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The optical properties of metal nanostructures result from their localized surface plasmon resonances (LSPRs). The exact spectral positions of LSPRs depend on the geometry of the particles and optical parameters of the surrounding material. In addition, when the nanoparticles are arranged in an array (metasurface) the LSPRs associated with isolated particles are affected by the presence of other array members. As a result, sharp spectral features related to surface lattice resonances (SLRs) can be observed. SLRs are sensitive to the angle of incidence of incoming light due to their association with the appearance and disappearance of diffraction orders in the optical response.
The resonances lead to strong local fields (hot spots) in the proximity of the particles. Such hot spots are particularly important when one wants to enhance nonlinear optical effects, for example second-harmonic generation (SHG). In the case of SHG, the sample needs to be also non-centrosymmetric, which can be fulfilled by using, e.g., V-shaped nanoparticles. In this paper, we show that SHG from arrays of V-shaped gold nanoparticles can be enhanced when the angle of incidence is matching optimal conditions for the excitation of SLRs.
Our sample consists of an array of V-shaped nanoparticles fabricated by standard electron-beam lithography and lift-off techniques. The gold particles are distributed in the array with a spacing of 500 nm and have the same dimensions to obtain a resonance close to fundamental wavelength for the polarization along the symmetry axis.
The SHG experiments were performed in transmission mode with the incident beam weakly focused on the samples. Polarizers and a half-wave plate were used to control the polarizations of the fundamental and second-harmonic beams. The SHG signal was collected by a photon-counting system for varying incident angles (rotation with respect to the symmetry axis (y) of the sample or orthogonal to it (x)).
SLRs can modify LSPRs leading to visible features in the SHG response. When the incident angle is increased, a redshift towards fundamental wavelength is observed together with narrowing of the resonance. Such improvement in the quality of the resonance results in stronger SHG. Thus, the sample needs to be rotated significantly to meet the optimal conditions for SHG enhancement, which can be as high as by a factor of four (projection x-z) or a factor of 10 (projection y-z) comparing to SHG measured at normal incidence. The maximum enhancement of the SHG signal is related to the SLRs which occur near incident angles that allow diffraction orders to propagate along the sample plane (in air or in the substrate).
Our investigation of the role of SLRs in SHG from metasurfaces shows that such resonances lead to prominent features in the angle-dependent SHG responses, which results in the enhancement of SHG by a factor of up to ten. In order to achieve the optimal conditions of SHG enhancement the sample needs to be rotated from normal incidence to match the angle that allows diffraction order to propagate in the substrate.
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