# Surface Plasmon Resonance-Based Fiber Optic Sensors Utilizing Molecular Imprinting
## 2 Surface Plasmon Resonance (SPR)
The dispersion curves showing the possibility of excitations by using the configurations compared with the direct light are shown in Figure 1d. For a prism-based coupling, the incoming light incidents at an angle of incidence, \(\theta_{res}\), and at the particular angle (greater than critical angle) at which the evanescent wave's wave vector matches that of the surface plasmon wave, the transfer of energy occurs, the condition being called \"resonance\", at which \(k_{EV@_{res}}=k_{SP}\). In the case of waveguides, whose structure is similar to that of an optical fiber, the guided modes, which generate evanescent wave to penetrate through the metal layer excite SPs [32; 36; 37]. The properties of light waves such as phase, polarization, intensity, etc., can be altered by the interaction of surface plasmon waves with the light which in turn changes the propagation constant of the SP. By using the dielectric medium as the sensing medium the mechanism of SPR can be used for sensing purposes, a first attempt of which was reported in 1983 [37]. For this, the condition for SPR needs the real part of dielectric constant of metal layer to be less than zero, and its absolute value to be greater than that of the prism and its own imaginary part. The analyte to be sensed is kept in contact with the metal layer and any change in its refractive index produces a change in the propagation constant of the surface plasmon wave. Hence, by satisfying the resonance condition, matching of momentum of surface plasmon wave and evanescent wave (due to total internal reflection), a shift in the resonance parameter is observed. The characterization of the sensor depends on the scheme used for the sensing purpose such as angular interrogation, spectral/wavelength interrogation, phase interrogation and intensity interrogation [31; 32; 36]. In angular interrogation method, a monochromatic light source is used and the angle of incidence of the light is varied to make the propagation constant of the evanescent wave equal to that of the surface plasmon wave. For spectral interrogation, the angle of incidence is kept fixed while the wavelength of the incident light is varied to obtain \(\lambda_{\textit{res}}\). The angle of incidence and wavelength of the incident light are kept fixed to realize the intensity interrogation. In phase interrogation method, the phase difference between p- and s-polarized lights is recorded. In the case of prism-based coupling discussed above, an angular interrogation scheme is used where at a particular angle of incidence the surface plasmons are excited showing a sudden dip in the light reflectance. The angle corresponding to a sharp dip in reflectance is termed the resonance  
Figure 1: The schematic of (**a**) grating-; (**b**) prism-; (**c**) waveguide-based coupling configurations for SPR generation and (**d**) the surface plasmon excitation possibilities by using evanescent wave in the dispersion curves. angle, \(\theta_{res}\). As the refractive index/dielectric constant of the sensing medium changes, the resonance angle changes. This is how the surface plasmon resonance-based sensors work [32]. In the case of systems having sensing layers and analyte, the interaction of analyte to be sensed with the sensing layer changes the propagation constant of the surface plasmon wave which depends on the depth to which the interaction has occurred and the effective refractive index change [36]. This is the principle of sensors based on the SPR technique. The prism-based coupling method is not compatible due to its bulky nature, moving mechanical and optical parts, inability to use it for remote sensing, etc. Hence, researchers started using optical fiber cores instead of prisms, where the evanescent wave can also be produced to excite surface plasmons. The fiber optic SPR sensors are low cost, have miniaturized probes, and are capable of online monitoring and remote sensing with biocompatibility.