Metal‐oxides are promising candidates to substitute silicon in intra‐chip optical interconnects, as they exhibit great electric field tuning capabilities. The development of crystal ion slicing of thin films from bulk crystals and the advances over epitaxial growth have allowed the integration of metal‐oxides on a single chip. In terms of performance, they possess strong electro‐optic response over broad bandwidths across near‐infrared. However, lattice and thermal expansion coefficient mismatch limits the compatibility with available substrates and other materials, while physical hardness makes high quality nanostructures difficult to implement. Here, a novel concept of electro‐optic (EO) switching is introduced: an adjacent BaTiO3 nanoparticle film to a plasmonic metasurface provides reflection changes up to 0.15% under 4 V of control signal for modulation frequencies up to 20 MHz, in the near‐infrared. The nanoparticle films show EO coefficients (37.04 ± 25.6 pm V−1) comparable to lithium niobate crystals, are deposited uniformly over large scale and on any type of substrate, while retain optical nonlinear properties (e.g. second‐harmonic generation). Photonic nanostructures such as metasurfaces incorporated with nanoparticle films can harness the multifunctional properties of metal‐oxides such as BaTiO3 to form a new family of switchable nano‐devices across the entire visible to near‐infrared part of the spectrum.

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Nonlinear crystals that have a noncentrosymmetric crystalline structure, such as lithium niobate (LiNbO3) and barium titanate (BaTiO3) exhibit nonzero second-order tensor susceptibilities (χ(2)) and linear electro-optic coefficients (r ij ). The constraints associated with top-down nanofabrication methods have led to bottom up approaches to harness the strong nonlinearities and electro-optical properties. Here, we present an overview of photonic assemblies made of randomly oriented noncentrosymmetric nanocrystals via bottom-up fabrication methods. In this configuration, nanocrystals can form objects with tunable dimensions, increased complexity, and a great span of symmetry level, ranging from thin layers to spheres. At the same time, according to their shape, photonic assemblies may support optical modes, that is, Mie or guided, which can tailor linear optical properties and enhance nonlinear and electro-optic responses. As a result, assemblies of noncentrosymmetric nanocrystals can form a disruptive platform to realize photonic integrated devices free of etching process and over large surface areas. Last, we foresee potential applications of noncentrosymmetric nanocrystals in various fields of nano-optics and sensing.

The aim of this work was to revisit second-harmonic generation (SHG) as a technique for the in situ analysis of oxide film formation on steel samples immersed in aqueous solutions and during electrochemical experiments. The measurements were compared to active reflectance spectroscopy (ARS), which is a more established in situ spectroscopy technique. Carbon steel electrodes were subjected to polarization in pH neutral and mild-alkaline electrolytes to provoke the formation of passive films. In situ SHG measurements allowed monitoring of compositional changes in the formed film, as the SHG intensity increased in the domain of stability of Fe 3 O 4 and decreased in the domain of stability of Fe 2 O 3 during potentiodynamic polarization. ARS, on the other hand, is suitable to monitor film thickness. ARS absorption increased along with film growth when moving in the anodic direction during potentiodynamic polarization. During 30 min cathodic polarization at −0.9 V Ag/AgCl sat. KCl in initially pH neutral the combination of SHG and ARS revealed the presence of an oxide film that was primarily composed of Fe 3 O 4 and with a thickness up to 0.7 nm.

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