Investigation of Second Harmonic Generation in Asymmetric Metal-Insulator-Metal Plasmonic Waveguides

In this study, the second harmonic generation in metal-insulator-metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Nonlinear processes such as second harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, it was shown that field enhancement in asymmetric metal-insulator-metal waveguides can result in large enhancement of SHG. Thus, a structure consisting of a MIM waveguide filled with lithium niobate and sandwiched between two same metals was first considered in this study. Thereafter, two different metals on both sides of the waveguide were used. It was shown in this study that this asymmetric device results in more than two orders of magnitude enhancement in SHG compared to a uniform slab of lithium niobate. For such structures, the field enhancement is due to the squeezing of the optical power from the wavelength-sized dielectric waveguide to the deep sub-wavelength MIM waveguide. So, the novelty of this paper is proposing a metal-LiNbO3-metal nanostructure with different top and bottom metals as plasmonic waveguides. Two different metals, gold and silver, are used as the metals of plasmonic waveguides, and the SHG is investigated in different structures. The interaction between interfering surface plasmonic polariton modes is studied. It is found that compared to the conventional symmetric metal-insulator-metal plasmonic waveguides, the asymmetric structure with different metals, silver-LiNbO3-gold, has higher SHG and longer propagation distance.