Broadband anti-reflection coating for sub-terahertz optics using dielectric multilayers.

Silica (SiO2) thin films with different refractive indexes as the anti-reflection coating for cadmium telluride (CdTe) solar cells were prepared by sol–gel method in this paper. In order to obtain the preparation process of SiO2 films with suitable refractive index, the different hydrolysis processes relied on the acid-base environment as well as the ratio of reactants was studied, respectively. It mainly includes that (I) the gel growth model in hydrogen or hydroxide ion environment provided by hydrochloric and ammonia. (II) The ratio of solvent, precursor and catalyst under base catalysis. (III) The weight and amount of polyethylene glycol molecular which is an additive agent to increase the porosity of sol film under acid catalysis. Based on the results above, a monolayer SiO2 coating with optimized refractive index was applied onto the glass substrate of CdTe solar cell, and the quantum efficiency has been enhanced at the band of 400–800 nm. After that, multilayer SiO2 coatings with high–low refractive indexes were designed according to the Fresnel theory to achieve wider spectrum (390–870 nm) enhancement. The broadband anti-reflection coatings (BARCs) were deposited on CdTe solar cell. The results were not consistent with expectations, which required further study about process matching between BARCs and device manufacture.

Preparation of broadband antireflective coatings with ultra-low refractive index layer by sol-gel method

Optical coatings with controllable ultralow refractive indices are of profound significance in optical areas. However, it remains a challenge to fabricate such coatings using a simple method. Here we develop an effective and simple approach to create ultra-low-index coatings. This approach was based on a modified sol-gel process, with a key process that involved the aggregation of silica nanoparticles via the addition of a polymer surfactant (e.g., polyvinylpyrrolydone) in sols before coating. The approach involves three steps: the synthesis of silica sols under ammonia catalysis in ethanol (Stöber method), the addition of polyvinylpyrrolydone in the silica sols to induce the aggregation of the silica nanoparticles, and the formation of ultra-low-index coatings by depositing the aggregated silica sols on substrates. Through varying the aggregation extent, this approach produced coatings with controllable refractive indices ranging from 1.17 to 1.07. To the best of our knowledge, the minimum index value of 1.07 from our coating is among the lowest refractive indices ever reported. The ultra-low-index coatings demonstrated excellent optical properties, with which perfect quarter-wavelength antireflection coatings (maximum transmittance ∼100%) and broadband antireflection coatings (transmittance >98% from 400 to 1100 nm) can be prepared. One advantage of the antireflection coatings is that their transmission is less dependent on the refractive index and the thickness of the stacking layer, which make it promising in large-scale production. Moreover, the coatings can be made hydrophobic (water contact angle 136°) by exposing the coatings to a hexamethyldisilazane atmosphere, exhibiting high environmental stability in a humid environment. The aggregation of silica nanoparticles in sol-gel processes provides a scalable alternative to the current approaches for creating ultra-low-index coatings.

Thermally stable nanoporous organosilicate thin films were realized by the microphase separation of pore-generating polymers mixed with an organosilicate matrix to be antireflective coatings (ARCs), for which a thin film with a refractive index (n) of 1.23 for zero reflection is required. The refractive index of such nanoporous organosilicate films can be tuned from 1.39 down to 1.23 by incorporating nanopores within the films. With a nanoporous single layer with n approximately 1.23, the light transmittance of the glass above 99.8% was achieved in the visible range (lambda approximately 550 nm). To overcome the limitation on the narrow wavelength for high transmittance imposed by a single antireflective nanoporous thin film, bilayer thin films with different refractive indices were prepared by placing a high refractive index layer with a refractive index of 1.45 below the nanoporous thin film. UV-vis transmittance of a glass coated with the bilayer films was compared with nanoporous single-layer films and it is demonstrated that the novel broadband antireflection coatings in a wide range of visible wavelength can be easily obtained by the organosilicate bilayer thin films described in this study. Also, ARCs developed in this study demonstrate excellent AR durability owing to the hydrophobic nature of the organosilicate matrix.

