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Stable nanosized particles of niobium and tantalum oxides were obtained by hydrolysis of the metal ethoxides in ethanol with triethylamine. These sols were amorphous hydrated oxides M2O5·nH2O. They were used to prepare thin porous films on SiO2 disks via spin-coating techniques. These coatings displayed thicknesses ranging from 100 to 300 nm and a refractive index of about 1.7. Laser damage tests at 1053-nm wavelength with a pulse duration of 3 ns were conducted on the single-layer systems. Ta2O5 coatings exhibited much higher threshold values (averaging 14.5 ± 2.1 J/cm2) than those of Nb2O5 (averaging 8.3 ± 1.6 J/cm2). Buildup of multilayer stacks of high- and low-index materials was prevented because of internal stress.

Nanosized needles of ScO(OH) and lozenge shaped platelets of γ-ScO(OH) were prepared by hydrothermal syntheses (200 °C, 10 h) using ScCl3·6H2O as starting material. The influence of hydrothermal conditions on the nature of the phase, the morphology and the particle size was investigated. Characterization was achieved by XRD, TEM, FT-IR and TGA. Increasing the acidity of the reaction medium increased the particle size whereas increasing the concentration of the ScCl3 solutions favored the formation of the platelet type ScO(OH). The NaCl and NH4Cl mineralizers increased agglomeration between particles as well as the temperatures of transformation into oxides (480 to 530 °C). The point of zero charge was determined and found to be 6.7 for scandium oxide and 5.7 for the scandium oxohydroxide as prepared. Stabilization of the colloidal suspensions of ScO(OH) was achieved in aqueous-alcoholic media at pH = 3 by using [3-(2-aminoethyl)aminopropyl]trimethoxysilane (DAMO). The stabilization is discussed in terms of surface coordination chemistry. These hybrid sols were used to obtain coatings of crystalline scandia on glass substrates by spin-coating. Thermal decomposition of scandium formate deposited by dip-coating via aqueous alcoholic media with PVP as an additive was an alternative for producing coatings of amorphous scandia with good optical properties.

CEA Limeil-Valenton has embarked in a project called 'laser megajoule' (LMJ) consisting of the construction of a 2-MJ/500- TW (351-nm) pulsed Nd:glass laser and devoted to inertial confinement fusion research in France. Room temperature and atmospheric pressure deposited sol-gel coatings for antireflective (AR), highly reflective (HR) and polarizer uses, and silicone films for environmental protection have displayed remarkable optical and laser strength performance. Such coatings can be applied onto large area and at a low cost compared to conventional vacuum deposition techniques. CEA Limeil-Valenton is also maintaining sustained efforts to promote the sol-gel technology in other areas.

The Centre d'Etudes de Limeil-Valenton is currently involved in a project which consists of the construction of a 2 KJ/500TW pulsed Nd:glass laser and which will be devoted to inertial confinement fusion research in France. With 240 laser beams and almost 10 000 m2 in coated area required, the proposed megajoule-class laser will be the largest laser system ever built in the world. The proposed laser conceptual design necessitates 44-cm by 44-cm by 6-cm cavity-end mirrors representing more than 50-m2 of coated area. These dielectric mirrors are made of quarterwave stacks of SiO2 and ZrO2-PVP and are prepared from colloidal suspensions. After a sustained search effort, we have prepared mirrored coatings with up to 99 percent damage thresholds ranging 8-10 J/cm2 at 1053- nm wavelength and with 3-ns pulse duration were achieved. Mirrors with good coating uniformity and weak edge-effect could be produced by dip-coating and laminar-flow coating at room temperature and atmospheric pressure. To satisfy the coating demand, coating equipments and a 1000-m2 sol-gel coating production plant are planned to be built at REOSC- Group SFIM company in order to start producing within schedule and cost goal.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

It is well established by manufacturers and users that optical coatings are generally prepared by the well known Physical Vapor Deposition (PVD) technology. In the authors' opinion sol-gel technology is an effective and competitive alternative. The aim of this paper is to emphasize on the sol-gel thin film work carried out at CEA Limeil-Valenton and concerning the technology for high power lasers. We will briefly discuss the chemistry of the sol-gel process, the production of optical coatings and the related deposition techniques. Finally, the paper describes performance of sol-gel optical coatings we have developed to fulfill the requirements of a future 1.8 MJ I500TW (351 nm) pulsed Nd:glass laser so-called << LMJ << (Laser MegaJoules). This powerful laser is to be used for our national Inertial Confinement Fusion (ICF) program, to demonstrate at the laboratory scale, ignition of deuterium-tritium fusion fuel. Moreover, the aim of this article is, hopefully, to provide a convincing argument that coatings and particularly optical coatings, are some of the useful products available from sol-gel technology, and that exciting developments in other areas than high power laser technology are almost certain to emerge within the coming decade. Keywords : sol-gel, oxides, colloidal suspensions, optical coatings, laser damage

