Resonance effects evanescent waves

The same observations are true for the evanescent wave of the modes in the surrounding medium. Moreover, these changes in the field distributions are accompanied by an increase of the effective refractive indices of the cladding modes, as will be clarified later. For this reason and by virtue of (3.1), the resonance wavelengths of the coated LPG are expected to blue-shift in response to an overlay thickness change. 

Lezec HJ, Thio T (2004) Diffracted evanescent wave model for enhanced and suppressed optical transmission through subwavelength hole arrays. Opt Express 12 3629-3651 Flammer PD, Schick IC, Collins RT, Hollingsworth RE (2007) Interference and resonant cavity effects explain enhanced transmission through subwavelength apertures in thin metal films. Opt Express 15 7984-7993... 

Structure A is the simplest structure formed by fusion sphcing a section of a coreless fiber to two pieces of continuous multimode fibers. In this sensor, different modes propagating in the multimode fiber expand to the surface of the coreless fiber in the central region and after reflection from the coreless fiber-environment boundary, get coupled back into the second multimode fiber fused to the coreless fiber. The evanescent waves at the coreless fiber-environment boundary excite plasmon resonance is the metallic nanoparticles formed on the surface of the coreless fiber section, as shown in Figure 3 A. This structure is not very effective for sensing purposes as there are a lot of... 

At certain incident angles the wavelength of the light matches the plasmon frequency and resonance occurs. This reduces the intensity of the reflected light, and the magnitude of the effect depends on the refractive index of the material into which the evanescent wave penetrates. 

Instead of the dielectric/dielectric interface used in evanescent wave sensors, it is possible to arrange a dielectric/metal/dielectric sandwich layer such that when monochromatic polarized light (e.g., from a laser source) impinges on a transparent medium having a metalhzed (e.g., Ag or Au) surface, light is absorbed within the plasma formed by the conduction electrons of the metal. This results in a phenomenon known as surface plasmon resonance (SPR). When SPR is induced, the effect is observed as a minimum in the intensity of the light reflected off the metal surface. 

Mainly electrochemical (amperometric, potentiometric, impedimetric, or conductometric) and optical (IR, Raman, fluorescence, absorption, reflection, evanescence field, or surface plasmon resonance) transducers are used as the basis for biosensors. However, beside these there are other, less often employed transducers that make use of the piezoelectric effect, surface acoustic waves, or detection of heat generated in enzyme reactions [40, 41]. In the context of this work, the focus is on the specific features of electrochemical transducers. An overview showing the different fields of apphcation can be found in Sect. 2.11.1.5 (Table 2). 

---