We present strong enhancement of third harmonic generation in an amorphous silicon metasurface consisting of elliptical nano resonators. We show that this enhancement originates from a new type of multi-mode Fano mechanism. These ‘Super-Fano’ resonances are investigated numerically in great detail using full-wave simulations. The theoretically predicted behavior of the metasurface is experimentally verified by linear and nonlinear transmission spectroscopy. Moreover, quantitative nonlinear measurements are performed, in which an absolute conversion efficiency as high as ηmax ≈ 2.8 × 10−7 a peak power intensity of 1.2 GW cm−2 is found. Compared to an unpatterned silicon film of the same thickness amplification factors of up to ~900 are demonstrated. Our results pave the way to exploiting a strong Fano-type multi-mode coupling in metasurfaces for high THG in potential applications.

We present the enhancement of frequency doubled and tripled light in zinc oxide (ZnO) by utilizing arrays of Mie-resonant dielectric nanoantennas. The cylindrical antennas exhibit resonances at the fundamental excitation wavelength, enabling strong localization of light and redistribution of the electric field inside the structures. Linear transmission spectroscopy reveals the resonant behavior of the fabricated nanoantennas and confirms the performed theoretical simulations. Nonlinear spectroscopic measurements show direct dependency between the resonant behavior and the amplification of nonlinear signals. Comparing the unpatterned ZnO thin films with the dielectric nanostructures shows a maximum enhancement by factors of about 22 for SHG and 7 for THG from resonant excitations of the nanoantennas.

We show the effective generation of second harmonic generated light in thin zinc oxide (ZnO) films by using double-resonant plasmonic nanoantennas. The designed structure consists of three gold rods with two localized surface plasmon resonances at ω and 2ω. Two of the nanoantennas are designed to be resonant for the fundamental frequency ω in order to provide a strong localization of light. The third antenna lies in between the two fundamental antennas separated by a small gap (≈30nm). Due to a strong overlap between the second harmonic resonance at 2ω and the fundamental resonance, the overall far-field radiation is significantly increased. Second harmonic generation spectroscopy measurements show an enhancement by a factor of nine compared to the emission from single dipole rods. Additionally, by optimizing the lattice constant for the nanoantenna arrays, the overall second harmonic response from the ZnO was significantly increased by a factor as large as 70, which is a great improvement for comparable plasmonic nanoantennas on thin zinc oxide.

Electric-field-induced second harmonic generation (EFISH) as a third order nonlinear process is of high practical interest for the realization of functional nonlinear structures. EFISH in materials with vanishing χ(2) and non-zero χ(3) offers huge potential, e.g., for background-free nonlinear electro-optical sampling. In this work, we have investigated SiO 2 as a potential EFISH material for such applications using DC-electric fields. We were able to observe significant second harmonic generation (SHG) in comparison to the background SHG signal. The fundamental excitation at 800 nm results in a SHG signal at 400 nm for high applied DC electric fields, which is a clear indication for EFISH. Additionally, we were are able to precisely model the EFISH signal using time-domain simulations. This numerical approach will be of great importance for efficiency enhancement and prove as a valuable tool for future device design.

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