# Microchannel-Embedded D-Shaped Photonic Crystal Fiber-Based Highly Sensitive Plasmonic Biosensor
## 1 Introduction  
Surface plasmon polaritons (SPPs) are propagating electromagnetic waves that include plasma oscillations of negatively charged particles in metal and lightwave in dielectric/air. In recent years, SPP-based optoelectronic devices have received a lot of interest for their ability to detect a very little variation of analyte refractive index (RI) and provide high sensitivity, immunity to electromagnetic interference, label-free and real-time detection [1, 2]. The surface plasmon resonance (SPR) occurs at a particular frequency (i.e., resonant wavelength) and similar polarization state when the surface plasmon (SP) is excited by the evanescent electric field and the input-guided lights are phase matched. The resonant wavelength of the SPR-based sensing device is extremely responsive to the analyte in contact and a little alteration of the sensing medium RI results in resonant wavelength shifting; therefore, a qualitative measure of the wavelength shift allows to track unknown analyte RI. Different optical structures, for example, the prism-based structure proposed by [PERSON], fiber Bragg grating, optical fiber, or microstructured optical fiber, are used that increase the momentum of the light to excite SPPs at the metal-dielectric/metal-air boundary by evanescent wave [3]. The mechanical components of the prism-based sensors are larger in size and associated with moving parts, which limitstheir applications for the remote sensing operation. Optical fiber-based sensors have great advantages over the prism-based sensors for their small size, flexible nature, and remote sensing operation [4; 5]. For example, an SPR-based optical-fiber sensor with metallic grating is shown in [4] with a maximum sensitivity of 25 \(\upmu\)m/RIU for RI sensing range from 1.33 to 1.38. However, limited structural parameters of these sensors prohibit their wider operations, and to work as a sensor, cladding needs to be removed. Among different SPR-based sensing configurations, the SPR integrated photonic crystal fiber (PCF) sensors allow endless ways to couple strongly core-guided light with the SP by reshaping structural parameters based on sensing applications and overcome the limitations of conventional systems [6; 7]. Due to miniaturization, real-time sensing, flexible design, and remote sensing properties, PCF-based SPR sensors can be applied for many useful sensing applications [7; 8; 9]. The operation of the PCF-based SPR sensors is primarily classified depending on the material deposition either outside on the dielectric of PCF or inside the airhole wall of PCF. Metal layer deposition on the internal wall of PCF leads to good detection of analyte due to easy coupling of the core mode with SPP mode [10]. However, metal deposition on the wall of a tiny air hole and infiltration of liquid analyte is laborious. On the other hand, metal film deposition over the dielectric surface of PCF makes the sensor fabrication easier, which opens the possibility of novel designs of PCF sensors such as D-shaped sensors, exposed-core sensors, U-shaped sensors, and so on [11; 12; 13]. The selection of plasmonic metal is crucial to improve the sensitivity of these sensors. Silver, aluminum, copper, and gold are frequently used as sensing media [14]. Though silver has lower optical damping and a narrow loss spectrum, it exhibits oxidation difficulty in the surroundings aqueous medium [15]. Gold shows superior performance among all these metals due to being chemically inert, biocompatible, having larger peak shift, and being stable in an aqueous environment [14; 16]. Most of the substances of interest have an RI less than 1.40. Different biological and chemical samples have an RI in the range of 1.40 to 1.44 [17; 18]. For example, the RI of kerosine is 1.44, different alcohols have higher RI, such as 1-hexanol which has an RI of 1.416 and 4-methyl-2-pentanol which has an RI of 1.41. Moreover, mammalian (e.g., bovine, porcine, canine, and human) tissues have RIs higher than1.40.