Enhancing nonlinear optical response of resonant gold nanostructures via lattice interactions
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Metal nanostructures are of interest because of their potential applications in optical metamaterials, which are artificial materials that can have physical properties not found in nature. Such nanostructures are also interesting for nonlinear optics, because they can be manufactured for the desired symmetries for specific nonlinear effects, and because the presence of plasmon resonances can greatly enhance the nonlinear responses. Additionally, surface lattice resonances (SLR), arising from coupling between particles, can further modify the response. In this work, the optical properties of metamaterials consisting of arrays of L-shaped gold nanoparticles of different sizes are studied. The nanoparticles are organised in square lattices with different dimensions of the unit cells and orientations of the particles in the array. Variation of such parameters allows to obtain structures supporting SLRs at different wavelengths. The linear properties of the structures are studied using extinction spectrometry and the nonlinear optical properties by measuring second-harmonic generation (SHG). The results presented in this work provide strong evidence for the importance of SLRs. The measured extinction spectra show that presence of SLR near a plasmon resonance leads to an improvement of the quality of the resonance, which increases the SHG response via boosted resonance enhancement. However, due to an oversight in the design of the extinction setup, typical spectral features of SLRs were not observed, and conclusive results could not be achieved. The results are still promising, and the effects of SLRs warrant further studies.