Publication
Second-harmonic generation (SHG) and other second-order nonlinear processes do not occur in bulk gold due to its centrosymmetric crystal structure. However, the centrosymmetry is broken at the surface, giving rise to non-zero surface susceptibility tensor χ(2)surf . In particular, {111} gold surfaces possess C3V symmetry (Fig. 1a), which results in a characteristic six-fold pattern of the SHG signal, that can be co- or cross-polarized, depending on the sample orientation angle (Fig. 2a). The SHG signal contains two contributions: isotropic (stemming from broken centrosymmetry in surface-normal direction) and anisotropic (due to centrosymmetry broken along 11-2 crystal axis) [1]. In this work, we fabricate metasurfaces on chemically synthesized monocrystalline gold flakes [2] that take advantage of the intrinsic anisotropy of the large {111} surface [3]. The metasurfaces – arrays of V-shaped grooves with subwavelength periodicity – are fabricated using focused ion beam (FIB) milling (Fig. 1 c and d). Intriguingly, FIB fabrication does not diminish the crystal structure and yields structures with outstanding quality. Tailoring the geometric dimensions of the grooves allows to tune the spectral position of the localized plasmonic resonance, which allows to considerably enhance the SHG signal [4]. Remarkably, isotropic and anisotropic SHG contributions are extremely sensitive to the near-field distribution at the resonance (Fig. 1 g and h), which allows to selectively amplify or reduce co- or cross-polarized SHG signals. We substantiate our results with numerical simulations, which show excellent agreement with the experiment (Fig. 1 e and f). These results will serve as a guide for exploiting the tensorial nature of nonlinear susceptibility in gold and the development of compact nonlinear devices that operate in reflection mode.