nanoparticle and anti-reflection coating (ARC) parameters. The best-case parameters increase the number of photons absorbed by up to 3.3%. We give a complete description of the dominating light-matter interaction mechanisms leading to the enhancement and reveal that the increase is due to the nanoparticles optically exhibiting both particle- and thin-film characteristics, which has not been demonstrated in earlier works.

Enhanced laser-induced damage performance of all-glass metasurfaces for energetic pulsed laser applications

To fabricate optical components with surface layers compatible with high-power laser applications that may operate as antireflective coatings, polarization rotators, or harness physical anisotropy for other uses, metasurfaces are becoming an appealing candidate. In this study, large-beam (1.05 cm diameter) 351-nm laser-induced damage testing was performed on an all-glass metasurface structure composed of cone-like features with a subwavelength spacing of adjacent features. These structures were fabricated on untreated fused silica glass and damage tested, as were structures that were fabricated on fused silica glass that experienced a preliminary etching process to remove the surface Beilby layer that is characteristic of polished fused silica. The laser-induced damage onset for structures on untreated fused silica glass was 19.3J⋅c m -2, while the sample that saw an initial pretreatment etch exhibited an improved damage onset of 20.4J⋅c m -2, only 6% short of the reference pretreated glass damage onset of 21.7J⋅c m -2. For perspective, the National Ignition Facility operational average fluence at this wavelength and pulse length is about 10J/c m 2. At a fluence of 25.5J⋅c m -2, the reference (pretreated) fused silica initiated 5.2 damage sites per m m 2, while the antireflective metasurface sample with a preliminary etching process treatment initiated 9.8 damage sites per m m 2. These findings demonstrate that substrate-engraved metasurfaces are compatible with high energy and power laser applications, further broadening their application space.

Resonant Anti-Reflection Metasurfaces for Infrared Transmission Optics

A fundamental capability needed for any transmissive optical component is anti-reflection, yet this capability can be challenging to achieve in a cost-effective manner over longer infrared wavelengths. We demonstrate that Mie-resonant photonic structures can enable high transmission through a high-index optical component, allowing it to function effectively over long-wavelength infrared wavelengths. Using silicon as a model system, we demonstrate a resonant metasurface that enables a window optic with transmission up to 40% greater than that of unpatterned Si. Imaging comparisons with unpatterned Si and off-the-shelf germanium optics are shown as well as modulation transfer function measurements, showing excellent performance and suitability for imaging applications. Our results show how resonant photonic structures can be used to improve optical transmission through high-index optical components and highlight their possible use in infrared imaging applications.

A Bioinspired Ag Nanoparticle/PPy Nanobowl/TiO2 Micropyramid SERS Substrate

In this paper, the micropyramid structure was transferred to the TiO₂ substrate by soft imprinting. Then, the PPy nanobowls were assembled onto the surface of the TiO₂ micropyramids through the induction of the PS template. Finally, a layer of Ag nanoparticles was deposited on the surface of PPy nanobowls to form a novel Ag nanoparticle/PPy nanobowl/TiO₂ micropyramid SERS substrate. Its structure is similar to the bioinspired compound eyes. This substrate exhibited excellent antireflection, ultra-sensitivity, excellent uniformity, and recyclability. The concentration of R6G molecules can be detected as low as 10^-9 mol/L, and the Raman enhancement factor can reach 3.4 × 10⁵. In addition, the excellent catalytic degradation performance of the substrate ensures recyclability. This work proves that the micropyramid structure can be applied to other SERS materials besides silicon by the above methods, which broadens the selection range of composite SERS materials.

The advent of thermoplasmonic membrane distillation

Freshwater scarcity is a vital societal challenge related to climate change, population pressure, and agricultural and industrial demands. Therefore, sustainable desalination/purification of salty/contaminated water for human uses is particularly relevant. Membrane distillation is an emerging hybrid thermal-membrane technology with the potential to overcome the drawbacks of conventional desalination by a synergic exploitation of the water-energy nexus. Although membrane distillation is considered a green technology, efficient heat management remains a critical concern affecting the cost of the process and hindering its viability at large scale. A multidisciplinary approach that involves materials chemistry, physical chemistry, chemical engineering, and materials and polymer science is required to solve this problem. The combination of solar energy with membrane distillation is considered a potentially feasible low-cost approach for providing high-quality freshwater with a low carbon footprint. In particular, recent discoveries about efficient light-to-heat conversion in nanomaterials have opened unprecedented perspectives for the implementation of sunlight-based renewable energy in membrane distillation. The integration of nanofillers enabling photothermal effects into membranes has been demonstrated to be able to significantly enhance the energy efficiency without impacting on economic costs. Here, we provide a comprehensive overview on the state of the art, the opportunities, open challenges and pitfalls of the emerging field of solar-driven membrane distillation. We also assess the peculiar physicochemical properties and synthesis scalability of photothermal materials, as well as the strategies for their integration into polymeric nanocomposite membranes enabling efficient light-to-heat conversion and freshwater.

