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\begin{document}

\title{Metal-insulator-metal waveguide based plasmonic sensors: Fantasy or
truth- A critical review}



\author[1]{Muhammad Ali Butt}%
\affil[1]{Politechnika Warszawska Instytut Mikroelektroniki i Optoelektroniki}%


\vspace{-1em}



  
  \date{May 16, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
There is a hysteria over plasmonic sensors based on
Metal-insulator-metal (MIM) waveguides and researchers around the globe
are extensively studying these devices over the past two decades for
diverse sensing applications for instance temperature, refractive index,
pressure, and biochemical applications among others. The sensors based
on MIM waveguides (WGs) are compact and offer unmatchable sensitivity as
assessed with dielectric waveguide-based sensors along with an
extraordinary figure of merits ( \emph{FOM}). However, the major concern
is that most of these sensors are only numerically simulated/analyzed
and there is no experimental demonstration accessible to date.
Therefore, in this mini-review, these highly alluring sensor designs are
critically analyzed to identify if they are practically feasible or just
a fantasy among the scientific community.%
\end{abstract}%



\sloppy


\textbf{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 \selectlanguage{greek}μ\selectlanguage{english}m. 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 \selectlanguage{greek}λ/\selectlanguage{english}2n, where n is the refractive index of the core and \selectlanguage{greek}λ \selectlanguage{english}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 \selectlanguage{greek}λ/\selectlanguage{english}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.

\textbf{Hosted file}

\verb`image1.emf`  available at \url{https://authorea.com/users/518234/articles/592467-metal-insulator-metal-waveguide-based-plasmonic-sensors-fantasy-or-truth-a-critical-review}

Figure 1. (a) 3D model of a MIM WG structure, which constitutes a thin
metal film deposited on a substrate. And a narrow slit (few nm) is
inscribed in the thin-film which provides a way for the SPP to propagate
in a low-index medium, (b) a cross-sectional view of the E-field
distribution taken at the output of the MIM WG, (c) The relationship
between the real part of the effective index and the ambient refractive
index.

In this mini-review, the recent advances in MIM WG-based plasmonic
sensors are investigated and trying to figure out the possible causes
which limit the experimental demonstration of such attractive devices.
To the best of the author's knowledge, almost all the reports are
devoted to the numerical analysis of the nanoscale sensing devices
without addressing the real challenges that can arise during
nanofabrication, sensing mechanisms (by using microfluidic channels),
and optical characterization. There are more than \textasciitilde{}6000
reports published up till now on MIM WG devices indexed in the Scopus
database. Butt et al. have also published more than 30 papers on the
numerical study of the MIM WG-based plasmonic sensors. Most of the MIM
WG-based sensors are based on unique resonant cavity schemes (as
discussed later) where the gap between the bus WG and the cavity is
\textasciitilde{}10 nm-50 nm, furthermore, the additional geometric
parameters such as the width of the bus WG, defects or baffles are also
in nm scale. This raises a question on the method of fabrication,
reproducibility, and light coupling methods during device
characterization.

\textbf{High-performance plasmonic sensor designs and sensing
applications}

In the past two decades, MIM plasmonic WG devices are numerically
simulated for temperature, refractive index, pressure, filtering, and
biochemical sensing applications. Since they may be used to monitor
solution concentration and pH level, as well as other biological and
chemical parameters, refractive index sensors have been the subject of
much research. By stimulating the sensing element with light that
creates SPs that are focused on the metal's surface, an EM field is
created. The MIM WG's effective refractive index (n\textsubscript{eff})
changes when a substance under examination comes into connection with
the sensor, which causes resonance wavelength to experience a redshift.
Several vital factors must be considered while designing sensing
devices. The ability to recognize variations in the ambient refractive
index is the most often employed performance trait of plasmonic sensors.
This is frequently expressed in terms of the bulk refractive index
sensitivity (\emph{S} ), which is defined as:

