Refractive index, photonic bandgap

FIGURE 4.2. Illustration of a two-dimensional photonic bandgap structure in which the white regions possess a high refractive index and the black regions represent air holes. Such structures can have a complete photonic bandgap under appropriate conditions. When a defect is introduced in the perfectly periodic lattice, a microcavity can be created. 

Fig. 2 Various types of hollow-core photonic bandgap fibers, (a) Photonic crystal fiber featuring small hollow core surrounded by a periodic array of large air holes, (b) Microstructured fiber featuring medium-sized hollow core surrounded by several rings of small air holes separated by nano-size bridges, (c) Bragg fiber featuring large hollow core surrounded by a periodic sequence of high and low refractive index layers...

Here,/rad is a radiation factor that describes changes in the confinement losses of a photonic bandgap-guided mode due to spectral shift of a fiber bandgap caused by changes in the real part of the refractive index of an analyte. 

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