Formation and Depth Profiling Analysis of Multilayers Derived from Sol-Gel Processing Using Dynamic SIMS, RBS, and TEM

Silica sol-gel single layer AR coatings are used in high peak power pulsed lasers due to their high laser induced damage threshold (LIDT) and their low refractive index (1.22). We have used sol-gel processing to spin and dip coat multilayers of alternating high index (zirconia/hafnia) and low index (base catalysed silica) sol-gels on to fused silica substrates. When tailored at the correct thickness these stacks have shown >95% reflectivity at 355 nm and normal incidence whilst retaining a high LIDT. Depth profiling using Dynamic Secondary Ion Mass Spectrometry (DSIMS) and Rutherford back scattering (RBS) through these multilayer coatings has revealed the effect of increasing the number of layers in the stack. Results are presented for both spin and dip coated multilayers and a significant difference in the interfacial boundary is seen between the two coating processes. These differences are related to changes in the LIDT of the coatings. Individual layer thicknesses were estimated using this technique and compared to values gained from UV-Visible spectroscopy. TEM analysis of an ion-milled cross-section of the multilayers was performed showing the colloidal silica coatings and the depth of interpenetration of the interfaces.     

Sol–gel preparation of double-layer tri-wavelength antireflective coating

Double-layer tri-wavelength antireflective (AR) coating effective simultaneously at 351, 527 and 1,053?nm has been designed and prepared by the sol?Cgel process. The refractive index and film thickness of bottom layer and up layer are 1.27 and 113?nm, and 1.17 and 245?nm, respectively. The bottom layer with refractive index of 1.27 was prepared from a mixture of acid-catalyzed and base-catalyzed silica sols, and the up layer with refractive index of 1.17 was prepared from polypropylene oxide modified silica sol. It was found that the addition of polypropylene oxide into the sol significantly decreased the refractive index and increased the hydrophobicity of the AR coating. The obtained tri-wavelength AR coating gives very high transmittance of 99.7, 99.1 and 98.0?% at 351, 527 and 1,053?nm, simultaneously.     

Highly transparent superhydrophobic thin film with low refractive index prepared by one-step coating of modified silica nanoparticles

An easy and effective method to prepare superhydrophobic thin film has been developed. The film with optically transparent and low refractive index was composed by one-step coating with modified silica nanoparticles. The silica nanoparticles were prepared by sol–gel process of hydrolysis and condensation of alkoxysilane compounds and then surface modification silica nanoparticles, 50 ± 10 nm, were accomplished using methoxytrimethylsilane (MOTMS). Water contact angle of film increased with the weight of MOTMS of silica sol. When the weight of MOTMS was optimized, the water contact angle and sliding angle of film were 152.8° and less than 10°, respectively. The transmittance of film was also increased as compared to the un-coated microscope glass slide, from 91 to 93.5 %. The refractive index of the film was approximately 1.09 as measured by ellipsometer. The superhydrphobic thin film was also successfully made by using spray coating and the water contact angle of this film was more than 160°. Surface morphology of difference coating methods, dip and spray, were studied. Our result suggests that the film can be applied for superhydrophobicity and optical applications.     

Fabrication of PZT/CuO composite films and their photovoltaic properties

In this paper, we reported the design and preparation of a double-layer antireflective (AR) coating, which possessed relatively high transmittance at 351, 527, and 1053?nm. The refractive indices and film thicknesses of the under layer and upper layer of the simulated AR coating were determined as 1.27, 95?nm and 1.18, 106?nm, respectively. The under layer of the double-layer coating dip-coated from a mixture of base-catalyzed and acid-catalyzed silica sols had a refractive index of 1.27. The upper layer fabricated by the deposition of methylated silica nanoparticles by simply adding methyltriethoxysilane into the base-catalyzed silica sols possessed a refractive index of 1.18. The hydrophobicity of coatings could be dramatically improved with the water contact angle increasing from 23.4° to 150.0°, and the refractive indices of the pure base-catalyzed silica coatings were easily decreased from 1.20 to 1.12 through the surface treatment of silica nanoparticles. Thus, we have successfully prepared a double-layer AR coating, which had a high transmittance of 99.8%, 96.1%, and 99.7% at 351, 527, and 1053?nm, respectively.