\emph{S} =[?]\selectlanguage{greek}λ\selectlanguage{english}\textsubscript{res}/[?]n,

where [?]\selectlanguage{greek}λ\selectlanguage{english}\textsubscript{res} is the change in the wavelength at which SP
is excited and [?]n is the change in the ambient refractive index. A
plasmonic sensor's capacity to detect slight variations in the
refractive index is directly related to \emph{S} and, additionally,
contrariwise proportional to the width of the resonant feature (spectral
dip or peak=\emph{FWHM} ) being recorded. The aggregate of these
variables is frequently described as the figure of merit (\emph{FOM} )
and is stated as:

\emph{FOM} =\emph{S} /\emph{FWHM} ,

Even though the \emph{FOM} is quite high in certain articles, it is
interpreted in a different way in each work and is typically described
to as \emph{FOM*.} For example, in , a great value of\emph{FOM}
*=2.33\selectlanguage{ngerman}×10\textsuperscript{4} is derived by means of the formula \selectlanguage{english}[?]R/(R\selectlanguage{english}[?]n)
at a constant wavelength, where R is the reflection rate in the sensor
structure and \selectlanguage{english}[?]R signifies the fluctuation in reflection intensity
caused by changes in the ambient refractive index (\selectlanguage{english}[?]n). \emph{FOM*}
=4.05x10\textsuperscript{4} is achieved in by utilizing the equation
\selectlanguage{english}[?]T/T\selectlanguage{english}[?]n, where T stands for the transmittance in the suggested structures
and \selectlanguage{english}[?]T/\selectlanguage{english}[?]n for the transmission shift at a fixed wavelength brought on by
a change in refractive index.

A discrete state and a continuous state can couple and interfere with
one another, leading to a phenomenon known as Fano resonance . In
metallic nanostructures, Fano resonance has been investigated as the
ideal property for bypassing the diffraction limit of light brought on
by SPPs . To create Fano resonance, many plasmonic structures, including
rectangular cavities , plasmonic nanoclusters , nanoslits , and MIM WG
structures , have been suggested. Because of the abrupt and asymmetric
line structure of plasmonic sensors based on Fano resonance, which
allows the transmission spectrum to be quickly lowered from peak to
trough, it is predicted that these sensors will be very sensitive. The
transmission spectrum's \emph{FWHM} is quite small, which greatly
enhances the sensor's detecting resolution. A few of the several Fano
resonance-based refractive index sensor architectures that have been
published in 2022 are stated here. In terms of \emph{FOM} , Fano
resonant systems may be superior to the more traditional Lorentz
resonant systems; however, the systems' ability to translate
improvements in\emph{FOM} into the ability to detect smaller changes in
the bulk refractive index may be constrained by the insufficient
contrast of plasmonic spectral features.

Figure 2 demonstrates the several designs of the MIM WG-based plasmonic
sensors employed for refractive index and temperature sensing
applications that are numerically investigated by the researchers.
Several reports proposed a use of defects, baffles, and nanodots as a
mechanism to enhance the sensitivity by narrowing down the path of the
SPs which results in the enhanced light-matter interaction. However,
this requires extremely high-resolution lithographic process and precise
etching of the unwanted metal layer to create such designs. The sensing
performance of all the devices is extremely high, however, none of the
researchers have studied the mechanism of light coupling and the losses
associated with it to such nanoscale WGs. Therefore, how many of these
sensor designs can be practically realizable is a tough question for the
experimental groups to obtain devices with sub-nanoscale footprints.
Moreover, electron beam lithography (EBL) is specifically used to
pattern nanoscale structures. The main benefit of EBL is that it has a
sub-10 nm precision for writing customized patterns. Due to its high
resolution and slow throughput, this type of direct writing is only
appropriate for the creation of photomasks, small quantities of
semiconductor devices, and research and development. Table 1
demonstrates the sensing capabilities of some of the prominent MIM
WG-based plasmonic sensing devices which are either simulated via the
Finite element method (FEM) or finite difference time domain (FDTD)
method. In comparison to a 3D model, almost all the researchers choose a
2D model since it requires less processing time and produces more
precise results.

