Optimizing of the novel asymmetric plasmonic waveguide with two identical gratings to increase the SHG efficiency

In this paper a novel plasmonic waveguide is proposed to increase the second harmonic generation (SHG) efficiency by considering asymmetric plasmonic waveguide geometry and two identical grating separated with each other by spacing. The proposed structure consists of two different metals on both sides of lithium niobate. By using two different metals the nonlinear susceptibility of the waveguide would be increased noticeably causing to increase SHG process. On the other hand, it consists of two identical gratings on one side. By two identical gratings, the pump beam is coupled to two opposing SPP waves, which interfere with each other and results in SPP standing wave in the region between the two gratings. The needed phase matching condition is satisfied between the fundamental waveguide mode at the fundamental frequency and second order waveguide mode at the second harmonic frequency (SHF) by an appropriate design of the waveguide geometrical parameters. The details of structure including the metals of top and bottom, distance between two gratings, depth, and the duty cycle of gratings will be optimized to reach the highest SHG efficiency and the highest SHF optical power. It will be shown that by optimizing the geometry of proposed structure and using different metals, field enhancement in proposed waveguide can result in large enhancement of SHG. The SHG signal generated in proposed waveguide is more than four orders of magnitude higher than those previously reported. The device length is shorter than 3 µm and the normalized SHG conversion efficiency comes up to more than 9 × 10⁷ W⁻¹ cm⁻².