H. Angus MacLeod

ASSEMBLY AND MOLECULAR ORGANIZATION OF SELF-ASSEMBLED LIPID BILAYERS ON SOLID SUBSTRATES MONITORED BY SURFACE PLASMON RESONANCE SPECTROSCOPY

The structural properties of lipid films, made from a squalene/butanol solution containing varying amounts (0-15 mg/ml) of egg phosphatidylcholine and deposited on a thin metallic silver layer, were investigated using surface plasmon resonance (SPR) spectroscopy. Optical parameters (thickness, refractive index and extinction coefficient) of such supported self-assembled lipid membranes were obtained from a theoretical analysis of the experimental SPR curves. The mass of the lipid membrane and the area and volume occupied by one lipid molecule were also calculated. The results were consistent with the formation of durable and homogeneous lipid bilayers on the solid substrate, and indicated similarities in structural properties between the present lipid bilayers and freely suspended and Langmuir-Blodgett bilayer membranes. Such bilayers represent a simple model for biological membranes, as well as providing a means of immobilizing proteins for various practical applications, including receptor-based sensors and molecular devices. The results confirm the value of the SPR technique for investigating the properties of thin biomolecular dielectric films deposited on a metal surface.

Optical constants derivation for an inhomogeneous thin film from in situ transmission measurements

The optical constants of a thin film on its structure. A technique, based on transmission measurements carried out in vacuo, has been developed to derive the profiles of the refractive index and extinction coefficient. The interpretation of the profiles gives information on the layer structure in vacuo. The technique can be used as a means of monitoring the variations of the optical constants with changes in the deposition parameters. This paper presents the technique, which is based on an envelope method, and gives some experimental results.

Development of an automated scanning monochromator for monitoring thin films.

A scanning monochromator system for the monitoring of thin-film deposition in a box coater is described. The system employs data from both a quartz crystal oscillator and a wideband transmission spectrometer. The spectrometer uses a holographic grating as its dispersive element and a CCD array to collect the data. All data are sent to a microcomputer where the information is displayed, stored, and analyzed. Several applications, including measurement of optical constants of inhomogeneous films and characterization of moisture adsorption, are discussed.

Interference filters as nonlinear decision-making elements for three-spot pattern recognition and associative memories.

Simple patterns consisting of three spots (V and Gamma) have been recognized by dividing, shifting, and recombining beams onto bistable ZnS interference filters. This experiment demonstrates AND-gate operation, cascading, and a moderate amount of parallelism, but a laser power of several watts was required and the response times were several milliseconds. An associative memory for fingerprint identification has been constructed using a VanderLugt correlator and an interference filter as a reflective thresholding device.

Moisture-dependent anisotropic effects in optical coatings

Anistropic effects which depend on or are influenced by adsorbed moisture are discussed for films deposited at oblique incidence. Form birefringence reduces as water enters voids in a coating and the process of water transport is itself anisotropic. In some films water patches with an elliptical shape are formed during an early stage of the adsorption process. The distribution of optical radiation scattered from moisture patches in such a film is expected to be anisotropic. As metal–dielectric–metal filters were used in some of the observations, anisotropy of reflectance was investigated for both silver and aluminum.

Nonlinear Behaviour of Optical Coatings Subjected to Intense Laser Irradiation

Abstract Optical filters subjected to intense laser irradiation can experience changes in their optical properties due to nonlinearities in the optical constants of their materials. The origin of the nonlinearity can be due to the dependence of refractive index on temperature or electric field. A simulation of both cases has been developed and is presented here. The model is then applied to two particular designs, showing that reflecting components can be turned into transmitting devices if sufficient input power is used. The result is relevant to the case of high-energy laser mirrors and to rugate filters.

Reflectance optimization of second-surface silvered glass mirrors for concentrating solar power and concentrating photovoltaics application

Methods developed to maximize the overall reflectance of the second-surface silvered glass used in concentrating solar power (CSP) and concentrating photovoltaics (CPV) solar systems are reported. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.5% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 3.5% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric enhancement, was measured at National Renewable Energy Laboratory to have 95.4% solar weighted reflectance. For comparison, nonenhanced, wet-silvered reflectors of the same 4-mm thickness show reflectance ranging from 91.6% to 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. Finally, an experiment capable of determining which junction limits the triple junction cell is demonstrated.

Second-surface silvered glass solar mirrors of very high reflectance

This paper reports methods developed to maximize the overall reflectance second-surface silvered glass. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.4% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 2.7% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric boost, was measured at NREL to have 95.4% solar weighted reflectance. For comparison, non-enhanced, wetsilvered reflectors of the same 4 mm thickness show reflectance ranging from 91.6 - 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending.

Feature issue on optical interference coatings

The feature issue on Optical Interference Coatings, stimulated by the June 1992 Topical Meeting, covers the wider field of optical surface treatments after polishing. It is the latest in a series that has been running every four years since 1976.

Optical interference coatings: introduction by the feature editors

This introduction briefly reviews the history of the series of topical meetings on optical interference coatings and defines the scope of the features in this 15 July 1989 issue on the papers from the Fourth Topical Meeting.