Hannu Husu

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. The approach opens new ways for tailoring the nonlinear responses of metamaterials and their tensorial properties.

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. This is explained by the character of plasmon oscillations supported by the two shapes. Our numerical simulations for both linear and second-order responses display unprecedented agreement with measurements.

Nanoimprint fabrication of gold nanocones with ~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.

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.

Chiral coupling in gold nanodimers

Second-harmonic generation from T-shaped gold nanodimers with nanogaps has different efficiencies for left- and right-hand circularly polarized fundamental light. The difference arises from the chiral symmetry breaking of the dimers due to nonorthogonal mutual orientations of the horizontal and vertical bars of the T shape and depends on the gap size. Unexpectedly, the smallest gap with presumably the strongest coupling gives rise to only a small difference. All results can be explained by considering the distribution of the polarized local field in the structure.

Inhomogeneities in the nonlinear tensorial responses of arrays of gold nanodots

We use second- and third-harmonic-generation microscopy to address the tensorial nonlinear responses of individual particles in an array of cylindrical gold nanodots. The responses in both orders exhibit widely-variable, polarization-dependent differences between individual nanodots and thereby indicate tensorial inhomogeneities in the sample. The result provides clear evidence that the second-order response, which is forbidden by symmetry for ideal particles, must arise from small-scale, symmetry-breaking features. A similar result for the third-order response, which is allowed for ideal particles, suggests that both nonlinear responses are dominated by strong variations in field localization around the small-scale features differing among individual nanodots.

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. The results arise from long-range diffractive coupling between the particles, as determined by the dependence of the array unit cell size on particle ordering.

Metamaterials with Tailorable Nonlinear Optical Properties

Second-order nonlinear processes such as second-harmonic generation (SHG) require noncentrosymmetric structures. The development of second-order metamaterials, however, has been hampered by symmetry breaking due to sample defects and shape distortions. The resulting outcomes can be interpreted in terms of effective higher-multipole (magnetic and quadrupole) effects that strongly modify the radiative properties of the samples.

Gap-dependent chiral coupling in T-shaped gold nanodimers

The mutual non-orthogonal orientations of its horizontal and vertical bars make T-shaped gold nanodimers chiral. Because of the broken symmetry second-harmonic generation from the structure has different efficiencies for left- and right-hand circularly-polarized fundamental light. The chiral signature arises from the coupling between the bars. One would therefore assume that the chiral signature is largest when the gap size is very small, because then the coupling is presumably the strongest. Counter-intuitively, the measurement results show a very small chiral signature for the smallest gap. To explain the results, one needs to consider the distribution of the local field in the unit cell of the structure.

Second-harmonic generation driven by asymmetric local fields in T-shaped gold nanodimers

We demonstrate both experimentally and computationally that SHG from arrays of T-shaped gold nanodimers with differing nanogap sizes results from asymmetry in the local field distribution rather than strict dependence on the nanogap size. Normal-incidence SHG measurements reveal that the SHG responses depend non-trivially on the nanogap size. Calculations show that strong orthogonal polarization components, which are not present in the exciting field, are induced, and that these induced components yield the dominant SHG response. The calculations also reveal that field enhancement is roughly independent of nanogap size and persists even for large nanogap sizes. A simple phenomenological model wherein the local surface susceptibility of the nanodimer interacts with the local field distribution along the nanodimer perimeter qualitatively explains the experimental results with good agreement.