Thermally-curable nanocomposite printing for the scalable manufacturing of dielectric metasurfaces

Metasurfaces consisting of artificially designed meta-atoms have been popularized recently due to their advantages of amplitude and phase of light control. However, the electron beam lithography method for metasurface fabrication has high cost and low throughput, which results in a limitation for the fabrication of metasurfaces. In this study, nanocomposite printing technology is used to fabricate high-efficiency metasurfaces with low cost. To demonstrate the efficiency of the proposed fabrication method, a metahologram is designed and fabricated using a nanocomposite. The metahologram exhibits conversion efficiencies of 48% and 35% at wavelengths of 532 and 635 nm, respectively. The nanocomposite is composed of polymers with nanoparticles, so durability tests are also performed to evaluate the effects of temperature and humidity on the metasurfaces. The test verifies that at temperatures below the glass transition temperature of the base resin, the nanostructures do not collapse, so the efficiency of the metasurfaces remains almost the same. The surrounding humidity does not affect the nanostructures at all. Hence, the durability of the nanocomposite metasurfaces can be further enhanced by replacing the base resin, and this nanocomposite printing method will facilitate practical metasurface use at low cost.

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.

A mixture-density-based tandem optimization network for on-demand inverse design of thin-film high reflectors

Deep learning (DL) has emerged as a promising tool for photonic inverse design. Nevertheless, despite the initial success in retrieving spectra of modest complexity with nearly instantaneous readout, DL-assisted design methods often underperform in accuracy compared with advanced optimization techniques and have not proven competitive in handling spectra of practical usefulness. Here, we introduce a tandem optimization model that combines a mixture density network (MDN) and a fully connected (FC) network to inversely design practical thin-film high reflectors. The multimodal nature of the MDN gives access to infinite candidate designs described by probability distributions, which are iteratively sampled and evaluated by the FC network to allow for rapid optimization. We show that the proposed model can retrieve the reflectance spectra of 20-layer thin-film structures. More interestingly, it reproduces with high precision the periodic structures of high reflectors derived from physical principles, even though no such information is included in the training data. Improved designs with extended high-reflectance zones are also demonstrated. Our approach combines the high-efficiency advantage of DL with the optimization-enabled performance improvement, enabling efficient and on-demand inverse design for practical applications.

Cellulose Nanomaterials in Interfacial Evaporators for Desalination: A "Natural" Choice

Herein, the recent advances in realizing highly efficient cellulose-based solar evaporators for alleviating the global water crisis are summarized. Fresh water scarcity is one of the most threatening issues for sustainable development. Solar steam generation, which harnesses the abundant sunlight, has been recognized as a sustainable approach to harvest fresh water. In contrast to synthetic polymeric materials that can pose serious negative environmental impacts, cellulose-based materials, owing to their biocompatibility, renewability, and sustainability, are highly attractive for realizing solar steam generators. The molecular and macromolecular features of cellulose and the physicochemical properties of extracted cellulose nanoparticles (cellulose nanocrystals and cellulose nanofibrils (CNF)) and natural cellulose materials (wood and bacterial nanocellulose (BNC)) that make them attractive as supporting substrate materials in solar steam generators are briefly discussed. Recent progress in designing highly efficient cellulose-based solar evaporators, including utilizing extracted cellulose nanoparticles via bottom-up assembly CNF, natural cellulose materials with intrinsic hierarchical structure (wood and BNC), and commercial planar cellulose substrates (air-laid paper, cellulose paper, and cotton fabric) is reviewed. The outstanding challenges that need to be addressed for these materials and devices to be utilized in the real-world and in overcoming global water crisis are also briefly highlighted.

