John Roy Sambles

THE ELECTRICAL RESISTIVITY OF GOLD FILMS

We present measurements of the d.c. electrical resistance of three classes of pure epitaxial gold films in the thickness range 30 to 900 nm. The combination of diverse morphological techniques with temperature-dependent data from 2 to 300 K enables us to apply a new theory based on those of Mayadas and Shatzkes for grain-boundary scattering, and of Soffer for surface scattering. Gold, evaporated and annealed on mica substrates, produced (111) films which gave a mean microscopic surface roughness to Fermi wavelength ratio, r, of 0.05 and a grain-boundary reflexion coefficient, Rg, of 0.45. On KBr substrates, samples, prepared similarly, formed two distinct types of (100) film. Use of reflexion high energy electron diffraction and electron microscopy showed that the grain structure of these types of film differed; however, both gave an r of 0.1. An Rg of 0.10 was determined for one type but remained unknown for the other. Our results show how previous workers, often relying on the validity of Fuchs’s theory and the misapplication thereof, have failed to present convincing evidence for specular surface scattering. We demonstrate the necessity for, experimentally, morphological observations and measurements over a wide temperature range, and, theoretically, the use of a method that combines the effects of both grain-boundary and angular-dependent surface scattering.

Accurate design of a noncollinear acousto-optic tunable filter

The analysis of a noncollinear acousto-optic tunable filter commonly found in the literature is based on an approximation for the birefringence of the interaction material. It is demonstrated that this approximation leads to a significant error in calculating the optimum incident angle for a device, which can seriously degrade its performance if a fixed output beam angle is required. An alternative, exact analysis is presented and compared with experimental results.

Microwave liquid crystal wavelength selector

The combined use of a zero-order grating with liquid crystals to control microwaves is presented. A nematic liquid crystal is aligned in the 75 μm gaps of a grating comprised of 1-mm-thick aluminum slats. A set of resonant transmission frequencies is recorded for microwaves incident on this structure with their electric field perpendicular to the slats. These resonances are due to the excitation of coupled surface plasmons. A voltage applied between adjacent pairs of slats causes the liquid crystal to realign, allowing sensitive control of the transmitted microwave frequencies.

Determination of the permittivity of nematic liquid crystals in the microwave region

Two 3 mm thick microscope glass plates, having one face plus their two long edges coated by a thick metallic film, are spaced 75 μm apart by mylar spacers. Because of the metallic coatings on the inner faces the structure acts as a single metallic slit. The space between the two coated plates is filled with aligned nematic liquid crystal (E7, Merck/BDH) and the cell is inserted in an absorber aperture. This single metallic slit geometry supports resonant modes when microwaves are incident with their polarization (E-field) perpendicular to the slit. The structure gives a set of Fabry-Perot-like resonant transmission frequencies. These frequencies move when a voltage is applied between the two plates, the liquid crystal being first aligned homogeneously, then realigning homeotropically with the applied field. By minotoring these changes a fast and easy to use procedure for determining the permittivity and its anisotropy for nematic liquid crystals in the microwave region has been developed. The parameters determined for E7 are εe = 3.17 (ne = 1.78 ± 0.01) and εo = 2.72 (no = 1.65 ± 0.01), (Δn ≈ 0.13) in the 40.0–60.0 GHz region.

Broadband polarization-converting mirror for the visible region of the spectrum

A mirror structure that enables the polarization of linearly polarized light to be rotated by 90 degrees over the entire visible region of the spectrum is presented. Theoretical modeling is used to show that this phenomenon occurs for light that is normally incident upon a metal grating consisting of a series of high and narrow ridges that are oriented at 45 degrees to the polarization angle. This broad polarization-conversion band is shown to arise from mode mixing of surface plasmon polariton-mediated polarization-conversion bands and interference-mediated polarization-conversion bands.

Microwave liquid-crystal variable phase grating

A voltage-controlled variable phase grating, at microwave frequencies, is described and its response characterized. It comprises a stack of 71 aluminium strips of 1mm thickness separated by 75μm spaces, filled with aligned nematic liquid crystal. For microwaves polarized normal to the grating strips there are a set of resonant transmitted frequencies. By varying the voltages applied across the liquid crystal layers and their distribution, a variable phase microwave grating is realized. This allows low-voltage control of output beam profile and intensity.

Optical non-linearity in liquid crystals using surface plasmon-polaritons

The authors present a detailed report of the observation of a strong, thermally induced optical non-linearity in a system comprised of a silver film and a nematic liquid crystal, in which the surface plasmon-polariton (SPP) mode may be excited. Various measurements of the reflected intensity from the experimental system as a function of the incident angle, including a novel AC-on-DC probe technique, reveal that the observed sharp optical switching is due to warming of liquid crystal, close to the metal/liquid crystal interface, into the isotropic phase through loss of energy of SPPS in Joule heating of the silver film. The many factors contributing to the observed reflectances are investigated and lead to a detailed model of the non-linearity for which a computer model is constructed. Conclusions drawn from the results of this work have serious implications for the use of SPPS in non-linear optics.

Determination of microwave permittivities using a metallic slit

A procedure for determining the microwave permittivity of small quantities of material is described. A single metallic slit is built comprising a pair of 3 mm thick aluminium plates separated by spacers of less than 100 μm thickness. The material to be characterized fills the space between the two plates and the slit is inserted in an absorber aperture. When microwaves are incident on this structure with their electric field perpendicular to the cell walls a set of Fabry–Perot-like resonant transmission peaks is recorded as a function of incident wavelength. By using these Fabry–Perot-like resonances a fast and easy-to-use procedure for determining the permittivities of materials in the microwave region has been developed. To demonstrate this technique, the microwave permittivity of four different materials, some of which may otherwise be difficult to characterize, have been determined at the microwave region (50.0–75.0 GHz).

Angle–frequency relationship for a practical acousto-optic deflector

A number of designs have been proposed for acousto-optic deflectors that use anisotropic acousto-optic diffraction in TeO2. The interaction geometries commonly used permit acceptable deflection angles by ensuring that, to first order, the required angle of incidence of the optical beam is fixed. For these interaction geometries, a complicated relationship between the deflection angle and the acoustic frequency is predicted. However, a simple linear relationship is observed experimentally. It is shown that this discrepancy is due to the angular spread in the acoustic beam, which is necessary for the operation of a practical acousto-optic deflector.

Polarization-Conversion Guided Mode (PCGM) technique for exploring thin anisotropic surface layers

A Polarization-Conversion Guided Mode (PCGM) technique has been developed to quantify optical anisotropy as low as 10-5 for a surface layer only 10 nm thick. The optical geometry consists of an index-fluid matched prism-coupler and an air-gap waveguide comprising the thin sample on a glass plate as the incident surface with a gold reflector forming the other surface of the guide. This allows non-destructive characterization of the optical anisotropy of surface layers. The polarization conversion signal is extraordinarily sensitive. Thus the influence of the polarization purity of the incoming beam, very small twists and/or tilts between the normal to the prism bottom surface and the sample plane, have all been analyzed in detail to allow extraction of the sought for information about the thin layer. Rubbed polyimide thin films and incline-evaporated SiOx layers, both used for liquid crystal alignment, have been examined by this PCGM technique to demonstrate its power.