Keith L. Lewis

Optical interference filters with continuous refractive index modulations by microwave plasma-assisted chemical vapor deposition

Optical interference filters that have continuously modulated refractive indices throughout their thicknesses (rugates) are designed and fabricated using a microwave plasma-assisted chemical vapor deposition technique. Examples are shown of single- and double-response narrowband reflection filters made from variable composition silicon oxynitride deposited on silica. Reflectivities in excess of 99.9% have been realized with bandwidths in the 2 to 10% range.

Advances in ultradurable phosphide-based broadband antireflection coatings for sand and rain erosion protection of infrared windows and domes

Pilkington Optronics (Barr & Stroud), in conjunction with the Defence Research Agency (Malvern, UK), has an ongoing development program for ultradurable coatings. Such coatings enhance environmental durability of infrared (IR) transmissive windows and domes for severe environments, such as those encountered in airborne systems. In particular, these coatings are required to provide protection against high velocity rain and sand impact. This program has to date produced one of the most effective sand and rain protective coatings, based on phosphide materials, specifically boron and gallium phosphide. The phosphide coatings are incorporated into anti-reflective (AR) multilayers, providing high transmission over the required IR waveband. Such AR coatings have been shown to be very effective in protecting windows and dome materials from rain and sand/dust impact damage. Results of single and multiple water jet impact tests, whirling arm rain erosion and sand erosion are presented. Current performance of AR coatings incorporating BP or GaP is presented. Combining BP and GaP in a composite structure, thereby maximizing IR transmission while maintaining sand and rain erosion protection, is described.

Non-Linear Effects in Thin Film Fullerene Structures

The fullerene materials are well known for their non-linear behaviour arising from excited state absorption processes. In solution, the processes can set in at low irradiance levels, although evidence is building that the magnitude of the effects is dependent on the solvent used in the experiments and the role of charge transfer processes. In evaporated films of C 60 , the solvent is absent and reduced non-linearities are seen as irradiances approach the laser damage threshold of the films. When the chromophor is incorporated into polymer films, non-linear processes are found at the same levels as the solution case provided that some residual solvent remains in the composite. Excited state absorption spectra suggest that some ionisation of the C 60 may be occuring.

1.064-μm laser damage studies of silicon oxy-nitride narrow band reflectors

In a paper presented at the 1992 Boulder Damage Symposium, we discussed the role of electric field effects, defect type, surface roughness, film thickness and coating absorption on the laser damage thresholds of sinusoidally modulated, plasma deposited, silicon oxy-nitride narrow band reflectors. We concluded that the damage threshold, which was essentially constant at 2 J/cm2 at the test wavelength of 0.532 micrometers , was defect dominated. A sizeable fraction of the damage events occurred at a particular type of defect--a hemispherical hillock feature typically 5 micrometers in diameter as identified by SEM and interferometric surface profiling. We postulated that this defect initiated damage because of either a microlensing effect or an enhanced electric field effect. We have since measured the laser damage thresholds of all these samples at 1.064 micrometers , and found significant variations in the damage thresholds, which were a factor of three higher on average than those at 0.532 micrometers . The microlens model presented can explain damage thresholds up to a factor of four higher at the longer wavelength, and predicts a minimum nodule height for increased damage susceptibility. The minimum nodule height is dependent on the wavelength and the coating average index. The wavelength scaling of the fluence enhancement and the minimum nodule height imply that nodule initiated damage will become an even more serious problem as the wavelength approaches the UV.

Ultradurable phosphide-based antireflection coatings for sand and rain erosion protection

Optical, mechanical and erosion protective characteristics of boron and gallium phosphide have been evaluated as single films and within anti-reflection multilayers. These coatings are shown to combine broad-band infra-red transmission with environmental durability, specifically in relation to abrasion resistance and elevated temperature performance up to 500 degree(s)C. Rain erosion protection of all common IR optical materials is demonstrated from single water jet impact and whirling arm tests. Protective characteristics in relation to solid particle impact are described. Productionizing of phosphide coating processes is well advanced in relation to control, scaling and handling of hazardous feedstocks.

TfI8: Pulsed plasma deposition of lead lanthanum zirconate titanate(PLZT) and lead lanthanum titanate(PLT)

Abstract Thin films of ferroelectric lanthanum-modified lead zirconate titanate (PLZT) and lanthanum-modified lead titanate (PLT) have been prepared using a novel high-powered pulsed plasma deposition technique. This method uses a microwave-induced intense discharge in a low pressure gas containing organometallic precursors of each of the required metal atoms, together with carbon dioxide and argon. In the discharge, the precursors dissociate to produce atoms which then recombine on the substrate to form the oxide film. Results are presented on the composition and crystal form of the films as the deposition conditions are varied. By careful control of deposition temperature, it is shown that a microcrystalline perovskite phase may be grown on a wide variety of substrate materials.

