Synthesis and second-harmonic generation studies of noncentrosymmetric gold nanostructures

Noncentrosymmetric gold nanoparticle structures were prepared in porous anodic aluminum oxide films via a modified template synthesis procedure. The noncentrosymmetric structures are defined by two gold particles in close proximity (ca. 22 +/- 8 nm end to end) within a single host oxide pore, one having average dimensions of a = 37 +/- 6 nm and b = 26 +/- 3, and the other having dimensions of a = 27 +/- 5 nm and b = 26 +/- 3 nm, where a is the axial length and b is the diameter of the quasi-cylindrical structures. Linear UV/vis polarization spectra of the gold particle/porous alumina film composites show plasmon resonance bands whose a incidence angle dependence is similar to composites containing centrosymmetric gold structures, where the incidence angle is defined with respect to the surface normal. Second harmonic generation (SHG) studies using an incident wavelength of 780 nm indicate that SHG intensities under s-polarization are low and independent of incidence angle (theta) for composites containing centrosymmetric and noncentrosymmetric gold nanostructures. However, in p-polarization, both composites show an increase in SHG counts with theta, with the noncentrosymmetric structures showing a higher SHG signal than their centrosymmetric counterparts. These results are consistent with local-field enhancements arising from long particle axis dipolar plasmon resonances.