Optical probes of organic thin films: Photons-in and photons-out

The theory of the Brillouin scattering from the surface of a semi-infinite medium, developed previously, is extended to coated surfaces of cubic materials. The Brillouin cross section is obtained for any scattering geometry and for p and s polarisation of the incident and scattered light. The formulae for the cross section contain contributions from the ripple and the elasto-optic scattering mechanisms for the film as well as for the substrate. The theory is applied to a silica film deposited on crystalline silicon and the behaviour of the cross section against the film thickness is investigated. The authors find that there are strong interference effects among the various contributions to the cross-section, so that the intensity of the spectral lines dramatically oscillates by changing the thickness. Although in general the ripple scattering mechanism is the dominant one it is found that the elasto-optic coupling in the silica film is not negligible and sometimes is even prevailing. The discrete phonon spectrum (Rayleigh, Sezawa, Lamb modes) and the continuous spectrum (resonances, mixed modes) are both studied.

http://iopscience.iop.org/article/10.1088/0022-3719/16/9/018/pdf

Theory of Brillouin scattering from surface acoustic phonons in supported films

A first-order perturbation theory describing the deflection of guided optical waves by sinusoidal surface corrugations on a slab waveguide is presented. The wave equation is satisfied everywhere in space, and the tangential fields are continuous across all interfaces to order δ0k, where k is the optical wave vector and δ0 is the maximum surface displacement. Analytical expressions are derived for synchronously generated normal modes for the geometries TEm → TEm′, TEm → TMm′, TMm → TMm′, and TMm → TEm′. Comparison with normal-mode analyses for the backscattering problem show agreement for TEm → TEm for both the local- and ideal-mode approximations. However, for TMm → TMm the results differ when ideal modes are used to describe the incident fields.

Scattering of guided waves by surface periodic gratings for arbitrary angles of incidence: perturbation field theory and implications to normal-mode analysis

Normal mode and total field analysis techniques for calculating the fields generated by a polarization source are discussed. The amplitudes of the normal modes are evaluated using a Green’s-function approach and the results are compared to a previous treatment by Yariv. The generated fields are also calculated by solving the polarization driven wave equation subject to the usual electromagnetic boundary conditions at all of the pertinent interfaces. It is shown that these two approaches yield equivalent results for a simple case, and their relative merits for solving problems in different regions of a waveguide are discussed.

Comparison of normal mode and total field analysis techniques in planar integrated optics

Nonlinear Optical Response of Metal Surfaces

A new approach to the problem of light scattering by surface acoustic waves in thin-film optical waveguides is presented. In this analysis electromagnetic fields are found to rigorously satisfy both the polarization driven wave equation and the electromagnetic boundary conditions at the frequencies of interest. A component of the solution field is found to evolve with propagation distance into a linearly growing guided wave mode that dominates the solutions under conditions that can be defined in terms of waveguide mode parameters. Analytical formulas for the growth coefficient are obtained for both the elasto–optic and corrugation scattering mechanisms for the geometries TEm → TEm, TEm → TMm, TMm → TMm and TMm → TEm The present results are found to be consistent with previous normal mode analyses of this phenomenon for the cases in which comparison is possible. Numerical calculations for thin films of As2S3 and Corning 7059 glass on fused silica substrates indicate that the elasto-optic effect does not always dominate the scattering cross section and that the corrugation mechanism must often be taken into account.

Scattering of guided optical beams by surface acoustic waves in thin films

A series of experimental results on Brillouin scattering in thin film optical waveguides is discussed for two scattering geometries and for three types of planar waveguides. The results of these experiments are compared with the theory presented in the previous two papers.

V. C-Y. So

Papers

Scattering of guided optical beams by surface acoustic waves in thin films

Brillouin scattering in thin film optical waveguides. III. Experiments