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We experimentally and theoretically investigate the influence of alumina coating on the second-harmonic generation (SHG) from split-ring resonator shaped air apertures engraved in a gold film, which are also termed as complementary split-ring resonators (CSRRs). By coating the CSRR arrays with alumina film of certain thickness, we precisely tune their electric diploe resonances (EDRs) to overlap the fundamental wavelength (FW) and realize the EDR enhanced SHG process. On this basis, by shortening the arm length of the CSRRs and then coating them with a certain thickness of the alumina film, we have achieved an SHG enhancement of nearly 1.2-fold in experiment and 8-fold in simulation compared to the CSRR array with an unshortened arm length. We attributed it to the improvement of the magnitude of the effective nonlinear source due to the realization of a doubly-resonant condition. As a flexible method, dielectric coating not only is beneficial to precisely and dynamically optimize the linear and nonlinear properties of the as-fabricated nanoscale devices but also can play the role of a protective layer, which can partially improve the damage threshold of these plasmonic nanoscale devices.

Nonlinear optical processes are promising for many applications, and recently great attention has been paid to improve the nonlinear efficiency of plasmonic metasurfaces. Here, we propose a hybrid structure consisting of a gold split-ring resonator (SRR) based nonlinear metasurface on top of a dielectric waveguide layer. By adjusting the periodicity of the SRR array, we demonstrate that the coupling between the magnetic surface plasmons (MSPs) of the SRRs and the waveguide modes could greatly enhance the second harmonic generation (SHG) intensity. Compared with the conventional SRR arrays, the hybrid structures could provide an extra enhancement in the SHG intensity of more than one order of magnitude.

We report experimental investigation of the effect of the cobalt hydroxide (Co(OH)2) decoration on the photocatalytic activity of the titanium dioxide (TiO2) films. We demonstrate that the decoration of Co(OH)2 is able to red-shift the absorption edge of the TiO2 film from UV to the visible light region, enhance the optical absorption, and facilitate the charge separation and transfer. As a result, the Co(OH)2-TiO2 composite film is found to exhibit much larger photocurrent and higher performance in photocatalytic degradation of methyl orange than the pure TiO2 film.