A double‐layer double‐wavelength antireflective (AR) coating with 100% transmittance at both 1064 nm and 532 nm, which is very important in high power laser systems, is designed using thin film design software (TFCalc). The refractive indices for the bottom and top layers of the designed AR coating are about 1.30 and 1.14. A simple, template‐free sol‐gel route is proposed to prepare the superhydrophobic ORMOSIL (organically modified silicate) thin film, which has an ultralow refractive index, by silica particle surface modification using hexamethylisilazane (HMDS); this treatment decreases the refractive index of the silica thin film from 1.23 to 1.13. The formation mechanism of the ultralow refractive index thin film is proposed. The particle surface modification with HMDS significantly improves the hydrophobicity of the coated film; the water contact angle of the film increases from 23.4° to 160°. The bottom layer, which has a refractive index of 1.30, is prepared from acid‐catalyzed and base‐catalyzed mixed silica sol. A double‐layer silica AR coating is obtained with transmittances of 99.6% and 99.8% at 532 nm and 1064 nm, respectively.

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

Graded refractive index layer systems for antireflective coatings and rugate filters deposited by reactive pulse magnetron sputtering

Estimation of the porosity and refractive index of sol–gel silica films using high resolution electron microscopy

Al2O3/TiO2/Al2O3 multilayer antireflection stacks used for broadband antireflection coatings and TiO2 and Al2O3 single layers were characterized by a novel combination of an X-ray reflectivity layer extraction algorithm, X-ray reflectivity simulations, spectroscopic ellipsometry, broadband optical reflectance, glancing incidence X-ray diffraction and transmission electron microscopy. Single-layer TiO2 and Al2O3 antireflection coatings were also measured by glancing incidence small angle X-ray scattering. Layer thicknesses and surface and interface roughnesses with a few A precision, densities normalized to bulk densities with less than one percent uncertainty, and differences in pore-induced scattering were assessed. These parameters are key to understanding the role of deposition and processing conditions on the performance of broadband antireflection coatings. Additionally, the effect of air annealing at 400 °C for 10 min and 40 min on the structural and optical properties of single-layer TiO2 and Al2O3 antireflection coatings was also revealed.

Broadband antireflective coatings in the optical communication band deposited by ion-assisted reactive magnetron sputtering

www.advopticalmat.de Inevitable Fresnel refl ection from the interface between two optical media with different refractive indices ( η ) can decrease light absorption in photovoltaic devices and can cause undesirable glare in fldisplays. Thus, an effective antirefl ection coating (ARC) for a higher optical transmittance can contribute to the accomplishment of improved contrast and brightness in display devices and higher levels of energy conversion effi ciency in solar panels. Traditional ARCs improve optical transmittance by introducing destructive interference between refl ected light beams from the air-fi lm and fi lm-substrate interfaces. An ideal ARC should satisfy the requirements of a quarter-wavelength fi lm thickness and a fi lm η of ( η s ) 0.5 , where η s denotes the η of the substrate. [ 1 ] However, an ARC with a homogeneous η can achieve high transmittance only in narrow wavelength and incidence angle ranges, which are fi xed by its thickness and refractive index. In contrast, an ARC with a gradually changing η , also known as a graded index, can avoid the formation of an interface with a sharp contrast in η . As a result, the Fresnel refl ection can be effectively minimized in a wide wavelength range and high optical transparency can be achieved. In an ideal gradedindex anti-refl ection coating (GIARC), the η should gradually vary from that of the substrate material, onto which the GIARC is coated, to the η of air ( ∼ 1), necessitating the use of a material with a widely tunable η . Theoretical studies showed that a quintic profi le of η varying from that of the substrate (typically 1.5‐2) to 1 is best for achieving the lowest refl ectance (<0.1% in the visible range). [ 2 ] While dense materials generally have an η larger than ∼ 1.35, the extensive variability of indices has been demonstrated by nanoporous materials for the applications for GIARC. [ 2‐6 ] Various methods, such as lithography, obliqueangle deposition, catalytic etching, chemical vapor deposition, and a sol-gel process, have been reported. [ 2‐5 ] However, these methods may lack cost-effectiveness and high-throughput capabilities due to the requirements of vacuum processing, multiple etching steps, or because of the limited controllability of η . Recently, highly porous block copolymer (BCP) thin fi lms were applied to the fabrication of ARC due to the excellent low-cost solution processability and facile tunability of η when controlling the fraction of the pores generated by the selective removal of one block. [ 7‐11 ] Kim and co-workers reported a wide tuning range of η between 1.17 and 1.42 in nanoporous polystyrene (PS) thin fi lms obtained by removing poly(methyl methacrylate) (PMMA) in PS-PMMA BCPs. The stacking of three PS layers with gradually changing porosity achieved a minimum refl ectance of less than 0.1%. Han and coworkers demonstrated that a mixture of PS-PMMA BCPs and PMMA homopolymers led to a graded distribution of PMMA along the vertical direction of the thin fi lms and that a gradient η can be obtained for the remaining PS nanostructures after the removal of PMMA, which can reduce the number of processing steps including spin-coating and acid etching. However, these approaches may be associated with challenges related to the low chemical and mechanical stability of the porous organic thin fi lms and the

