Contents I. Introduction
Surface plasmons (SP) can be perceived as waves of electron density that propagate at the interface between a metal and a dielectric. Surface plasmon waves are produced when an incoming photon from a light beam causes free electrons to resonate at the surface of the metal sheet. Due to the oscillation of the electrons some of the energy of incident light is lost. The loss becomes maximum when the wavelength of the incident light matches with wavelength of surface plasmons. This wavelength at which highest propagation loss is obtained is called resonant wavelength. As surface plasmon is sensitive to any change in the dielectric medium, tiny change of refractive index (RI) of dielectric medium (analyte) causes a shift in the resonant wavelength. The resultant shift in resonant wavelength can be utilized to identify an unknown analyte [1], [2], [3]. Since surface plasmon resonance (SPR) based sensors offer high sensitivity and effective sensing approach, they have been applied successfully in a variety of fields like food and water testing [4], chemical detection [5], gas sensing [6], bioimaging [7], disease detection [8], and drug detection [9] etc. Currently, most of the SPR sensors are based on the conventional Kretschmann configuration, where plasmonic metals are deposited on one side of the prism and light passes through the prism. Although prism based SPR sensors provide reliable performance, a major constraint of these devices is their bulky configuration and expensive optical and mechanical components [10]. To overcome these constraints, optical fiber based SPR sensors have been introduced. They have improved advantages like simple and lightweight structure, in situ monitoring, reduced confinement loss, remote sensing, and incessant analysis. However, they provide limited alternatives when it comes to incorporating novel design and tuning the crucial design parameters. In sharp contrast to optical fiber based SPR sensors, photonic crystal fiber (PCF) based SPR sensors can provide unprecedented advantages as they can integrate properties of both the optical fibers and photonic crystals, which leads to a set of characteristics that simply cannot be achieved just using the classical fibers [11]. PCFs are lightweight and compact in nature, and they offer greater flexibility in design. The most important and attractive feature of the PCF is its excellent ability of controlling light propagation by simply changing the design parameters such as number of air holes, position and size of the holes, and pitch. By controlling the light propagation through tailoring the design, a strong coupling between the surface plasmon polariton (SPP) mode and core mode can be achieved. This results in a sharp loss peak and superior sensitivity [12].
