Ian M. Thomas

High laser damage threshold porous silica antireflective coating

A quarterwave-thick narrow-bandwidth antireflective coating for fused silica optical components and KDP crystals has been developed. The coating consists of porous silica prepared from a silica sol in ethanol. It is applied by dip or spin from a solution at room temperature and requires no further treatment. The laser damage threshold levels are about equal to the surface damage thresholds of the uncoated substrates.

Optical properties and laser demonstrations of Nd-doped sol-gel silica glasses

Abstract Nd-doped sol-gel silica glasses possessing optical quality that was adequate for a simple demonstration of laser action have been prepared. The highest quantum yield and longest emission lifetime obtained were 55% and 425 μs, respectively, for an Nd doping level of 0.36 × 1020 cm−3. The optical scattering loss was found to be on the order of 4.5%/cm. The sol-gel samples were prepared using a novel combination of techniques, including, for example, the employment of a particulate silica filler material to ensure crack-free drying, and the addition of propylene oxide to reduce the gelation time.

Method for the preparation of porous silica antireflection coatings varying in refractive index from 1. 22 to 1. 44

Coatings of refractive index that vary between 1.22 and 1.44 were prepared from combinations of a colloidal silica suspension and a polysiloxane solution. The polysiloxane acted both as a binder and a filler for the colloidal silica particles. Increasing ratios gave coatings of lower porosity, hence higher refractive index, and better abrasion resistance. This variation in refractive index allowed highefficiency quarter-wave antireflection coatings to be prepared on substrates whose index varied from 1.45 to 2.1. The laser damage thresholds of all the coatings were >50 J/cm(2) at a wavelength of 1.06 microm with a 10-ns pulse length.

Surface chemistry and trimethylsilyl functionalization of Stöber silica sols

Abstract Various silica sols, with different surface chemistries, were reacted in solvent dispersions with hexamethyldisilazane (HMDS) or ethoxytrimethylsilane (ETMS) to produce hydrophobic, trimethylsilyl (TMS) functionalized sols. 1H and 29Si nuclear magnetic resonance were used to quantify the surface species and the TMS surface coverage. The amount of TMS surface coverage, which ranged from 5% to 33%, was a strong function of the starting silica-surface chemistry and the HMDS reaction time. Sols with a greater hydrogen-bonded silanol surface (as opposed to an ethoxy surface or isolated silanol surface) resulted in greater TMS coverage. HMDS reacts with both the solvent (ethanol) and the silica surface. Reaction rate measurements suggested that the silica surface reacts with HMDS at short times (minutes) and then with ETMS, which is a product of the HMDS/ethanol reaction, at long times (days). High TMS coverage is required for sol stability in non-polar solvents; the colloid size was found to increase in decane for sols with poor TMS coverage. In addition, coatings made from TMS sols showed an 80× slower remaining ethoxy-surface hydrolysis rate upon exposure to humidity than untreated sols. These TMS sol films will be utilized as anti-reflection coatings on moisture sensitive optics (e.g., potassium dihydrogen phosphate (KDP) crystals) used in high-peak-power laser systems.

Random phase plates for beam smoothing on the Nova laser

We discuss the design and fabrication of 80-cm-diameter random phase plates for target-plane beam smoothing on the Nova laser. Random phase plates have been used in a variety of inertial confinement fusion target experiments, such as studying direct-drive hydrodynamic stability and producing spatially smooth x-ray backlighting sources. These phase plates were produced by using a novel sol-gel dip-coating technique developed by us. The sol-gel phase plates have a high optical damage threshold at the second- and third-harmonic wavelengths of the Nd:glass laser and have excellent optical performance.

Single-layer TiO 2 and multilayer TiO 2 –SiO 2 optical coatings prepared from colloidal suspensions

Single-layer coatings on fused silica substrates have been prepared from aqueous colloidal suspensions of titania at room temperature. These coatings were porous and had a refractive index of 1.9. Multilayer high reflectivity dielectric coatings were also prepared by laying down quarterwave thick alternating coats at room temperature of this titania with silica, prepared from a colloidal suspension of silica. Single-shot laser damage threshold measurements at 1064-nm wavelength with a pulse length of 1 ns were carried out for both the titania and titania-silica systems. These averaged 7.5 and 6.2 J/cm(2), respectively.

Non-Newtonian flow effects during spin coating large-area optical coatings with colloidal suspensions

Multilayer sol‐gel optical high reflectors with greater than 99% reflection have been prepared on substrates up to 20 cm in diameter by spin coating silica/alumina colloidal suspensions. These coatings are radially nonuniform, owing to the extensive shear‐thinning rheology of the high‐index alumina suspension. To a large degree the film thickness nonuniformity can be compensated for by the reflection bandwidth. The rheological properties of the alumina suspension under steady shear have been measured. The low‐shear reduced viscosity and the shear‐thinning time constant are shown to vary exponentially with φ2, where φ is the solids volume fraction. At φ=0.1 the sol has effectively gelled. A model for spincoating with a non‐Newtonian fluid has been developed that uses the Carreau rheology model to fit the measured viscometric data. Modeling and experimental results show that as long as these non‐Newtonian effects are sufficiently large (as in this case) the radial film uniformity is determined only by these...

Porous fluoride antireflective coatings

Quarterwave antireflective coatings of CaF(2) and MgF(2) have been prepared on fused silica and calcium fluoride substrates from colloidal suspensions of the relevant fluorides in methanol. These coatings have high optical efficiency and laser damage thresholds at 350 nm in the 6-8-J/cm(2) range for single shots of 0.6-ns pulse length and 20-25 J/cm(2) for 25-Hz multishots of 25-ns pulse length.

Optical coatings prepared from colloidal media

Abstract 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.

Synthesis of lanthanide and lanthanide-silicate aerogels

In this paper we report on the preparation of mixed lanthanide-silicate and pure lanthanide aerogels from the chlorides of erbium, praseodymium, and neodymium. A two-step sol-gel method is described for preparing the mixed aerogels, using a sub-stoichiometric amount of water in the first step to prepare a partially condensed silica-lanthanide precursor. The pure lanthanide aerogels are prepared directly from the chlorides using propylene oxide as a scavenger for reaction generated hydrochloric acid. The aerogel microstructures vary from colloidal for the pure lanthanide and high weight percent lanthanide-silicate aerogels to polymeric for the low weight percent lanthanide-silicate aerogels. This change in microstructure is also indicated by the BET analyses, which show that the surface areas decrease with increasing lanthanide concentrations. In general, we measured reductions of lanthanide contents during the supercritical drying step due to insufficient linking and subsequent “washing out” of the lanthanides from the gels. Also, the retention efficiency for the lanthanide improves with higher silica concentrations, making quantitative doping by this method practical only for the lower lanthanide concentrations.