In connection with recent theoretical predictions, enhancement of optical second harmonic generation (SHG) by diffractive coupling to the silver surface-plasmon polariton (SPP) mode is shown experimentally to be maximized on a biperiodic corrugated surface. Optimized first- order diffractive coupling of incident radiation to the SPP maximizes resonance enhancement of the surface-localized electromagnetic field. Through the nonlinear susceptibility of the silver surface, an SPP-enhanced, evanescent, second harmonic wave is coherently generated which is selectively scattered into the second harmonic specular order by the second spatial harmonic in the surface profile. Biperiodic surfaces with appropriately optimized spatial harmonic composition are shown to provide enhancements in second harmonic relection of up to 10$+4$/ over the corresponding flat surface response. Using the hologrpahic technique of Breidne et al. (Fourier blaze holography), biperiodic surfaces were fabricated which consisted of a superposition of an 833 nm fundamental and a phase- and amplitude-controlled second spatial harmonic. The results of angle-resolved SHG experiments are presented along with atomic force microscopy line scans of the surface profiles. The effect of coupling to the SPP mode at both the incident and second harmonic frequencies is also discussed.
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