  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.
