Introduction
The study of surface plasmon polaritons (SPPs)-based nanophotonics has rapidly increased over the past two decades. Well beyond the diffraction limit, these electromagnetic (EM) waves can be steered by metallic nanostructures as they travel over metal-dielectric surfaces. The progress of highly integrated photonic signal-processing systems, sensors, and optical imaging techniques with nano resolution are all made possible by these extraordinary capabilities. Thanks to their special qualities, plasmonic-based devices have attracted a lot of attention in recent years, significantly increasing the sensitivity of photonic sensors. Expectations regarding the role of metals in the creation of novel optical devices based on plasmonic phenomena, including SPPs, are being changed by ongoing advances in nanofabrication. These have potential applications in a variety of fields, including nanophotonics, biosensing, electronics, imaging, and many more.
Among other plasmonic waveguides, metal-insulator-metal (MIM) WG architecture is one of the most widely utilized plasmonic-based nanostructures for the creation of integrated optical circuits. MIM WGs are plasmonic structures that have two metal claddings around an insulator as shown in Figure 1 (a). The primary characteristics of this system are its straightforward construction and potential to restrict light at the subwavelength level. The E-field distribution taken at the output of the MIM WG is shown in Figure 1 (b). The light injected in this WG propagates in the air and is fully exposed to the ambient medium. When there is a slight change in the refractive index in the ambient medium, this brings a large shift in the effective index of the propagating mode as shown in Figure 1 (c). Another appealing plasmonic WG structure that has been employed in both active and passive devices is an insulator-metal-insulator (IMI), which sandwiched metal between two insulator claddings. Since IMI WGs have substantially lower propagation losses than MIM WGs, they are often utilized to transmit near-infrared optical power across distances greater than 10 μm. A TM mode that is equal to a dielectric mode can be sustained if the symmetry criterion is strictly satisfied. However, its use is constrained at deep subwavelength scales due to the absence of mode confinement.
Heisenberg’s uncertainty principle states that further reduction in the dimension of dielectric WGs will inevitably result in cut-off because the mode size of a dielectric WG is affected by the diffraction limit (close to λ/2n, where n is the refractive index of the core and λ is the incident wavelength in a vacuum). The size of the focused point was shown to be almost directly related to the radius of the nanosphere. Thus, when the radius of the silicon nanosphere is 2 nm, light is constrained to a focused point with a size of λ/373. Due to this restriction, novel WG structures and materials are being investigated. The light may be contained on a deep subwavelength scale by WGs based on surface plasmons (SPs) that can sustain a propagation mode that is closely coupled to metallic surfaces. As a result, plasmonics has drawn a lot of interest due to their potential for overcoming diffraction limitations. To further identify the optical characteristics of plasmonic systems, theoretical, computational, and numerical simulation tools like COMSOL and Lumerical have been developed. The rapidly growing discipline of plasmonics combines basic study with practical applications in physics, engineering, chemistry, biology, food science, medicine, and environmental sciences.