Reflection losses at optical interfaces are detrimental for many applications in optics and optoelectronics. Antireflection coatings (ARCs) are typically based on the principle that light reflected at the ARC’s interfaces interferes destructively to maximise light transmission. Advances in the fabrication of porous thin films have enabled to achieve refractive indices low enough to meet conditions for zero-reflectance off substrate materials like glass. Yet, many widespread applications require robustness and self-cleaning of ARCs, as well as low processing temperatures and compatibility with plastic substrates. In this chapter a new concept is presented which relies on the use of a high molecular weight block copolymer system with a large volume fraction of the hydrophobic block in combination with silica sol-gel chemistry. Spontaneous dense packing of colloidal micelles results in the co-assembly of the inorganic material in an inverse-opal like morphology. Subsequent removal of the polymer host leads to robust and continuous inorganic films with refractive indices as low as 1.13. The polymer-driven self-assembly route to ultralow refractive index films enables to replace up to 50 w% silica with high refractive index photocatalytic TiO2 nanocrystals, which freely disperse within the inorganic network. The resulting ARCs show good optical and self-cleaning properties and can be processed onto flexible plastic substrates.

Broadband antireflection coatings for terahertz (THz) components are extremely important in the application of THz technology. We present an in-depth materials study on the broadband antireflection design based on the concept of impedance matching. Several selected conductive materials are tested and compared. They can all be used as THz broadband antireflection coatings regardless of their nature of conductivity. We provide a facile method to approximately predict the impedance matching condition for various conductive materials based on their dc real conductivity. The theoretical predications are in good agreement with experimental results and published data. Doped metal oxides or polymers are recommended as good candidates for fabricating antireflection coating in THz range. The thickness-independent dc real conductivity enables better control on individual tuning parameter—film thickness. Given a substrate, THz transmission in antireflection coating has a theoretical upper limit which is further reduced with non-negligible imaginary conductivity.

Broadband antireflection (AR) coatings are essential elements for improving the photocurrent generation of photovoltaic modules or the enhancement of visibility in optical devices. In this paper, we report a hybrid nanostructured antireflection coating combination that is a clean and efficient method for fabricating a nanostructured antireflection coating (ARC). A multilayer thin-film was introduced between the ARC and substrate to solve the significant problem of preparing nanostructured ARCs on different substrates. In this way, we rebuilt a gradient refractive index structure and optimize the antireflective property by simply adjusting the moth-eye structure and multilayers. Subwavelength-structured cone arrays were directly patterned using a self-assembled single-layer polystyrene (PS) nanosphere array as an etching mask. Nanostructure coatings exhibited excellent broadband and wide-angle antireflective properties. The bottom-up preparation process and hybrid structural combination have the potential to significantly enhance the broadband and wide-angle antireflective properties for a number of optical systems that require high transparency, which is promising for reducing the manufacturing cost of nanostructured AR coatings.

The study attempts to investigate the significant spatiotemporal variations in the continental United States’ streamflow as a response to large-scale climate signals across multiple spectral bands (SBs). Using non-parametric (long-term) trend and (abrupt) shift detection tests, coupled with discrete wavelet transform, 237 unimpaired streamflow stations were analyzed over a study period of 62 years (1951 to 2012), looking at the water year and seasonal data, along with three discrete SBs of two, four, and eight years. Wavelet coherence analysis, derived from continuous wavelet transform, determined the association between the regional streamflow patterns and three large-scale climate signals, i.e., El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multi-decadal Oscillation (AMO), across continuous SBs ranging from two to 16 years. The results indicated significant positive (negative) trends and shifts in the northeastern and north-central (northwestern) regions with an increase in the number of stations as the SBs increased. The spatiotemporal association between regional streamflow and the climate signals varied significantly (from no correlation, Rn2~ 0, to perfect correlation, Rn2~ 1.0) even amongst adjacent regions. Among the climate signals, ENSO showed the highest association (Rn2~ 1.0), having a consistent phase relationship with the regional streamflow patterns, especially in the higher SBs. PDO (with the least influence among the three signals) and AMO showed stronger associations, mostly in the lower SBs. These results may help explain the teleconnections between the climate signals and the US streamflow variations across multiple SBs, which may lead to improved regional flow regulations.