Closed-Surface Multifunctional Antireflective Coating Made from SiO2 with TiO2 Nanocomposites

An SiO₂-TiO₂ closed-surface antireflective coating was fabricated by the one-dipping method. TiO₂ nanoparticles were mixed with a nanocomposited silica sol, which was composed of acid-catalyzed nanosilica networks and silica hollow nanospheres (HNs). The microstructure of the sol-gel was characterized by transmission electron microscopy. The silica HNs were approximately 40–50 nm in diameter with a shell thickness of approximately 8–10 nm. The branched-chain structure resulting from acidic hydrolysis grew on these silica HNs, and TiO₂ was distributed inside this network. The surface morphology of the coating was measured by field emission scanning electron microscopy and atomic force microscopy. After optimization, transmittance of up to 94.03% was obtained on photovoltaic (PV) glass with a single side coated by this antireflective coating, whose refractive index was around 1.30. The short-circuit current gain of PV module was around 2.14–2.32%, as shown by the current-voltage (IV) curve measurements and external quantum efficiency (EQE) tests. This thin film also exhibited high photocatalytic activity. Due to the lack of voids on its surface, the antireflective coating in this study possessed excellent long-term reliability and robustness in both high-moisture and high-temperature environments. Combined with its self-cleaning function, this antireflective coating has great potential to be implemented in windows and photovoltaic modules.

Use of nanostructured alumina thin films in multilayer anti-reflective coatings

A new method for modification of planar multilayer structures to create nanostructured aluminum oxide anti-reflection coatings is reported. The method is non-toxic and low-cost, being based on treatment of the coating with heated de-ionized water after the deposition of aluminum oxide. The results show that the method provides a viable alternative for attaining a low reflectance ARC. In particular, a low average reflectivity of ∼3.3% is demonstrated in a broadband spectrum extending from 400 nm to 2000 nm for ARCs deposited on GaInP solar-cells, the typical material used as top-junction in solar cell tandem architectures. Moreover, the process is compatible with volume manufacturing technologies used in photovoltaics, such as ion beam sputtering and electron beam evaporation.

All-Nanoparticle Monolayer Broadband Antireflective and Self-Cleaning Transparent Glass Coatings

The vast majority of light-emitting diode and liquid-crystal displays, solar panels, and windows in residential and industrial buildings use glass panels owing to their high mechanical stability, chemical resistance, and optical properties. Glass surfaces reflect about 4-5% of incident light if no antireflective coating is applied. In addition to energy losses in displays, surface reflections diminish picture quality. Engineering of antireflective coatings can be beneficial for all types of glass screens, specifically for large screens and touch-screen devices when scratch-resistance and self-cleaning properties of the glass surface are also desired. A scalable and robust approach to produce antireflective coatings for glass surfaces with desired optical and mechanical properties is introduced in this work. The developed coating mimics the structure of a moth-eye cornea. The coating is a subwavelength-microstructured thin layer on the glass surface made of a monolayer of hemispherical silica nanoparticles obtained by hydrothermal fusion of spherical particles to the glass substrate. The sequence of the particle deposition in the layer-by-layer process is adjusted to balance attractive-repulsive interactions among nanoparticles and between the nanoparticles and the glass surface to generate coatings with a high surface coverage of up to 70%, which exceeds the 54.7% limit of the random sequential addition model. This level of surface coverage allows for a combination of properties beneficial for the described applications: (i) an average reflectance of 0.5 ± 0.2% for a visible and near-infrared optical spectrum, (ii) an improved mechanical stability and scratch resistance, and (iii) non-wetting behavior.

Nanoenabled Photothermal Materials for Clean Water Production

Solar-powered water evaporation is a primitive technology but interest has revived in the last five years due to the use of nanoenabled photothermal absorbers. The cutting-edge nanoenabled photothermal materials can exploit a full spectrum of solar radiation with exceptionally high photothermal conversion efficiency. Additionally, photothermal design through heat management and the hierarchy of smooth water-flow channels have evolved in parallel. Indeed, the integration of all desirable functions into one photothermal layer remains an essential challenge for an effective yield of clean water in remote-sensing areas. Some nanoenabled photothermal prototypes equipped with unprecedented water evaporation rates have been reported recently for clean water production. Many barriers and difficulties remain, despite the latest scientific and practical implementation developments. This Review seeks to inspire nanoenvironmental research communities to drive onward toward real-time solar-driven clean water production.