Laser damage in electron-beam-patterned surface resonant structures

Surface resonant structures can be used as spectral filtering elements. Both band-stop and band-pass designs are possible using capacitive and inductive mesh concepts respectively. With the current evolution in electron lithographic technology to allow the realisation of sub-micron features, it has become possible in recent years to produce resonant mesh arrays to high degrees of fidelity and performance over significant areas of optical substrates. These can provide, for example, responses in the infra-red spectral region using elements with a dimensional scale of a few microns. The unit cell design in the mesh can be relatively complex and can be tailored to provide specific spectral responses in the waveband range of interest. Such structures can find a wide variety of applications, especially as dichroic beam splitter elements for the separation of infra-red and RF radiation. The possibility of using such structures for spectral filtering has been known for many years, particularly for the microwave, millimeter and far infra-red wavelength regime. Some of the earlier applications in the far JR have been discussed by UJriCh [1,21 More recently, Byrne et al [31 fabricated capacitive and inductive mesh filters for the infra-red using electron beam lithography and demonstrated the bandstop and bandpass transmissive properties achievable. Byrne and coworkers used a lift-off technique to create capacitive mesh patterns having linewidths of less than O.25j.tm in O.lp.m thick aluminium and gold on calcium fluoride. Crossed capacitive dipoles of aluminium 2.6.tm in length with an aspect ratio of 10:1 produced a broad reflection band at 6.25tm. The measured bandwidth (FWHM) was about 2.5pm. For inductive designs, a two level lift-off approach was adopted, involving pattern transfer from an initial capacitive design using oxygen reactive ion etch techniques. A 1.8pm length slot with aspect ratio of 10:1 produced a resonance at 6.5.tm with peak transmittance of about 80% and bandwidth of 2.75p.m. Low levels of intrinsic absorption in the slot material of the inductive mesh are enhanced by the resonant effect of the mesh and reduce the level of transmittance achievable. It is of interest from the point of view of potential high power laser applications to explore the relationship between the level of optical absorbance and the ensuing laser damage threshold of such meshes. Mohebi [71 explored the response of wire grid polarisers to pulsed C02 laser irradiation at 10.6j.m and found that damage thresholds were largest for the case when the polarisation of the incident radiation was perpendicular to the wires. Antirerfiection coatings reduced the damage threshold. This work assesses the case of crossed dipole meshes and explores the role of absorption by incorporating weakly absorbing films in the meshes and by the addition of dielectric material between the absorber and the antenna plane. It also explores the role of resistive loss in determining the properties of capacitive meshes. The absorber used has optical constants of n = 2.9 - 0.25i which are typical of some transition metal oxides and chalcogenide materials.

Laser damage studies of silicon oxy-nitride narrowband reflectors

A series of sinusoidally modulated, plasma deposited, silicon oxy-nitride, narrow band reflectors have been examined with a view to understanding the relative roles of electric field effects, defect type, surface roughness, thickness, and coating absorption on the laser damage threshold. The damage threshold measurements were made at 0.532 micrometers with a range of spot sizes, a pulse length of 15 ns (full width at half maximum intensity), and each site was tested with 100 shots at a 10 Hz repetition rate. The damage threshold was essentially constant at around 2 J/cm2 for all the samples, and was defect dominated. Three types of topological defects were discovered using a WYKO three dimensional surface profiler, and one of the defect types was responsible for a large fraction of the damage events. It is postulated that this 5 micrometers hemispherical defect may behave either as a microlens which enhances the peak fluence that the underlying coating is subjected to, or as a center for enhanced electric field effects.

Boron phosphide IR coatings

Boron phosphide (BP) has outstanding mechanical, optical and thermal properties. It has been prepared as a single crystal and as a coating by various techniques. BP coatings have been shown to combine good broad-band transmission with exceptional durability and erosion protection of all common infrared optical materials. Their protective properties on various substrates have been assessed in detail in whirling arm rain erosion tests and by water jet impact measurements, and the thickness dependences determined. Comparisons have been drawn between BP and other durable coatings. The productionising of BP coating processes is well advanced after overcoming a number of problems, in particular the exceptional hazards presented by the feedstocks.

Interference filters using silicon/air micro-machined structures

Optical filters can be produced based on silicon/air micromachined structures. The various factors controlling the achievement of sharp resonance features will be explored. Performance levels of prototype devices for the infra-red will be described.