Grating-enhanced nonlinear excitation of surface polaritons: An electromagnetic study

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Abstract
This paper reports a new electromagnetic theory of Raman excitation of surface polaritons along a grating ruled on a nonlinear medium. The formalism presented here is valid for any groove depth and periodic profile. We find that an increase of the groove depth leads to a strong enhancement of the surface-polariton intensity, to a depression of the surface-polariton dispersion curve, and to a broadening of the surface-polariton linewidth. The theory reported here predicts the following surprising result: the existence of an optimum groove depth which gives the highest surface-polariton intensity. We show that the broadening is due to the fact that, in addition to the well-known dissipation process, surface polaritons may lose energy by coupling with other diffracted orders. We also show that the enhancement of the surface-polariton intensity with the groove depth is closely related to the existence of Wood anomalies of gratings. Finally, we compare our theoretical results with the corresponding ones reported by Ushioda et al. [Phys. Rev. B 19, 4012 (1979)], and find good agreement between our numerical results and the measurements reported by these authors.