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.