Surface-enhanced second-harmonic diffraction: Experimental investigation of selective enhancement

Recent theoretical predictions demonstrating large enhancements of second-harmonic diffraction through the control of surface harmonic composition when the surface-plasmon polariton (SPP) is excited on a corrugated silver surface have been investigated experimentally for the specular order. Interpretation of the mechanism of enhancement in terms of a three-step selective scattering process involving only two spatial harmonics has been found to be justified. Fourier blaze holography, which permits the phase- and amplitude-controlled superposition of multiple harmonics in photoresist, has been used to fabricate the nanostructured, biperiodic gratings. Surfaces have been characterized by atomic force microscopy and Heitmann's method. As predicted, surfaces with optimized spatial frequency composition equal or exceed randomly rough surfaces in the enhancement of second-harmonic reflection relative to a flat silver surface. Enhancement produced by simultaneous excitation of counterpropagating SPP modes at the pump and second-harmonic frequencies has also been detected by varying grating wave vector to tune through a double resonance condition. Factors affecting the quantitative agreement between enhancement measurements and theoretical predictions are briefly discussed.