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Room-Temperature Mirror Preparation Using Sol-Gel Chemistry and Laminar-Flow Coating Technique

The CEA/DAM megajoule-class pulsed Nd:glass laser devoted to Inertial Confinement Fusion (ICF) research will require 240 cavity-end mirrors. The approved laser design necessitates 42-cm × 46-cm × 9-cm highly-reflective (HR)-coated substrates representing more than 50 m² of coated area. Prototypes of these dielectric mirrors were prepared with interference quaterwave stacks of SiO₂ and ZrO₂-PVP (PolyVinylPyrrolidone) thin films starting from sol-gel colloidal suspensions (sols). Low refractive index material was based on nanosized silica particles and high refractive index coating solution was made of a composite system. The colloidal/polymeric ratio in the composite system has been optimized regarding refractive index value, laser damage threshold and chemical interactions have been studied using FT-IR spectroscopy. A deposition technique so-called Laminar Flow Coating (LFC) has been associated to sol-gel chemistry for HR laser damage-resistant sol-gel coating development. This novel coating method confirmed its main advantages compared to dipping or spinning processes: coating large flat square substrates at room temperature with small solution consumption, good thickness uniformity, weak edge-effects, induced stress-free coating, good optical properties and laser damage resistance fulfilling.     

Characterization of individual layers of an optical design based multilayered antireflection coating developed by sol–gel method

Multilayered non-quarter wavelength (design wavelength ??₀?=?908?nm) based antireflection coating (ARC) has been developed by sol?Cgel process. Simulation of a 3-layers ARC design with the materials of refractive index ranging between 1.15 and 1.65 has been done to fulfil the experimental processing for deposition of specific thickness and refractive index. Colloidal sol based silica, polymeric sol based silica and complexed solution based zirconia were used for the fabrication of AR layers. The microstructural studies of the individual layer and also of ARC using FESEM, EDX, AFM and XRD were done separately. Surface roughnesses were found gradually decreasing by successive deposition of zirconia, colloidal silica and polymeric silica layers. Optical performance of the synthesized ARC (%0.67, at ??₀) was close to that of (%0.63, at ??₀) the theoretically designed value.     

Properties and production of F-doped silica glass

Fluorine-doped silica glass is an optical material with high transmission in the vacuum ultraviolet region, specifically at 157 nm. This feature, along with low thermal expansion and ease of polishing make it the material of choice for the photomask substrate for the 157 nm lithography node. We report the synthesis of fluorine-doped silica glass using various dopants. Characterization of the glasses was achieved by measuring vacuum UV and IR spectra and by refractive index measurements. Transmission and refractive index are both found to depend on fluorine concentration.     

Preparation of nearly monodisperse multiply coated submicrospheres with a high refractive index

Nearly monodisperse SiO2/TiO2/SiO2 multiply coated submicrospheres with nearly monodisperse silica submicrospheres as cores, thick titania layers, and thin silica skin were prepared to increase the refractive index of complex submicrospheres while keeping their near monodispersity and perfect surface properties. Nearly monodisperse colloidal silica submicrospheres as cores with a diameter of 200 nm were synthesized by a seeding technique on the basis of the hydrolysis of tetraethyl orthosilicate (TEOS) in an aqueous ethanol solution with ammonia as catalyst. On the basis of the hydrolysis of tetrabutyl orthotitanate, a procedure combining continuous feeding with multistep coating was determined to prepare titania coatings about 40 nm thick and increase the refractive index of the complex submicrospheres. The hydrolysis of TEOS was still used to get the outmost silica coating about 10 nm thick on titania coated silica submicrospheres to eliminate random aggregation caused by the TiO2 surface properties of the TiO2/SiO2 complex submicrospheres during the final fabrication of photonic crystals.     