We investigate an innovative sol-gel method that allows coating of mineral or plastic substrates with colloidal oxide-based materials and/or inorganic-organic composites and/or hybrids. To date, we have developed narrow and broadband antireflective coatings, multilayered dielectric mirrors, and a few polarizing thin films. These sol-gel optical coatings have demonstrated adequate performance for use in our proposed 2MJ/600TW (351 nm_ Nd:glass laser. To prepare such optical thin films, sol-gel technology offers outstanding technical and economic advantages over physical vapor deposition. Sol-gel technology opens new prospects in a variety of coating applications, and few areas offer greater payoff for interaction across disciplines than that of sol-gel-derived thin films.

The adhesion of sol-gel antireflective porous silica coatings on vitreous optical substrates has been dramatically improved by exposure to ammonia vapors or a dip in basic solutions. The approximately 70 to 270-nm thick coatings consisted of monolayers of spherical, 20-nm diameter amorphous silica particles deposited from ethanolic colloidal suspensions by conventional liquid coating techniques. Although, the as-deposited coatings had only low adhesion and were easily damaged when cleaned by standard drag-wiping procedures, coatings exposed over 5 hours to ammonia vapors passed both adhesive-tape and moderate abrasive- resistance tests. The increase in strength was accompanied by a roughly 20% shrinkage of the original coating thickness but the antireflective properties were retained. Our explanation of this chemical effect is a base-catalyzed phenomenon leading to surface silanol condensation and hydrogen-bonding of neighbor silica particles. In addition, since this basic treatment enhanced the laser damage resistance, such strengthened antireflective coatings have been successfully evaluated on flashlamps used on Phebus, Europe's most powerful laser. This allows an increase of the laser-disk pumping efficiency.

The cost of a large laser system is a strong function of the overall electrical to final photon efficiency. To improve the stored energy and therefore the pumping efficiency of sophisticated and costly laser-glass amplifiers, we have developed a novel two-layer broadband antireflective coating for the blast-shield component. The blast-shield is an optic placed between the flashlamps and the laser disk amplifiers to prevent damage of laser disks by possible explosion of a flashlamp. The sol-gel antireflective coating was dip-coated at room temperature onto 8-cm diameter glass samples. The coating basically consisted of a halfwave- thick high-index material such as ZrO2-PVP (PolyVinyl Pyrrolidone) and a quarterwave- thick low-index material such as SiO2-siloxane. To improve the abrasion resistance of the coated part, a lubricating and water-repellent material was applied as a very thin overcoat. In addition to a 6.5 to 7.2% transmission gain over the spectrum of interest, the coating was moderately abrasion resistant and chemically durable. Flashlamp-induced damage to the antireflective coating for 1000 glow discharges at 10 to 12 J/cm2 were minimal and similar to uncoated parts.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe a new coating method Laminar Flow Coating (LFC) technique developed to obtain highly reflective (HR) laser damage resistant sol-gel multidielectric coatings. Such coatings are used in high-power lasers for inertial confinement fusion experiments (ICF). This technique uses substrates in an upside-down position and a travelling wave of coating solution is transported with a laminar motion under the substrate surface with a tubular dispense unit. This creates a thin-film coating by solvent evaporation. Satisfactory results have been obtained on 20-cm square glass substrates regarding the optical performances, the thickness uniformity, the edge-effects and the laser damage resistance. This deposition technique combines the advantages of both classical techniques: the non-exclusive substrate geometry such as in dip-coating and the small solution consumption such as in spin-coating.

A novel quarterwave-thick narrow-bandwidth antireflective coating has been developed for both plastic and vitreous substrates by a sol-gel route. This coating has revealed pronounced scratch- and climatic-resistance under adverse conditions. The single-layer coating consists basically of a composite material made of silica as the discontinuous phase and of a polytetrafluoroethylene-derived (Teflon1) organic polymer as the continuous phase. This leads to fluorine-containing colloidal silica product, or a so-called Flucosil coating. The coating is applied by spinning or dipping from specific solutions at room temperature followed by a mild and short heat treatment. In addition to remarkable abrasion and environmental resistance properties, such coatings have displayed excellent laser-induced damage threshold levels, surpassing uncoated substrates.

