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 (neff) 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 (S ), which is defined as:
S =∆λres/∆n,
where ∆λres 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 S and, additionally, contrariwise proportional to the width of the resonant feature (spectral dip or peak=FWHM ) being recorded. The aggregate of these variables is frequently described as the figure of merit (FOM ) and is stated as:
FOM =S /FWHM ,
Even though the FOM is quite high in certain articles, it is interpreted in a different way in each work and is typically described to as FOM*. For example, in , a great value ofFOM *=2.33×104 is derived by means of the formula ∆R/(R∆n) at a constant wavelength, where R is the reflection rate in the sensor structure and ∆R signifies the fluctuation in reflection intensity caused by changes in the ambient refractive index (∆n). FOM* =4.05×104 is achieved in by utilizing the equation ∆T/T∆n, where T stands for the transmittance in the suggested structures and ∆T/∆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 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 FOM , Fano resonant systems may be superior to the more traditional Lorentz resonant systems; however, the systems’ ability to translate improvements inFOM 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.