Second-harmonic generation from metal nanoparticles: resonance enhancement versus particle geometry.

We demonstrate that optical second-harmonic generation (SHG) from arrays of noncentrosymmetric gold nanoparticles depends essentially on particle geometry. We prepare nanoparticles with different geometrical shapes (L and T) but similar wavelengths for the polarization-dependent plasmon resonances. In contrast to recent interpretations emphasizing resonances at the fundamental frequency, the T shape leads to stronger SHG when only one, instead of both, polarization component of the fundamental field is resonant.

Enhancement of second-harmonic generation from metal nanoparticles by passive elements.

We prepare arrays of gold nanoparticles that include both noncentrosymmetric particles with a second-order nonlinear optical response (active particles) and centrosymmetric particles with no second-order response (passive particles). The plasmon resonances of the active and passive particles are at distinct wavelengths, yet the passive particles modify the electromagnetic modes of the structure in such a way that second-harmonic generation from the active particles is enhanced. Our results provide a completely new concept for optimizing the nonlinear responses of metamaterials.

Metamaterials with tailored nonlinear optical response.

We demonstrate that the second-order nonlinear optical response of noncentrosymmetric metal nanoparticles (metamolecules) can be efficiently controlled by their mutual ordering in an array. Two samples with minor change in ordering have nonlinear responses differing by a factor of up to 50. The results arise from polarization-dependent plasmonic resonances modified by long-range coupling associated with metamolecular ordering.

Spectral control in anisotropic resonance-domain metamaterials.

We introduce a concept to control the spectral and dichroic properties of metamaterials. The approach is based on anisotropic metal nanoparticles and on varying their mutual orientation in a periodic lattice. Even seemingly inconsequential changes in particle ordering strongly modify the dichroic properties of the arrays and result in either very narrow resonances or ultrabroad extinction ranges.

Particle plasmon resonances in L-shaped gold nanoparticles.

We present an extensive experimental and theoretical study of the particle plasmon resonances of L-shaped gold nanoparticles. For the small characteristic size of the particles, we observe more higher-order resonances than previously from related shapes, and show that a short-wavelength resonance arises from the particle arm width and is not the suggested volume plasmon. We interpret the resonances through the local vector electric field in the structure and by fully taking into account the particle symmetry.

Nanoimprint fabrication of gold nanocones with approximately 10 nm tips for enhanced optical interactions.

We show that nanoimprint lithography combined with electron-beam evaporation provides a cost-efficient, rapid, and reproducible method to fabricate conical nanostructures with very sharp tips on flat surfaces in high volumes. We demonstrate the method by preparing a wafer-scale array of gold nanocones with an average tip radius of 5 nm. Strong local fields at the tips enhance the second-harmonic generation by over 2 orders of magnitude compared with a nonsharp reference.

Local field asymmetry drives second-harmonic generation in non-centrosymmetric nanodimers.

We demonstrate that second-harmonic generation (SHG) from arrays of non-centrosymmetric T-shaped gold nanodimers with a nanogap arises from asymmetry in the local fundamental field distribution and is not related strictly to nanogap size. Calculations show that the local field contains orthogonal polarization components not present in the exciting field, which yield the dominant SHG response. The strongest SHG responses occur through the local surface susceptibility of the particles for a fundamental field distributed asymmetrically at the particle perimeters.