Electrooptical properties of gel-glass dispersed liquid crystals

Liquid crystal microdroplets can be trapped into silica gel-glasses by sol-gel processes. Several silica matrices containing different non-reacting organic radicals have been employed for preparing gel-glass dispersed liquid crystals (GDLC). The optical properties of thin GDLC layers may be modified by coating the material with transparent electrodes and applying external AC-voltage signals. Switching between opaque and transparent states is achieved. The sample transmission depends on the scattering properties of the material; no polarizers are required. The transmission of the transparent state is improved by matching the refractive indices of the gel-glass matrix and the liquid crystal. This can be achieved by adding high refractive index dopants to the starting mixture, or by substituting the non-reacting organic group of the silica matrix. Both approaches have been tested. Phenyl and methyl monomers have been used as well as titanium-based dopants. The resulting GDLC materials have been optimized by controlling the starting mixture composition, the reaction conditions, and the film preparation. The optics, dynamics, and stability of GDLC films are analyzed.     

Organopolysiloxanes as Chemically Sensitive Coatings for Optical Fibers

Various types of UV-curable organically modified siloxanes have been synthesized by the sol-gel method with the aim of fabricating chemically sensitive coatings for silica optical fibers. The refractive index of the coating material can be tailored in the range from 1.46 to 1.56 and sensitivity towards CO2 is achieved by incorporation of amino groups. The interaction of the cured layers with CO2 or with hydrocarbons has been studied in immersion experiments. Both the reaction of CO2 with incorporated amino groups and the penetration of hydrocarbons into the layer induce changes of the light absorption coefficient and the refractive index of the coating which are detected by measuring the output light intensity from the fiber.     

Polyvinylpyrrolidone/ZrO<Subscript>2</Subscript>-based sol–gel films applied in highly reflective mirrors for inertial confinement fusion

Polyvinylpyrrolidone (PVP)/ZrO₂-based hybrid thin films, with suitable properties for application in the high power lasers, have been prepared combining the advantages of both the sol–gel route and the organic-inorganic hybrid materials. By virtue of light scattering analysis, the chemical composition of the reaction system was fully optimized, endowing the films with good optical properties and high laser-induced damage threshold (LIDT). Light scattering studies also provided valuable structural information about the hybrid sols, which offered a better understanding of the structure and performance of the hybrid films. Our experiments showed that, in the hybrid sols, the incomplete substitution for the chelating ligands by the hydroxyls might considerably conceal and weaken the effect of PVP on the nucleation and growth of ZrO₂ particles. Thus, the incorporation of PVP only resulted in slight decreases in the refractive index and LIDT of the films. By energy relaxation through their flexible polymer chains, however, the addition of PVP could easily enhance the stress compatibility between the high- and low-index layers and then facilitate the deposition of the multi-layer highly reflective mirrors.     

Investigations of MgF2 Optical Thin Films Prepared from Autoclaved Sol

MgF₂ thin films with ultra low refractive indices were obtained by sol–gel method using sols prepared from magnesium acetate and hydrofluoric acid. The sols were autoclaved in a Teflon cell at 100–180°C for 24 h and then coated on SiO₂ glass or CaF₂ crystal substrates by spin coating. Subsequently, the samples were heat treated at 150°C for 1 h. The optical properties of the thin films, such as refractive index and transmittance, were investigated in the UV region, especially in the deep UV (DUV, below 250 nm) region. Though the coatings consist of single layers, both samples exhibited high transmittance. In addition, they exhibit such antireflection effect over a very wide range of wavelength. Such a good antireflection effect was produced only by low refractive indices. It has also been confirmed that, even in the range of vacuum UV (VUV, 200–150 nm), CaF₂ samples with antireflection coatings could keep such a good antireflection effect.     