Deep Convolutional Mixture Density Network for Inverse Design of Layered Photonic Structures

Machine learning (ML) techniques, such as neural networks, have emerged as powerful tools for the inverse design of nanophotonic structures. However, this innovative approach suffers some limitations. A primary one is the nonuniqueness problem, which can prevent ML algorithms from properly converging because vastly different designs produce nearly identical spectra. Here, we introduce a mixture density network (MDN) approach, which models the design parameters as multimodal probability distributions instead of discrete values, allowing the algorithms to converge in cases of nonuniqueness without sacrificing degenerate solutions. We apply our MDN technique to inversely design two types of multilayer photonic structures consisting of thin films of oxides, which present a significant challenge for conventional ML algorithms due to a high degree of nonuniqueness in their optical properties. In the 10-layer case, the MDN can handle transmission spectra with high complexity and under varying illumination conditions. The 4-layer case tends to show a stronger multimodal character, with secondary modes indicating alternative solutions for a target spectrum. The shape of the distributions gives valuable information for postprocessing and about the uncertainty in the predictions, which is not available with deterministic networks. Our approach provides an effective solution to the inverse design of photonic structures and yields more optimal searches for the structures with high degeneracy and spectral complexity.

Broadband control of the optical properties of semiconductors through site-controlled self-assembly of microcrystals

We investigate light-matter interactions in periodic silicon microcrystals fabricated combining top-down and bottom-up strategies. The morphology of the microcrystals, their periodic arrangement, and their high refractive index allow the exploration of photonic effects in microstructured architectures. We observe a notable decrease in reflectivity above the silicon bandgap from the ultraviolet to the near-infrared. Finite-difference time-domain simulations show that this phenomenon is accompanied by a ∼2-fold absorption enhancement with respect to a flat sample. Finally, we demonstrate that ordered silicon microstructures enable a fine tuning of the light absorption by changing experimentally accessible knobs as pattern and growth parameters. This work will facilitate the implementation of optoelectronic devices based on high-density microcrystals arrays with optimized light-matter interactions.

Broadband transparency with all-dielectric metasurfaces engraved on silicon waveguide facets: effect of inverted and extruded features based on Babinet's principle

Building blocks of photonic integrated circuitry (PIC), optical waveguides, have long been considered transparent. However, the inevitable Fresnel reflection from waveguide facets limits their transparency. This limitation becomes more severe in high-index waveguides in which the transparency may drop to 65%. We overcome this inherent optical property of high-index waveguides by engineering an appropriate facet landscape made of sub-wavelength artificial features unit cells. For this, we develop a semi-analytical formalism for predicting the metasurface parameters made of high-index dielectric materials, to be engraved on the facets of optical waveguides, based on Babinet's principle: either extruded from the waveguide facet or etched into it. Our semi-analytical model predicts the shape of anti-reflective metasurface unit cells to achieve transmission as high as 98.5% in near-infrared from 1 μm to 2 μm. This new class of metasurfaces may be used for the improvement of PIC devices for communication and sensing, where device transparency is crucial for high signal-to-noise ratios.

Broadband, High-Temperature Stable Reflector for Aerospace Thermal Radiation Protection

A simple and thermally stable photonic heterostructure exhibiting high average reflectivity (⟨R⟩ ≈ 88.8%) across a broad wavelength range (920-1450 nm) is presented. The design combines a thin, highly reflective and broadband metallic substrate (Ta) with an optimized dielectric coating (10 layers) to create an enhanced reflector with improved optical and thermal properties compared to its constituents. The heterostructure exhibits temperature-reversible reflective properties up to 1000 °C. In order to take advantage of the high reflectivity and temperature stable properties of this coating, in a wide range of non-photonic composite materials, we have fabricated heterostructure platelets as additives. By impregnating these additives into other types of materials, their response can be photonically enhanced.  Platelets of such a heterostructure have been introduced inside an organic matrix to increase its broadband reflection performance. The platelet-impregnated matrix displays an average reflectivity improvement from 5% to an average of 55% over a 1000 nm range, making it a suitable additive for next generation thermal protection systems (TPS).

