G.W. Bradberry

Determination of the Optical Constants and Thickness of a Highly Absorbing Film Using the Attenuated Total Reflection Technique

Abstract Theoretical analysis of the Kretschmann configuration for attenuated total reflection experiments shows that near the critical angle the reflectivity is strongly dependent upon the dielectric constants and thickness of a thin absorbing layer on the prism surface. Numerical calculations based on Fresnels equations illustrate this clearly for thin absorbing films. Using this property of highly absorbing films it has then been possible to determine the optical constants of a thin film of phthalocyanine over the visible region of the spectrum.

Prism Coupling to Long-range Coupled-surface Modes

Abstract Using the virtual mode theory for attenuated total reflection (ATR) experiments the long-range coupled-surface modes supported by symmetrically bound thin films have been examined to study the influence of prism coupling. Approximate analytic results show that in certain circumstances prism coupling may result in a decrease of mode loss from the unperturbed eigenvalue. This is well illustrated for the long-range surface exciton-polariton. Detailed numerical evaluations confirm the analytic results. The generally accepted conclusion that prism coupling always results in an increase in mode loss is thus not supported by these results which show convincingly that for more complex ATR geometries simplistic treatments are invalid.

The excitation of infra-red Surface Plasmon-Polaritons on refractory metals

Abstract Observations are reported on the attenuated total reflection excitation of surface plasmon polaritons (SPP) on the refractory metals tungsten and molybdenum. Using the Otto geometry with an air gap of order 10 μm as the tunnel barrier has allowed the excitation of SSPs on polished surfaces of bulk single crystals of these two metals. Reflectivities, monitored as a function of the angle of incidence of a laser beam of 3.391 μm wavelength, are fitted to Fresnels theory for reflection from parallel interfaces. From such fits the dielectric constants are determined, which accord reasonably with normal spectroscopic measurements.

Measurement of narrow linewidth with a Fabry-Perot interferometer of limited resolution

Abstract It is shown that the high extinction of a multi-pass Fabry-Perot interferometer may be exploited to measure the width of very narrow Lorentzian profiles. The technique is illustrated with measurements on re-orientational light scattering spectra of a nematic liquid crystal. Linewidths nearly two orders of magnitude narrower than the conventionally defined resolving limit are determined with an accuracy of a few percent.

Examination of the +1, −1 Surface Plasmon Mini-gap on a Gold Grating

Abstract An energy gap in the excitation of surface plasmons is found for light at normal incidence to a gold grating. This gap occurs at the crossing of the plus and minus first order surface plasmons. It arises directly as a consequence of distortion of the grating from sinusoidality, the first harmonic of the grating providing coupling between the plus and minus one orders. Experiments have been performed using both wavelength scans, where at a fixed angle of incidence the wavelength of excitation is varied, and angle of incidence scans, where for a fixed wavelength the angle of incidence is varied a few degrees either side of normal to the grating. By fitting the angular dependent reflectivity scans using grating modelling theory the gold grating is characterized at all wavelengths. This then allows a detailed comparison of the theoretical dispersion curve with that obtained experimentally. The agreement for both p-polarized light (for angle dependence with the plane of incidence normal to the grating...

Coupled Surface Plasmons at 3·391 μm

Abstract Using 3·391 μm wavelength radiation to excite surface plasmons means that the conventional dimensional constraints imposed upon the dielectric thickness required for coupled surface plasmons is relaxed. Results are presented and analysed for coupled surface plasmons using silver layers spaced by a variable air gap of several micrometres thickness. Adjusting the air gap allows the determination of the dispersion curve for the coupled plasmons as a function of gap thickness down to a gap of about 5 μm.

Experimental Observation of Surface Exciton-polaritons on Vanadium Using Infrared Radiation

Abstract Surface exciton-polaritons (SEPs) have been observed on the surface of vanadium at room temperature by the technique of attenuated total reflection spectroscopy using infrared radiation (He-Ne, 3·391 μm). The experimental results compare well with theoretical predictions. Relationships between the real and imaginary parts of the permittivity of the active medium which can support an SEP are discussed. The distribution of the electromagnetic field in the three regions, prism, air gap and sample, is calculated and illustrates the localization of the SEP at the vanadium surface.

Characterization of Thin Absorbing Films Using Infrared Surface Plasmons

Abstract A combination of infrared excitation of surface plasmons and guided mode resonances is used to accurately parameterize thin highly absorbing films. In most circumstances it is difficult to determine both the thickness and dielectric constants of such films even if they are metallic and therefore able to support surface plasmons. Indeed it is clearly shown how it is impossible to characterize highly absorbing metal films using the Kretschmann geometry. However using an experimental system combining the Kretschmann geometry with a small air-gap spacer between the thin layer of interest and a normal thick metal layer it is possible to uniquely characterize the thin layer.

Liquid layers bound to a gold surface supporting a surface plasmon-polariton

Abstract This paper presents a novel use of the surface plasmon-polariton at the surface of a gold film to allow a study of the progressive thickening of an organic liquid film condensing on the gold. The vapour is supplied by an effectively infinite liquid reservoir some 4 mm below the observation height. For methanol, n-hexane and n-heptane equilibrium thicknesses of order 16 nm, 26 nm and 28 nm respectively, are obtained, while for ethanol there appears to be continual growth to a bulk condensed phase. The data can be reconciled in the most general terms with theories of Van der Waals bonding between thin liquid layers and a plane surface. Currently however there is no detailed theory which would predict the striking difference in behaviour between methanol and ethanol.

Critical Edge Characterization of the Optical Tensor of a Uniaxial Crystal

The angle dependent reflectivity at the boundary between a uniaxial crystal and an isotropic fluid, which has a refractive index greater than the smaller index of the uniaxial material but less than the other index, is analysed extensively. Consequently the use of a prism and matching fluid with just such refractive indices allows a procedure for the complete characterization of a uniaxial medium to be established. This is undertaken by studying the reflectivity at the prism/fluid/crystal interface with an angle of incidence near to the pseudo-critical angle of the lower index in the uniaxial crystal and examining how the reflectivity varies with the rotation of the uniaxial crystal. Experimental results using a single crystal of calcite give very good agreement with the analysis and computer modelling of this situation and illustrates the use of the new technique.