SURFACE-ENHANCED 2ND-HARMONIC DIFFRACTION - SELECTIVE ENHANCEMENT BY SPATIAL HARMONICS

Surface-enhanced second-harmonic diffraction from a corrugated silver surface is studied as a function of the Fourier decomposition of the s urface profile. Numerical results are obtained using the reduced Rayle igh equations for the linear and second-harmonic fields. Scattering of the surface-plasmon-polariton (SPP) -enhanced, evanescent nonlinear p olarization wave into radiative channels is shown to provide a sensiti ve probe of spatial-harmonic content. The presence of a specific highe r harmonic in the surface profile allows preferential scattering of th e enhanced nonlinear polarization into a certain diffraction order whe re a higher-order scattering mechanism might otherwise be operative. T he propagating orders can thereby be selectively enhanced, in some cas es, by many orders of magnitude. Calculations are presented for symmet ric profiles and a range of grating periods. The nature of these selec tive enhancements suggests that optimized profiles for second-harmonic diffraction into a particular order can be formed by a superposition of two appropriately selected Fourier components. To explore this poss ibility, gratings with groove densities of 1200 and 1290 grooves/mm we re studied by first determining the enhancement of each of the propaga ting orders at their respective optimum groove depths, assuming a pure ly sinusoidal profile. A search for the maximum enhancement of each or der was then performed by varying the amplitudes of the grating fundam ental and relevant order-enhancing higher harmonic. For the two period s considered, optimized profiles were found. The degree of coupling to the SPP at the second-harmonic frequency is shown to be important in determining the optimized profile as demonstrated by the substantially different enhancing properties of these similar groove-density gratin gs.