H. J. Simon

Enhanced total reflection with surface plasmons

The total internal reflectance of a homogeneous amplifying medium in contact with a thin silver film which is evaporated on a high-index prism is determined theoretically with the help of the Fresnel formulas. At the angle of incidence for surface-plasmon excitation the reflectance is reasonantly enhanced when the gain of the amplifying medium exceeds a value determined by the absorption of the metal film. For any value of gain above threshold there exists a thickness of the silver film for which the reflectance has a singular point. A device utilizing this resonance would be possible near 1 μ where the absorption of the silver is at a minimun and high-gain dyes are available.

Temporal pulse reshaping with surface waves

The temporal reshaping of short optical pulses reflected from a metal film under the condition of surface-wave excitation of single- and multiple-boundary surface-plasmon modes is theoretically investigated. Reflected pulse shapes are calculated through evaluation of Fourier transform integrals both by direct numerical integration and by approximate pole expansion. The latter is particularly useful for the interpretation of the simple exponential time decay of the trailing edge of the pulse. Emphasis is placed on determination of the range of parameters for which pulse reshaping may be experimentally observed. Diffraction of the incident beam is a limiting factor.

Attenuated total reflectance from a layered silver grating with coupled surface waves

We consider the experimental and calculated reflectance of Nd:YAG laser radiation from a thin silver grating bounded by matched dielectric media. A grating with 1200 lines/mm and a peak-to-valley depth of 270 A is ionetched into a quartz crystal. A silver film is evaporated onto the quartz grating, and a second quartz crystal is optically contacted to the top grating surface to complete the layered structure. The reflectance of the grating layers is calculated by using reduced Rayleigh equations extended to the double-boundary geometry. For a silverfilm thickness of 520 A the splitting of the single-boundary surface-plasmon mode into two coupled modes is observed, and the experimental attenuated total reflection is in good agreement with that calculated. However, for the thinnest film studied, 140 A, the observed resonance is shallower and broader than that predicted because of scattering of the long-range surface-plasmon mode from the grating grooves.

Observation of long range surface plasmon decay length by optical second harmonic generation.

We report the observation of the spatial decay of the long range surface plasmon mode using optical second harmonic generation. The surface wave is excited at 1.06 microm with prism coupling in a multilayer geometry consisting of a quartz crystal substrate, a thin silver film, and an index-matched liquid. From the asymmetry in the reflected second harmonic profile a decay length of 0.8 mm is determined. This value is a factor of 3 smaller than that predicted due to the effect of surface roughness scattering on the propagation of the long range surface plasmon mode.

Observation of interference in reflection profile resulting from excitation of optical normal modes with focused beams

We report the observation of novel interference patterns in reflected beam profiles resulting from the excitation with a focused laser beam of both the long-range surface plasmon mode and leaky waveguide modes in a prism-coupled thin-film geometry. Many new features of the observation are in good agreement with the theory; namely, the greater than unity normalized peak interference intensities, the monotonically decreasing spacing between the periodic intensity variations, and the reshaping of the reflected profile due to propagation. Implications of these results for submicrometer patterning in nanoscience are pointed out.

Reflected second-harmonic generation with coupled surface-plasmon modes in Ag/liquid/Ag layers

We observed enhanced reflected second-harmonic generation (SHG) that was due to coupled surface-plasmon modes in thin silver films separated with a liquid dielectric layer whose thickness may be continuously varied. The observed angular SHG spectra resulting from excitation of the coupled fundamental surface-plasmon modes are in reasonable agreement with a multilayer plane-wave theory. In contrast to the highly resonant SHG associated with the familiar long-range surface-plasmon mode, here the coupling of the surface modes decreases the SHG resonance because of the lossy nature of these new coupled modes.

Simple diffractive theory of enhanced backscattering

A simple theory of enhanced backscattering from weakly rough dielectric and metallic surfaces is presented in which constructive interference of time-reversed scattering paths is explicitly displayed. We show that, when starting from a perturbative second-order electromagnetic theory, one may calculate the backscattering intensity distribution from a single analytic formula. Excellent agreement is obtained with published results that were achieved with extensive numerical simulations. In the case of scattering from dielectric surfaces or loss-less metallic surfaces, the angular width of the backscattering peak is determined solely by diffraction.

Laser profile reshaping in a Fabry-Perot thin film.

We report the observation of non-Gaussian reflected and transmitted laser spatial profiles resulting from the excitation of resonant modes below the critical angle for total reflection in a Fabry-Perot cavity formed by a thin ~7 um air film between two glass prisms. The observations of an interference minimum in the reflected profile and exponential decay in the transmitted profile are new and in excellent agreement with a complex pole theory. Extension of the theory to a larger ~0.7 m cavity at normal incidence predicts similar reshaped profiles.