Ultraviolet-transparent low-index layers for antireflective coatings

Nanostructured low-index layers are useful as the last layers of antireflective (AR) coatings because they can broaden their spectral ranges and improve the performance for oblique light incidence. Structuring of evaporated organic layers by plasma opens a route to produce inorganic interference stacks and low-index layers in the same vacuum process. The organic material uracil has been investigated as a template material for AR nanostructures. An additional plasma-treatment step was added to the manufacturing process, which decreases the organic fraction of the coating substantially. As a result, a better environmental stability and higher transmission in the ultraviolet range was achieved.

Design and development of an optical reflective notch filter using the ion assisted deposition technique with stepwise modulated thickness for avionics applications

This paper presents research work about the design and fabrication of a 44-layer optical reflective notch filter. The performance of the fabricated notch filter was studied at normal (0°) and oblique (45°) incidence angle. In addition, the paper also discusses a three-layer broadband antireflective coating on both sides of the multilayer stack to suppress the ripples in the passband region. The thickness-modulated reflective stack of the filter was designed by using the materials Al₂O₃ (1.63) and SiO₂ (1.46). Optimization of the multilayer stack was carried out by using the damped least-squares algorithm. The theoretical and experimental results from the ion-assisted e-beam deposited samples for single notch reflective filters are presented. Good agreement in the design and experimental results was observed when the deposition process was controlled by time of evaporation. Further, the filter was characterized for the optical properties by using a UV-VIS-NIR spectrophotometer, surface morphology and protective properties using field emission scanning electron microscopy, a coherence correlation interferometer, and water contact angle.

Nanostructured Hybrid-Material Transparent Surface with Antireflection Properties and a Facile Fabrication Process

Highly transparent optical surfaces with antireflection (AR) properties have the potential to increase the performance of a wide range of applications, such as windows for photovoltaic cells, photodetectors, and display screens among others. Biomimetic structures inspired by the moth-eye have attracted much attention as they can offer superior AR properties, which can generate broadband, omnidirectional optical transmission, and water-repellent self-cleaning behavior. However, many biomimetic surfaces suffer from time-consuming and complex processing, for example, electron beam and nanoimprint lithography, and/or sub-optimal mechanical reliability. In this paper, we introduce a hybrid material approach-nanostructured polyimide on a substrate-for demonstrating a surface with significant AR and hydrophobic properties together with low scattering (haze) and high mechanical resistance. As an example of applications, we demonstrate an indium tin oxide transparent conductive substrate with a large AR effect and optical transmission associated to the nanostructured polyimide coating. The proposed design and method based on conventional spin-coating and lithography-free metal dewetting have the potential to be a low-cost processing path of nanostructured AR transparent substrates.

Engineering Lateral Heterojunction of Selenium-Coated Tellurium Nanomaterials toward Highly Efficient Solar Desalination

Herein, a core-shell tellurium-selenium (Te-Se) nanomaterial with polymer-tailed and lateral heterojunction structures is developed as a photothermal absorber in a bionic solar-evaporation system. It is further revealed that the amorphous Se shell surrounds the crystalline Te core, which not only protects the Te phase from oxidation but also serves as a natural barrier to life entities. The core (Te)-shell (Se) configuration thus exhibits robust stability enhanced by 0.05 eV per Se atom and excellent biocompatibility. Furthermore, high energy efficiencies of 90.71 ± 0.37% and 86.14 ± 1.02% and evaporation rates of 12.88 ± 0.052 and 1.323 ± 0.015 kg m-2 h-1 are obtained under 10 and 1 sun for simulated seawater, respectively. Importantly, no salting out is observed in salt solutions, and the collected water under natural light irradiation possesses extremely low ion concentrations of Na+, K+, Ca2+, and Mg2+ relative to real seawater. Considering the tunable electronic structures, biocompatibilities, and modifiable broadband absorption of the solar spectrum of lateral heterojunction nanomaterials of Te-Se, the way is paved to engineering 2D semiconductor materials with supporting 3D porous hydrophilic materials for application in solar desalination, wastewater treatment, and biomedical ventures.

Optical modeling of random anti-reflective meta-surfaces for laser systems applications

Optical performance of anti-reflective random meta-surfaces are studied numerically for coherent beam propagation systems, such as lasers. A methodology for the modeling of such optics performance is developed and applied to study the reflectivity and laser beam quality degradation. These quantitative metrics and design considerations highlight that reducing the size of the meta-surface period much below the light wavelength is not necessarily required.

