Nanoporous Metallic Network as a Large-Scale 3D Source of Second Harmonic Light

Abstract

Second harmonic generation (SHG) is forbidden from most bulk metals, because metals are characterized by a net zero electric-field in equilibrium. This limit breaks down when reaching nanoscale dimensions, as have been shown for metallic nano-particles and nano-cavities. Yet, nonlinear response from a three-dimensional (3D) macroscale metallic piece comprising sub-optical wavelength features remains a challenge for many years. Herein, we introduce a largescale nanoporous metallic network whose building-blocks are assembled into an effective nonlinear conductive material, with a considerable conversion efficiency in a wide range of optical wavelengths. The high nonlinear response results from the network structure having a large surface area on which the inversion symmetry is broken. In addition, because of the 3D structure, hot-spots can be formed also in deeper focal plans of the metallic network, and thus can give rise to coherent addition of the signal. The solid connectivity between the nanoscale building-blocks also plays a role, because it forms a robust network which may respond in a collective manner to form very intense hot-spots. Broadband responses of the metallic network are observed both by SHG and cathodoluminescence (CL). The large-scale dimension and generation of randomized hot-spots make this 3D metallic network a promising candidate for applications like photocatalysis, sensing, or in optical imaging as structured illumination microscopy.