High refractive index hyperbranched polyvinylsulfides for planar one‐dimensional all‐polymer photonic crystals

We report on the growth and characterization of one‐dimensional (1D) planar all‐polymer photonic crystals (PhC) with high dielectric contrast (Δn = 0.3) prepared by spin coating using hyperbranched polyvinylsulfide polymers (HB‐PVS) as high refractive index material and cellulose acetate as low refractive index material. Solution processable HB‐PVS show a near ultraviolet absorption inducing an increased refractive index in the visible‐near infrared (n = 1.68, λ = 1000 nm). HBPVS:Cellulose Acetate Distributed Bragg Reflectors show a very clear fingerprint of the photonic band gap possessing the expected polarized dispersion properties as a function of the incidence angle. Moreover, engineered microcavities tuned on the weak fluorescence spectrum of the HB‐PVS show directional fluorescence enhancement effects due to spectral redistribution of the emission oscillator strength. The combination of all these properties testifies the high optical quality of the obtained photonic structures thus indicating HB‐PVS as an interesting material for the preparation of such PhC. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 73–80     

Sol-Gel Processing of Zirconia Coating for HR Mirrors with High Laser Damage Threshold

High laser-damage resistant coatings are very important in high power laser systems. In this study ZrO₂ thin films are prepared by sol-gel spin-coating technology from suitable zirconia aqueous colloidal suspensions containing nano-crystalline ZrO₂ at room temperature synthesized by a hydrothermal process from an inorganic precursor (ZrOCl₂·8H₂O). By adding a soluble organic binder PVP to the suspension prior to application, it is possible to substantially increase the coating refractive index and the abrasion-resistance as well as the laser damage threshold. The features of the coatings and the colloidal suspensions are investigated. Multilayer highly reflective dielectric coatings are also elaborated by applying quarterwave-thick alternating coatings of the binder-aided zirconia and silica, which is prepared with the sol-gel process from TEOS. To achieve 99% reflectivity, 19–21 layers are required. Single shot laser damage tests are carried out using a high power laser at 1064 nm wavelength with a pulse duration of 2.5 ns. The laser damage thresholds of 18 and 15 J/cm² are achieved for single ZrO₂-PVP coating and ZrO₂-PVP/SiO₂ multilayers respectively.     

Sol–gel preparation of antireflective coatings at 351 nm with different thickness and improved moisture-resistance

Antireflective (AR) coatings at 351 nm with different thickness were designed and prepared by sol–gel process using tetraethylorthosilicate as precursor and ammonia as catalyst. The parameters of these coatings, including film thickness and refractive index, were calculated by optical formula and the coatings were prepared accordingly. Sol dilution method was used to adjust the film thickness. The wavelengths of maximum transmission measured by UV–Vis spectrophotometer, were used to monitor the film thickness. It was found that AR coatings with higher thickness possess better abrasion-resistance. Hydroxyl terminated polydimethylsiloxane (PDMS) was added into pure silica sol to improve both the abrasion-resistance and moisture-resistance of AR coating.     

Preparation of thin silica films with controlled thickness and tunable refractive index

Silica films with controlled thickness and refractive index have been formed by the sequential adsorption of a cationic polyelectrolyte and silica sols. The conditions used to prepare the sol were varied, and allowed films with refractive indices as low as 1.16 to be obtained. The sequential adsorption technique allows the thickness of these films to be controlled in increments of 5-10 nm, depending on the desired refractive index. Scanning electron microscopy revealed that a low packing density of constituent silica particles was responsible for the low indices of these films. The as-adsorbed films are thermally robust; calcination at 500 degrees C resulted in only very small decreases in film thickness (by < or =1.8%) and refractive index (to as low as 1.14). After calcination, the silica films remained hydrophilic and sorbed water vapor from the atmosphere. As a result, the refractive indices of these films increased with increasing relative humidity (RH). The dependence of the refractive index on RH was eliminated by treating the calcined films with trimethylchlorosilane.     