Field Test of Self-Cleaning Zr-Modified-TiO2-SiO2 Films on Glass with a Demonstration of Their Anti-Fogging Effect

The number of commercial products claiming self-cleaning properties is rising and testing of long-term activity and durability of such coatings needs to be addressed more. The time-dependent changes of different characteristics like haze, transparency, and color are essential for transparent glazing materials. Herein, we aimed to examine whether the laboratory results obtained on the Zr-modified-titania-silica (TiZr) self-cleaning materials would translate to larger-scale outdoor-exposed testing. TiZr thin films were deposited via spraying onto float glass window surfaces and exposed into three different environments for 20 months. For comparison, a commercially available active SGG BIOCLEAN^TM glass and standard float glass were simultaneously exposed in the same conditions. It was shown that the self-cleaning property of either a commercial product or TiZr-coated float glass was not considerably effective in real field test conditions, although the previous laboratory tests showed pronounced photocatalytic activity of TiZr thin films. The inclination angle; however, was shown to have a considerable effect on the self-cleaning ability of samples, as did the rain patterns during the testing period. On the other hand, the anti-fogging effect of our TiZr material was very well expressed in controlled laboratory conditions (measuring droplet formation time) as well as in the real outdoor environment.

Wide-Angle Broadband Antireflection Coatings Prepared by Atomic Layer Deposition

A novel broadband antireflective coating with ultra-low residual reflectance for light incidence angles from 0° up to 60° is presented. The system consists of an interference multilayer coating made by atomic layer deposition (ALD) combined with a low- n nanoporous SiO₂ top-layer obtained by wet-chemical etching of an atomically mixed SiO₂/Al₂O₃ ALD composite. The average residual reflectance measured at normal incidence for double-sided coated B270 glass substrates is only 0.5% in a broad spectral range from 400 to 1100 nm. The average reflectance of the substrate considering both front and rear sides decreased in the visible spectral range of 420-680 nm from 9.9 and 15.8 to 0.4 and 1.8% at an oblique angle of incidence (AOI) of 45° and 60°, respectively, by applying the hybrid ALD antireflection coatings. The measured average transmittance reaches 99.5% at AOI 6° in the 400-950 nm spectral range. Measurements three weeks after preparation show only a small reduction of the average transmittance to 99.3% in this spectral range spanning 550 nm. Ten months later, the average transmittance is still 99.0%, whereby the sample handling might have also affected the performance. The hybrid ALD system shows excellent conformal AR performance on a strongly curved B270 aspheric lens with a diameter of 50 mm and a height of 25 mm. The presented process is a promising route toward omnidirectional AR coatings on complex 3D optics, which are increasingly important for consumer and high-performance optical systems.

Photothermal Membrane Water Treatment for Two Worlds

In meeting the increasing need for clean water in both developing and developed countries and in rural and urban communities, photothermal membrane water treatment technologies provide outstanding advantages: For developing countries and rural communities, by utilizing sunlight, photothermal membrane water treatment provides inexpensive, convenient, modular, decentralized, and accessible ways to clean water, which can reduce the consumption of conventional energy (e.g., electricity, natural gas) and the cost of clean water production. In developed countries and urban communities, photothermal membrane water treatment can improve the energy efficiency during water purification. In these water purification processes, the light absorption and light-to-heat conversion of photothermal materials are important factors in determining the membrane efficacy. Nanomaterials with well-controlled structure and optical properties can increase the light absorption and photothermal conversion of newly developed membranes. This Account introduces our recent work on developing scalable, cost-effective, and highly efficient photothermal membranes for four water purification applications: reverse osmosis (RO), ultrafiltration (UF), solar steam generation (SSG), and photothermal membrane distillation (PMD). By utilizing photothermal materials, first, we have demonstrated how sunlight can be used to improve the membrane's resistance to biofouling in RO and UF processes by photothermally induced inactivation of microorganisms. Second, we have developed novel SSG membranes (i.e., interfacial evaporators) that can harvest solar energy, convert it to localized heat, and generate clean water by evaporation. This desalination approach is particularly useful and promising for treatment of highly saline water. These new interfacial evaporators utilized graphene oxide (GO), reduced graphene oxide (RGO), molybdenum disulfide (MoS₂), and polydopamine (PDA). The solar conversion efficiency and environmental sustainability of the interfacial evaporators were optimized via (i) novel and versatile bottom-up biofabrication (e.g., incorporation of photothermal materials during bacterial nanocellulose (BNC) growth) and (ii) easy and cost-effective top-down preparation (e.g., modification of natural wood with photothermal materials). Third, we have developed membranes for PMD that incorporate photothermal materials to generate heat under solar irradiation, thus providing a higher transmembrane temperature difference and higher driving force for effective vapor transport, making the membrane distillation process more energy-efficient. Lastly, this Account compares the photothermal membrane applications, summarizes current challenges for photothermal membrane applications, and offers future directions to facilitate the translation of photothermal membranes from the laboratory to large engineered systems by improving their scalability, stability, and sustainability.

