Second harmonic generation in a graphene-based plasmonic waveguide

Lithium niobate nanophotonic structures have recently become a promising candidate for efficient nonlinear frequency-conversion processes. Here, the second harmonic generation in a graphene-based LN waveguide is theoretically proposed at the telecommunication band. The structure is able to gain high conversion efficiency due to the large nonlinear coefficient of LN and tight field confinement. The subwavelength mode confinement inside the LN layer is strongly influenced by the graphene conductivity. In the presented structure, the nonlinear interaction of propagating plasmons can be widely tuned by slightly change in the surface conductivity of graphene monolayer which is a promising feature for SHG applications in comparison to the conventional structures which rely on geometry variation. According to the results, SH intensity of $$I_{{{\text{SH}}}} = 0.09\,{\text{kW}}/{\text{cm}}^{2}$$ is observed at the fundamental wavelength of $$1550\,{\text{nm}}$$ with a 7% of nonlinear conversion efficiency. To analyze the geometrical parameters and show the tunability of the configuration, the effect of input frequency and waveguide length on SH output power are demonstrated at $$P_{{{\text{FF}}}} = 1W$$ and μc = 0.6 eV. The calculations reveal that the $$P_{{{\text{SH}}}}$$ becomes lower by lengthening the waveguide where the maximum output of $$P_{{{\text{SH}}}} = 72.5\,{\text{mW}}$$ is obtained at 1 μm-long waveguide.