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/518234/articles/592467-metal-insulator-metal-waveguide-based-plasmonic-sensors-fantasy-or-truth-a-critical-review}

\textbf{Figure 2.} MIM WG based plasmonic sensor designs based on
different cavity shapes, (a) side coupled square cavity, (b) end coupled
ring cavity, (c) asymmetric structure, (d) ring and a rectangular, (e)
side coupled ring and a defect , (f) end coupled square cavity, (g) side
coupled split ring cavity, (h) cavity with defect, (i) T-shaped cavity
with nanorods, (j) L-shaped cavity, (k)dual hexagonal cavities, (l) side
coupled ring with baffles, (m) horizontal eight shape, (n)horn shaped
cavity, (o) racetrack and circular cavity.

Table 1. Sensing characteristics of MIM WG-based plasmonic sensors\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
\begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
Author(s)\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
Application\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\emph{S}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\emph{FOM}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
Demonstration\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
Reference\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Xu et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1500 nm/RIU\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
75\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Kazanskiy et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index and temperature sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
700 nm/RIU and -0.35 nm/\textsuperscript{o}C\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
21.9 and 0.008\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Khonina et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Gas sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
135.95 pm/ppm\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Wang et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1114.3 nm/RIU\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
55.71\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Rohimah et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1333 nm/RIU\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
5876\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
Butt et al.\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
Refractive index and temperature sensing\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
1240 nm/RIU to 1350 nm/RIU, -0.58 nm/\textsuperscript{o}C to -0.64
nm/\textsuperscript{o}C\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
18.74 to 691\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Binfeng et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1488 nm/RIU\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Yang et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index and temperature sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1750 nm/RIU and 2.455 nm/\textsuperscript{o}C\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{-}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Palizvan et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Pressure sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
16.5 nm/MPa\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Wu et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index and temperature sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
3460 nm/RIU and 1.36 nm/\textsuperscript{o}C\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Jumat et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
3400 nm/RIU\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
36\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Butt et al.\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Refractive index sensing\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1065 nm/RIU\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
251.17\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Numerical\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Are MIM WG-based plasmonic sensors practically realizable?}

Semiconductors are significantly easier to microfabricate than metals
especially noble metals. With the use of sputtering or evaporation
methods, plasmonic metals may be deposited in vacuum environments.
Deposition rates are generally restricted to less than 1 nm/s to achieve
sufficient uniformity and quality. Due to the often-low necessary
plasmonics structure height, the modest deposition rate is not a major
issue. The electroplating process can produce faster growth rates, but
the surface quality may degrade, making it unsuitable for plasmon waves.
As there are few efficient ways to etch Cu, Ag, or Au, the pattern
transmission from the photoresist to metals is difficult for noble
metals. However, in Cl\textsubscript{2}-based settings, Al may be plasma
etched.

Plasmonic WGs are exceptionally lossy since they include metal, in
contrast to dielectric WGs where the propagation loss might be
negligible. Major ohmic losses that set a maximum propagation length
restriction on directed SP propagation. Various geometries have been
created employing arrays of nanosize dimension characteristics to
compensate for these losses. As plasmonic WGs, thin metal films with a
finite width embedded in a dielectric can be employed. Given that the
observed propagation length for operating with light with a wavelength
of 1550 nm is stated to be as long as 13.6 mm, this shape provides the
best propagation results for a surface plasmon-based WG. However, in
this plasmonic WG shape, the localization for both directions is on the
order of a few micrometers. One can narrow the wire's width to achieve
subwavelength localization and then utilize SPs to direct light beneath
the nanowire.

In general, propagation loss and mode confinement reflect a trade-off in
any plasmonic WG. The propagation loss increases with decreasing mode
size. The modes carried by plasmonic WGs often have more complicated
geometries. Therefore, assessing the mode confinement of plasmonic WGs
is a difficult process. The definition of the mode region should rely on
the specific application, it has been considered and agreed upon. Energy
dissipation affects both plasmonic and electronic circuits. The SP
propagation length, over which the SPP intensity falls to 1/e of its
initial value, can be used to represent the propagation
loss.\textbf{Figure 3} presents the trade-off between the device
sensitivity and other major factors such as footprint, optical losses,
and fabrication complexities.