High Photosensitive Sol-Gel Hybrid Materials for Direct Photo-Imprinting of Micro-Optics

Sol-gel derived inorganic-organic hybrid materials (HYBRIMERs) were found to have high photosensitivity arising from large changes in refractive index and volume upon exposure to light. These materials combined different photosensitive mechanisms due to the presence of polymers and silica in the HYBRIMER structure. Photo-induced densification of a germanium-doped HYBRIMER gave an increase in refractive index accompanying a volume contraction. A methacrylate HYBRIMER was decomposed by long UV illumination, giving a low refractive index and resulting in a reduction in film thickness. Also, photo-initiators were locked inside the methacrylate HYBRIMER during illumination, giving a simultaneous increase in the refractive index and film thickness. Direct photo-imprinting using the photosensitive HYBRIMER was demonstrated for simple fabrication of gratings, microlens, and waveguides using a photomask, a phase mask, an interferometer, and laser writing.     

Study of spin coated multilayer zirconia and silica films for non-quarterwavelength optical design based antireflection effect at 1,054 nm

A non-quarterwavelength optical design (design wavelength, λ_o = 1,054 nm) based antireflection (AR) coating was prepared by sol–gel spin coating technique. Two materials, zirconia and silica were chosen for the deposition of AR layers on borosilicate crown glass, refractive index (R. I. = 1.51). For this design, the bottom and middle layers were of zirconia with the R. I. range 1.941–1.958 while the top layer was of silica with R. I. 1.455. To understand the surface feature after each deposition, refractive index and physical thickness of the layers were measured ellipsometrically (λ = 632.8 nm) at different points over the area, 10 mm × 10 mm with an interval of 0.5 mm along the centre based perpendicular projection made on an imaginary chord. The surface feature was examined by plotting the measured values of the optical parameters against the displacement. The surface roughness decreased with increasing layers. This was verified by the study of AFM images of the layers. Specular reflection of the antireflection coated product at λ₀ was comparable to that of the theoretically simulated curve.     

Atomic force microscopy of coated glasses

Atomic force microscopy has been used to investigate the topology of alkoxide gel dip coatings on different substrates. Results of SiO(2) - TiO(2) - ZrO(2) (STZ) coatings are presented on float glass, on polished fused silica, on commercially coated insulating flat glass, and on PtRh. Consolidated STZ coatings display the so-called glass pattern with ripples equal or less than 2 nm high. The same pattern is seen on partially dense STZ coatings, as soon as the surface is stiff enough for scanning, and also on the bottom of a 50 nm deep sputtering crater in the consolidated coating. The vitreous STZ coating on the fire side of the float glass is as flat as the float glass itself. It has the same tendency to contamination. 100 nm wide and 50 nm deep polishing grooves on fused silica have been filled up with the 80 nm thick coating, only dips of a few nm remain. The trenches between the SnO(2) crystallites on the insulating flat glass were filled up and the roughness of the substrate was partially reduced. PtRh sheet remained rough even after the coating. On the partially densified STZ coating, sputtering generates a grained surface.     

Low Temperature Preparation of High Refractive Index and Mechanically Resistant Sol-gel TiO2 Films for Multilayer Antireflective Coating Applications

TiO₂ sol-gel thin films have attracted a large attention for applications which require high refractive index transparent layers. In this work, sol-gel TiO₂ layers were prepared by Aerosol-gel deposition followed by a thermal treatment procedure in air. Depending on the experimental conditions, abrasion resistant and high refractive index layers could be obtained after post-treatment at only 110°C. In this paper, the experimental parameters which allow the preparation of functional TiO₂ sol-gel layers at such low temperature are discussed. It is concluded that the preparation of high refractive index and mechanically resistant TiO₂ layers can be interpreted in terms of competition between polycondensation and densification mechanisms. This result allows to envisage the sol-gel processing at low temperature of multilayer antireflective coatings.