In recent years, plasmonic effects and devices have commanded increased attention, as they enable sub-wavelength photonics applications. The existence of surface plasmon polaritons (SPPs), which are essentially guided electromagnetic waves propagating along a metal–dielectric interface with strong near-field confinement, allows for efficient light–matter interactions (Prämassing et al., 2020; Gaur et al., 2021). For the guided mode, the maximum intensity occurs at the metal–dielectric interface where the field amplitudes decay exponentially in the direction perpendicular to the interface (Dionne et al., 2006). This so-called evanescent field represents the bound, non-radiative nature of SPPs (Barnes et al., 2003). A plasmonic device that has attracted particular attention is the plasmonic waveguide. A specific merit of SPP-based waveguides is that they can break the diffraction limit related to the dielectric waveguides, leading to strong field confinement (Sun et al., 2014). Among the plasmonic waveguides, plasmonic slot waveguides are particularly attractive due to the provided lateral confinement and the capability of guiding the mode in low-index materials, which can be an asset in the fabrication of miniaturized optical circuits. Therefore, compared with dielectric waveguides, they allow compact integration and short coupling lengths (Osowiecki et al., 2014). Furthermore, as plasmonic slot waveguides allow for a high-energy fraction in low-index material, they promise high sensitivity with respect to absorption in a gaseous sensing medium, acting as a low-index medium in such a sensor.

Traditionally, noble metals have been widely used as plasmonic materials due to their chemical stability, large plasma frequency and high electrical conductivity (Paulraj et al., 2020). However, they typically suffer from high intrinsic losses and high costs, hampering practical applications (Chen et al., 2017). As alternatives to noble metals, transparent conductive oxides (TCOs) can be considered. They not only show a low loss in the infrared range but their compatibility with standard microfabrication also makes them particularly appealing for the simple integration of plasmonic devices (Wang et al., 2017). Another feature of TCOs is that their optical and electrical properties can be tuned by changing the doping concentration. In addition, TCOs are often fabricated using thin-film technology and are utilized in optoelectronic applications, such as electrode materials in biosensors and other sensors (Sohn et al., 2011). Among TCOs, indium tin oxide (ITO) has attracted particular attention from many scientists and researchers due to its surface smoothness, mechanical stability, robustness in moist air and simple fabrication (Dao et al., 2019). Therefore, it has been used in many studies related to optical meta-surfaces (Shrestha et al., 2018), localized plasmonic nanoparticles (Xi et al., 2018) and active tuneable plasmonic devices (Liu et al., 2018). Moreover, ITO is an appropriate material for sensing applications owing to unique features such as high electrical conductivity, substrate adhesion, and electrochemical and physical stability (Aydin et al., 2017). However, as ITO is a non-stoichiometric compound, its optical properties crucially depend on the fabrication processes such as growth and deposition procedures as well as on annealing conditions (West et al., 2010). It has already been shown that sputtering power and annealing processes can affect the optical properties of ITO in the mid-infrared spectrum (Wang et al., 2017). ITO thin films can be produced using technologies such as magnetron sputtering, electron beam evaporation and chemical vapor deposition. Among these methods, magnetron sputtering is widely used due to the better film quality, faster deposition rate and the associated repeatability (Zhang et al., 2020).

Although ITO has been introduced as a desirable plasmonic material to replace noble metals, to the author's knowledge, it has neither been studied in detail theoretically nor experimentally for application in waveguiding systems to date. We consider ITO for waveguiding systems because it shows a high carrier density which makes it a suitable alternative plasmonic material in the near- and mid-infrared region. In this paper, a plasmonic slot waveguide based on ITO has been designed and numerically analyzed for sensing applications. Our sensor platform is designed for a wavelength of 4.26 µm, which corresponds to the absorption peak of CO2, and serves as an example of the application of infrared absorption sensing in gases. We used an ITO-based slot waveguide on top of 2 µm silica as a substrate, as illustrated in Fig. 1. In practice, this substrate can be an oxidized silicon wafer. In the simulation, the region to be occupied by the sensing medium in the final application is associated with a refractive index of 1, as we are studying the properties of the guided mode. The modal properties of the proposed structure have been investigated using the finite element method (FEM) implemented by the COMSOL Multiphysics software (COMSOL, 2022). The refractive index of SiO2 and ITO at 4.26 µm is nSiO2=1.38+0.00043i and nITO=1.9213+5.65i (Wang et al., 2017), respectively.

The preliminary results of this study were presented at the Sensors and Measurement Science International (SMSI) conference in 2021 (Saeidi et al., 2021a). In the conference paper, our sensor platform had been optimized based on the figures of merit (FOM) as a product of the evanescent field ratio and propagation length. In the current work, to consider the effect of group velocity, the confinement factor including both the evanescent field ratio and group velocity has been evaluated. Therefore, our sensor platform is optimized based on a FOM, which is defined as the confinement factor divided by the imaginary part of the effective mode index. Our aim is to optimize the waveguide geometries to reach the optimal propagation length and confinement factor simultaneously.

We investigated the feasibility of ITO as an alternative plasmonic material to replace noble metals for sensing applications in the mid-infrared region. Considering a specific example, an ITO-based plasmonic slot waveguide was designed and numerically analyzed for a wavelength of 4.26 µm, which is the absorption band of CO2. The plasmonic slot waveguide was optimized based on a figure of merit (FOM) composed of the product of the propagation length and the confinement factor, which are two crucial quantities in sensing applications. The achieved optimal FOM is 3.2, which is associated with 9 µm and 49 % for the propagation length and confinement factor, respectively. As the optical properties of ITO are highly dependent on the fabrication process such as growth and deposition process and also on annealing conditions, based on the adopted fabrication methods, the optical properties of ITO, such as the complex refractive index, can vary. Thus, the obtained FOM might change if another refractive index associated with another fabrication method is used. Nevertheless, the optimization approach introduced in this work can always be used to find the best geometrical parameters of any plasmonic slot waveguide.

As the proposed structure shows low propagation lengths, we finally have to conclude that ITO appears not to be an appropriate candidate for plasmonic waveguiding systems. However, based on the achieved high confinement factor, it could be an attractive material for applications in which a cavity or resonator would be used.