Ideal optical contrast for 2D material observation using bi-layer antireflection absorbing substrates

The capability to observe 2D materials with optical microscopy techniques is of central importance in the development of the field and is a driving force for the assembly and study of 2D material van der Waals heterostructures. Such an observation of ultrathin materials usually benefits from antireflection conditions associated with the choice of a particular substrate geometry. The most common configuration uses a transparent oxide layer with a thickness minimizing light reflection at the air/substrate interface when light travels from air to the substrate. Backside Absorbing Layer Microscopy (BALM) is a newly proposed configuration in which light travels from glass to air (or another medium such as water or a solvent) and the antireflection layer is a light-absorbing material (typically a metal). We recently showed that this technique produces images of 2D materials with unprecedented contrast and can be ideally coupled to chemical and electrochemical experiments. Here, we show that contrast can be optimal using double-layer antireflection coatings. By following in situ and with sub-nm precision the controlled deposition of molecules, we notably establish precisely the ideal observation conditions for graphene oxide monolayers which represent one of the most challenging 2D material cases in terms of transparency and thickness. We also provide guidelines for the selection of antireflection coatings applicable to a large variety of nanomaterials. This work strengthens the potential of BALM as a generic, powerful and versatile technique for the study of molecular-scale materials and phenomena.

Improvement of Terahertz Photoconductive Antenna using Optical Antenna Array of ZnO Nanorods

An efficient terahertz (THz) photoconductive antenna (PCA), as a major constituent for the generation or detection of THz waves, plays an essential role in bridging microwave-to-photonic gaps. Here, we propose an impressive approach comprising the use of arrayed zinc oxide nanorods (ZnO NRs) as an optical nanoantenna over an anti-reflective layer (silicon nitride) in the antenna gap to boost the photocurrent and consequently the THz signal. The numerical approach applied in investigating the optical behavior of the structure, demonstrates a significant field enhancement within the LT-GaAs layer due to the optical antenna performing simultaneously as a concentrator and an antireflector which behaves as a graded-refractive index layer. ZnO NRs have been fabricated on the PCA gap using the hydrothermal method as a simple, low cost and production compatible fabrication method compared to other complex methods used for the optical nanoantennas. Compared to the conventional PCA with a traditional antireflection coating, the measured THz power by time domain spectroscopy (TDS) is increased more than 4 times on average over the 0.1–1.2 THz range.

Investigation of TiO2 Thin Film Deposited by Microwave Plasma Assisted Sputtering and Its Application in 3D Glasses

TiO2 deposition using separate regions for sputtering and oxidation is not well investigated. We optimized process parameter for such as oxygen flow and microwave power to produce high quality TiO2 filters for Stereo/3D imaging applications. This deposition technique was chosen for its unique advantages: high deposition rates while increasing the probability of obtaining stoichiometric oxides, reduces possibility of target poisoning and provides better stability of process. Various characterization methods, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman, X-ray diffraction (XRD), transmission spectroscopy, were used in compliment to simulations for detailed analysis of deposited TiO2 thin films. Process parameters were optimized to achieve TiO2 films with low surface scattering and absorption for fabricating multi-passbands interference filter for 3D glasses. From observations and quantitative analysis of surfaces, it was seen that surface roughness increases while oxygen flow or microwave power increases. As the content of anatase phase also increases with higher microwave power and higher oxygen flow, the formation of anatase grains can cause higher surface roughness. Optical analysis of samples validates these trends and provided additional information for absorption trends. Optimized parameters for deposition process are then obtained and the final fabricated 3D glasses filters showed high match to design, within 0.5% range for thickness error.

