Designing Mid-Infrared Gold-Based Plasmonic Slot Waveguides for CO2-Sensing Applications

Plasmonic slot waveguides have attracted much attention due to the possibility of high light confinement, although they suffer from relatively high propagation loss originating from the presence of a metal. Although the tightly confined light in a small gap leads to a high confinement factor, which is crucial for sensing applications, the use of plasmonic guiding at the same time results in a low propagation length. Therefore, the consideration of a trade-off between the confinement factor and the propagation length is essential to optimize the waveguide geometries. Using silicon nitride as a platform as one of the most common material systems, we have investigated free-standing and asymmetric gold-based plasmonic slot waveguides designed for sensing applications. A new figure of merit (FOM) is introduced to optimize the waveguide geometries for a wavelength of 4.26 mu m corresponding to the absorption peak of CO2, aiming at the enhancement of the confinement factor and propagation length simultaneously. For the free-standing structure, the achieved FOM is 274.6 corresponding to approximately 42% and 868 mu m for confinement factor and propagation length, respectively. The FOM for the asymmetric structure shows a value of 70.1 which corresponds to 36% and 264 mu m for confinement factor and propagation length, respectively.

Automated summary

Plasmonic slot waveguides offer high light confinement but suffer from significant propagation loss due to the presence of metal. Balancing the trade-off between confinement factor and propagation length is essential for optimizing waveguide geometries.