We describe a new coating method "Laminar Flow Coating"* (LFC) technique developed to obtain highly reflective (HR) laser damage resistant sol-gel multidielectric coatings. Such coatings are used in highpower lasers for inertial confinement fusion experiments (ICF). This technique uses substrates in an upsidedown position and a travelling wave of coating solution is laminary transported under the substrate surface with a tubular dispense unit. This creates a thin-film coating by the solvent evaporation. Satisfactory results have been obtained onto 20-cm square glass substrates regarding the optical performances, the tickness uniformity, the edge-effects and the laser damage resistance. This deposition technique combines the advantages of both classical techniques : the substrate non-exclusive geometry such as in dip-coating and the small solution consumption such as in spin-coating. The association of sol-gel colloidal suspensions and LFC coating process has been demonstrated as a promising way to produce cheap specific optical coatings1.

In a collaborative effort with some french industrial and university laboratories and also in contact with our colleagues from the Lawrence Livermore National Laboratory (LLNL), we have developed at Limeil-Valenton National Laboratory (LVNL) various laser-performant and inexpensive dielectric mirrors made of a resin-replicated substrate and a specific colloidal sol-gel coating. To date, we have obtained 8" to 12" diameter molded optical surfaces with a flatness as good as A./10 peak to valley at 633-nm wavelength with a roughness ranging 10-15 A RMS. We have been able to acheive coatingreflectivity exceeding 99 % using room temperature spin-coating deposition of alternate silica and alumina quarterwave layers, and conserving the original substrate flatness. Such sol-gel coated replicated mirrors averaged 13.5 i/cm2 in a single shot irradiation mode and 15.1 i/cm2 with a laser-annealing procedure at 1064-nm and with 3-ns pulses. The present cost of such a product is about 4-6 times cheaper than a similar component conventionally e-beam coated.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Pursuing our official collaborative ICF-laser search and development program at Limeil National Laboratory (LNL) with the Lawrence Livermore National Laboratory (LLNL) for high damage threshold optical coatings, we have now formally established that Sol-Gel highly reflective coatings prepared from colloids are superior to those prepared from solutions of precursor materials. Thus, single layer coatings of bohemite A1203.H20 have been prepared on fused silica and BK-7 substrates from aqueous colloidal suspensions of hydrated alumina at room temperature. Such coatings were porous and therefore revealed a measured refractive index of about 1.44, lower than the relevant dense material. These single coats when laser damage tested, have exhibited thresholds, under S-on-i irradiation mode (S shots at the same fluence onto a selected site) of respectively i2-i4 J/cm2 with 3-ns pulses and 35-50 i/cm2 with-i6 ns pulses at 30-Hz. Multilayer, high reflectivity, dielectric coatings were also prepared at room temperature by laying down quarterwave-thick, alternating coats of this alumina with silica also prepared from colloidal sol, and having a refractive index of 1.22. To achieve 99% reflectivity 32 to 36 total layers were required. Such HR-coatings revealed damage thresholds as good as those of single layers of the constituting oxides in the same laser conditions.

Hydrolysis of niobium and tantalum pentaethoxides in ethanol and in the presence of basic (ammonia, tetramethylammonium hydroxide, di and triethylamine) or acidic (nitric or hydrochloric acids) additives was investigated. Triethylamine gives monodispersed, small particles of amorphous M2O5,nH2O (M = Nb, Ta). These colloidal suspensions were used to obtain thin films by spin-coating techniques. The coatings display a thickness of 100–300 nm and a refractive index around 1.7. Laser damage tests at 1064 nm wavelength with a pulse length of 3 ns were carried out on the single layer systems. The threshold values (one-onone) are in favor of the Ta2O5 coatings with an average of 14.5 ± 2.1 J/cm2 , by comparison with 8.3 ± 1.6 J/cm2 for the Nb2O5 films.

In our collaborative continuing ICF-laser search program with the Lawrence Livermore National Laboratory for high damage threshold optical coatings, we have now established that sol-gel HR-coatings prepared from colloidal oxide suspensions are superior to those prepared from solutions of precursor materials. Our investigations of single coatings of TiO 2 , ZrO 2 , HfO 2 , ThO 2 and Al 2 O 3 · H 2 O have revealed that those from Al 2 O 3 · H 2 O and ThO 2 have the highest laser damage threshold at 1064 nm wavelength. We have prepared highly reflective (HR) coatings with various oxide pairs by stacking the different high-index materials and SiO 2 , also prepared from suspension, in the conventional manner, and have achieved reflectivity of 99% with 15–35 layers depending of the oxide association choice. Disappointingly, the damage thresholds of these HR coatings have only been about half those of single coats of the component oxides.