Discussion of Two Ways of Optically Modeling Indium–Tin–Oxide Layers in Slot Waveguides for Waveguide Analysis

Slot waveguides (e.g., metal-insulator-metal (MIM) waveguides) with an indium-tin-oxide (ITO) layer in their slot regions are promising as compact intensity modulators. To analyze such waveguides, a uniform-accumulation layer model is usually used. In this model, the ITO layer is treated as a stack of a layer with untuned permittivity and a layer with permittivity tuned by driving voltage, which is called an accumulation layer. However, rigorous analysis requires a gradient-index layer model in which the ITO layer is treated as a layer with a continuously varying permittivity distribution. It is necessary to check whether the uniform-accumulation layer model results in correct analysis in comparison with the gradient-index layer model. This paper analyzes an ITO-based MIM-type slot waveguide using the two models. Compared with the analysis based on the gradient-index layer model, the analysis based on the uniform-accumulation layer model becomes incorrect if the driving voltage is large, particularly when it is around the value for which the real part of the accumulation layer permittivity is equal to zero. Therefore, this paper shows that the uniform-accumulation layer model should be used just for small driving voltage with the accumulation layer thickness determined appropriately. This paper may correct misunderstanding of the properties of ITO-based slot waveguides.