Enhanced four-wave mixing in silicon nitride waveguides coated with indium tin oxide operating at epsilon-near-zero

We present a theoretical investigation of four-wave mixing (FWM) enhancement in silicon nitride (Si₃N₄) waveguides incorporating an indium tin oxide (ITO) layer operating at epsilon-near-zero (ENZ). Nonlinear numerical simulations were employed to optimize the hybrid waveguide geometry and quantify improvements in conversion efficiency with respect to pump power while accounting for absorption introduced by the incorporation of a lossy ENZ medium. The ENZ-tuned ITO layer provides strong field enhancement and a substantial increase in the effective nonlinear coefficient, resulting in a pronounced increase in FWM efficiency. However, a trade-off exists between enhancement and attenuation, limiting the waveguide length over which efficiency improvements occur. For instance, enhanced output idler power persists in an ITO-coated Si₃N₄ waveguide relative to the uncoated case up to ∼40 µm with a maximum improvement of 15 dB at ∼6 µm. Micro-ring resonators were also compared, resulting in an improvement of ∼1.6 dB in idler power for the ITO case. Design optimization is expected to yield improved performance. The proposed platform offers a promising route towards efficient compact non-linear waveguide devices for applications such as the generation of continuous-variable squeezed light, thereby advancing scalable quantum photonic technologies.