Lianghong Yan

Ultralow-refractive-index optical thin films through nanoscale etching of ordered mesoporous silica films

A great deal of intensive research has been conducted to obtain high-quality transparent ultralow-refractive-index and ultralow-dielectric-constant thin films for microptics and microelectronics applications. Here, we report a simple procedure to prepare highly porous silica thin films with high optical quality and water resistance through nano-etching of mesoporous silica films followed by fluoroalkylsilane surface modification. The films possess an ultralow refractive index of 1.03 (800 nm) and an ultralow dielectric constant of 1.30 (100 kHz), to our knowledge the lowest values ever reported in thin film materials. The films are superhydrophobic (water contact angle=156  deg), thus exhibit high moisture stability.

Effect of Hydrophobic Modification on the Durability and Environmental Properties of Porous MgF2 Antireflective Films

Conventional porous MgF2 antireflective films show poor environmental durability due to its hydrophilic nature. To improve the film environmental durability, hydrophobic porous MgF2 antireflective films are prepared by a methyl silicone-modified MgF2 sol. Effects of the methyl silicone proportions on the film properties are examined in order to control the wettability, peak transmittance, and environmental durability of the films. The modification of methyl silicone can increase the film hydrophobicity, thus decreases the adsorption of water, and improves the environmental durability. However, large amounts of methyl silicone could increase the film refractive index, resulting in the decrease in peak transmittance for films on BK7 glass. Films with 40% silicone show water contact angle of 110°, peak transmittance of 98.7%, and good environmental durability. The results of accelerated test show that the environmental durability of the porous MgF2 films modified with 40% methyl silicone is six times higher than that of the conventional films.

Design and sol–gel preparation of a six-layer tri-wavelength ORMOSIL antireflective coating for a high power laser system

A six-layer tri-wavelength antireflective (TWAR) coating possessing nearly 100% transmittance simultaneously at 1064 nm, 532 nm and 355 nm was designed and realized by a simple sol–gel process. This TWAR coating consisted of six alternating layers of two quarterwave low refractive index components. The refractive indices of these two these two components are 1.21 and 1.14. Organically modified silicate (ORMOSIL) thin films with a refractive index of 1.21–1.13 were prepared by using tetramethylorthosilicate (TEOS) and dimethyldiethoxysilane (DDS) as co-precursors. A discussion of the crack problem on the interfaces of the multi-layer sol–gel silica AR coating was proposed, which was very important in the preparation of the multi-layer AR coating with high transmittance. Finally, the designed six-layer ORMOSIL TWAR coating was realized with a transmittance of 99.7%, 99.5% and 99.2% at 355 nm, 532 nm and 1064 nm, respectively. This is the best TWAR coating so far and will find great application in a high power laser system.

A simple method to control the microstructure and properties of sol–gel silica antireflective coatings

To control the microstructure and properties of sol–gel silica antireflective (AR) coatings, a simple PMHS (poly(methylhydrogen)siloxane) surface modification method was applied and investigated. This is accomplished simply by immersing the AR coating into the modifier solution containing PMHS, hexane and the Karstedt catalyst and then immediately withdrawing it at a desired withdrawal rate. A systematic study was carried out to investigate the effect of PMHS on the microstructure and optical, hydrophobic and mechanical properties of sol–gel silica AR coatings. It was found that PMHS surface modification greatly increased the hydrophobicity and mechanical properties of sol–gel silica AR coatings by increasing their water contact angles from 27.2° to about 120° and Youngs modulus from 317 MPa to 482 MPa. Most importantly, the transmittance of AR coatings can be tailored well between 96.4% and 100% by controlling the PMHS chain length, PMHS concentration and withdrawal rate. Speculation and verification of the PMHS surface modification process was discussed in detail after SEM characterization and optical simulation by TFCalc™ software. Finally, with this simple method, a hydrophobic sol–gel silica AR coating with excellent transmittance of 99.9%, improved mechanical properties and AR stability was realized, which can find great applications in high power laser fusion systems.

Preparation of silica coatings with continuously adjustable refractive indices and wettability properties via sol–gel method

Silica coatings with continuously adjustable refractive indices and wettability properties were prepared through a sol–gel base-catalyzed process. Adjustment of the molar ratio of water (H2O) to tetraethylorthosilicate (TEOS) was utilized to change the hydrolysis degree of the precursors, and hence change the morphology of the silica particles. With the increase in the H2O/TEOS molar ratio, the morphology of the silica particles changed from a linear net-work structure to a bead-like structure and then to a granular particle structure. A particle growth mechanism was proposed and verified by characterization. As the H2O/TEOS molar ratio increased from 0.3 to 21.0, the refractive indices of the silica coatings increased from 1.132 to 1.328. Meanwhile, a varied H2O/TEOS molar ratio also modulated the surface wettability of the silica coatings. The static water angle of the silica coatings decreased from 145° to 6° by increasing the H2O/TEOS molar ratio from 0.3 to 21.0. Different hydrophilic and hydrophobic coatings could be obtained by simply controlling the H2O/TEOS molar ratio. Silica coatings with different refractive indices and hydrophobic (or hydrophilic) properties were obtained at different H2O/TEOS molar ratios.

Preparation of sol-gel ZrO2/SiO2 multi-layer film based on UV-irradiation treatment

The transmittances of ZrO2/SiO2 25-layer film with and without UV-irradiation was compared with theoretical model. With UV-irradiation, infiltration was controlled in ZrO2/SiO2/ZrO2 three-layer film, and the refractive index had been increased by the treatment.

Hydro-oleophobic Silica antireflective films with high laser damage threshold

Hydro-oleophobic Silica antireflective films with high laser damage threshold were prepared by sol-gel process. The highest water contact angle of 136° and oil (peanut) contact angle of 93° were obtained. The highest optical transmittance and laser damage threshold (351nm, 1ns) were 99.5% and 6 J /cm2 respectively.