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/518234/articles/592467-metal-insulator-metal-waveguide-based-plasmonic-sensors-fantasy-or-truth-a-critical-review}

\textbf{Figure 3} . Sensitivity, fabrication complications, losses, and
footprint comparison of plasmonic WGs with dielectric or hybrid
plasmonic WGs.

The main benefit of the plasmonic WGs is that the operational wavelength
of light does not influence its size. This makes it possible to create
plasmonic WGs with lengths on the order of several tens of nanometers,
something that is not feasible with dielectric WGs. Additionally,
exceptionally strong light confinement and effective interaction with
nanomaterials that have various special properties become conceivable. A
three-dimensional plasmonic mode converter is necessary since such WGs
cannot readily be included in optical integrated circuits. The plasmonic
WG is made up of a MIM WG, a Si-wire WG, and a plasmonic mode converter
section. On an SOI substrate, the silicon ridge WG, and the silicon
section of the plasmonic mode converter are constructed, followed by the
formation of the metal components using electron beam lithography and
evaporation. This set of procedures makes use of standard methods for
fabricating nanostructures and is hence compatible with those applied to
other silicon photonic devices. For device integration, this makes it
possible to integrate with other optical components on the same
substrate. For a MIM WG with a core size of 50 nm \selectlanguage{ngerman}× 20 nm, a plasmonic
mode converter with an air gap width of 40 nm has been developed as
shown in Figure 4 (a). The E-field distribution in the whole plasmonic
device and at different segments of the WG is presented in Figure 4 (b).

\textbf{Hosted file}

\verb`image4.emf`  available at \url{https://authorea.com/users/518234/articles/592467-metal-insulator-metal-waveguide-based-plasmonic-sensors-fantasy-or-truth-a-critical-review}

\textbf{Figure 4} . Demonstration of a plasmonic WG, (a) SEM image of
the MIM WG coupled to a plasmonic mode converter, (b) E-field
distribution in the plasmonic mode converter-full device (top), the
E-field distribution in the different parts of the device (bottom).

The air gap width has a significant impact on the mode converter's
coupling efficiency (CE). Without the air gap, good CE cannot be
achieved, but the CE is significantly increased by adding a tiny air gap
of roughly 20 nm. But when the air gap width rises, the CE steadily
declines until it reaches the same level as when there is no air gap at
a width of 100 nm. Additionally, without the airgap, the metal portion
of the mode converter absorbs or scatters most of the propagating light.
Most of the light is scattered when the gap is large because the side
lobes of the silicon ridge WG cannot be properly drawn to the metal.

Despite its apparent simplicity, the procedure detailed here is
difficult due to the positional precision needed for fabrication at a
scale of ca. 20 nm or less. With the use of NTT's advanced nanostructure
production methods, this procedure is feasible. The integration of WGs
with a deep-subwavelength core to optical integrated circuits is made
possible by the creation of the first extremely efficient 3D mode
converter. This accomplishment not only reduces the size of traditional
devices but also lays the path for the possible use of novel devices
with never-before-seen features in conjunction with nanomaterials. It is
expected that more experimental research groups will join hands to find
a low-cost and easy solution for the realization of these exceptional
plasmonic sensor designs which are extensively published without
experimental demonstration.

\textbf{Conclusion}

Plasmonic sensors based on MIM WGs are exceedingly attractive due to
their small footprint and can offer extremely high sensitivity which
cannot be obtained with conventional dielectric WG sensors. The triumph
of such devices remains in the straightforward, accurate, less complex,
low-cost fabrication and its compatibility with the existing platforms.
Moreover, flexible sensor designs are required where minor variations in
the geometric parameters due to fabrication imperfections should not
deteriorate its sensing performance. Hence, Dr. Butt, the author of this
paper believes that the plasmonic mode converter proposed by Ono M et
al. at NTT is valuable for the realization of such a plasmonic WG
structure, however, it is still in its infancy and entails a long way to
find alternate ways to ensure the reproducibility of such devices with
low technology demand.

\textbf{Acknowledgment}

The author would like to thank the Warsaw University of Technology for
continuous support in the completion of this work.

\textbf{Conflict of interest:} There is no conflict of interest.

\textbf{Data availability statement:} The data that support the findings
of this study are available from the corresponding author upon
reasonable request.

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