Enhanced Transmission and Self-Cleaning of Patterned Sapphire Substrates Prepared by Wet Chemical Etching Using Silica Masks

Highly transparent and superhydrophilic sapphire with surface antireflective subwavelength structures were prepared by wet etching using colloidal monolayer silica masks. The film thicknesses of the silica masks were adjusted by the volume concentrations of polystyrene spheres. The evolution of etching morphologies of sapphire was studied, and antireflective concave pyramid nanoarrays on sapphire substrates were designed by calculation and were then prepared. The transmission and wettability of as-obtained patterned sapphire substrates were also investigated. As for sapphire with optimum surface concave micropyramid arrays, average visible transmittance can reach 91.7%, which is apparently higher than that of flat sapphire (85.5%). Moreover, the concave pyramid arrays can significantly increase the surface hydrophilicity of sapphire, exhibiting a water contact angle of 12.6° compared with 62.7° of flat sapphire. The proposed method can be an excellent strategy for preparing antireflective and self-cleaning concave micropyramid subwavelength structures on sapphire without complicated equipment and expensive raw materials.

Graphene-based stretchable and transparent moisture barrier

We propose an alumina-deposited double-layer graphene (2LG) as a transparent, scalable, and stretchable barrier against moisture; this barrier is indispensable for foldable or stretchable organic displays and electronics. Both the barrier property and stretchability were significantly enhanced through the introduction of 2LG between alumina and a polymeric substrate. 2LG with negligible polymeric residues was coated on the polymeric substrate via a scalable dry transfer method in a roll-to-roll manner; an alumina layer was deposited on the graphene via atomic layer deposition. The effect of the graphene layer on crack generation in the alumina layer was systematically studied under external strain using an in situ micro-tensile tester, and correlations between the deformation-induced defects and water vapor transmission rate were quantitatively analyzed. The enhanced stretchability of alumina-deposited 2LG originated from the interlayer sliding between the graphene layers, which resulted in the crack density of the alumina layer being reduced under external strain.

Backside absorbing layer microscopy: Watching graphene chemistry

The rapid rise of two-dimensional nanomaterials implies the development of new versatile, high-resolution visualization and placement techniques. For example, a single graphene layer becomes observable on Si/SiO₂ substrates by reflected light under optical microscopy because of interference effects when the thickness of silicon oxide is optimized. However, differentiating monolayers from bilayers remains challenging, and advanced techniques, such as Raman mapping, atomic force microscopy (AFM), or scanning electron microscopy (SEM) are more suitable to observe graphene monolayers. The first two techniques are slow, and the third is operated in vacuum; hence, in all cases, real-time experiments including notably chemical modifications are not accessible. The development of optical microscopy techniques that combine the speed, large area, and high contrast of SEM with the topological information of AFM is therefore highly desirable. We introduce a new widefield optical microscopy technique based on the use of previously unknown antireflection and absorbing (ARA) layers that yield ultrahigh contrast reflection imaging of monolayers. The BALM (backside absorbing layer microscopy) technique can achieve the subnanometer-scale vertical resolution, large area, and real-time imaging. Moreover, the inverted optical microscope geometry allows its easy implementation and combination with other techniques. We notably demonstrate the potentiality of BALM by in operando imaging chemical modifications of graphene oxide. The technique can be applied to the deposition, observation, and modification of any nanometer-thick materials.

Broadband Anti-Reflective Coating Based on Plasmonic Nanocomposite

We report on the fabrication, the characterization, and the optical simulation of a gold–silica nanocomposite and present its integration into a broadband anti-reflective coating (ARC) for a silicon substrate. The two-layer ARC consists of a nanocomposite (randomly distributed gold cluster in a silica matrix) and a pure silica film. We capitalize on the large refractive index of the composite to impose an abrupt phase change at the interface of the coating to diminish the light reflection from the substrate through the ultrathin nanocoating. The average reflectivity of the silicon can be reduced by such a coating to less than 0.1% in the entire visible spectrum. We experimentally and numerically prove that percolated nanocomposites with an overall thickness of 20 nm can provide anti-reflectivity up to near infrared (NIR). The ARC bandwidth can be shifted more than 500 nm and broadened to cover even the NIR wavelength by changing the volume filling fraction of the gold clusters. The angular sensitivity of thin ultrathin antireflective coating is negligible up to 60°. The present ARC could find applications in thermo-photovoltaics and bolometers.