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Non-linear optics via waveguide geometry

The project concerns the study of non-linear effects in a waveguide geometry. The interest lies in the possibility of concentrating energy and of increasing the local field via the excitation of electromagnetic resonance, like plasmon-polaritons or guided waves. The project will have a special interest in gratings and corrugated structures and the prospects for second-harmonic generation, four-wave mixing and other non-linear effects in signal processing which are becoming increasingly realistic.

The scattering of light, by isolated bumps in waveguides has been analysed both linearly and nonlinearly. Multistable behaviour has been predicted for the nonlinear case which augurs well for device prospects. Nonlinear boundary conditions at curved vacuum-metal interfaces has been investigated thoroughly, together with second-harmonic generation at randomly rough metal surfaces. Precise rules governing polarisation conversion in second-harmonic generation have been found. A variety of nonlinear waves and solitons in superlattice structures have been demonstrated together with their stability regimes, with respect to sound radiating into the structures and for propagation along the superlattice. Optical nonlinearities of a quantum well, embedded into a microcavity, may be enhanced, compared to the bulk so that self-induced transparency of pulses propagating along such cavities can be demonstrated. Phenomena like X cascaded bistability and X cascaded soliton propagation have been found. Multistable behaviour using a film of highly nonlinear material inside a waveguide to increase nonlinear interaction efficiency has also been shown to exist. Second-harmonic generation in optical resonators like a grating coupler has been shown to provide the feedback necessary for optical bistability. This new type of bistability occurs on both sides of the resonance maximum, for certain offset conditions in the incident angle. Physical insight into this new effect has been obtained by comparing two approaches. One is based upon rigorous integration of Maxwell's equations and the other on a coupled-mode analysis.

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