**Pankaj Kumar Choudhury**

**1**

**Chapter 1**

**1. Introduction**

of optical frequencies.

properties of high-index core optical fibers.

**2. Periodic band-gap structures**

trends in this specialized area are also touched upon.

Introductory Chapter: Photonic

Mirrors are important optical devices as these are greatly useful in applications, such as imaging, solar energy collection, and filtering. Also, these are indispensable in lasers in which the cavities constitute one of the components of prominence. Metallic and dielectric are the two common types of mirrors highly used in optics. Metallic mirrors reflect electromagnetic waves over a broad range of frequencies. But, these are not suitable for operations in the infrared (IR) and optical frequency regimes due to high absorption of optical power, thereby causing loss. As such, the bandwidth of operation of metallic mirrors (or guides) remains limited. However, dielectric medium-coated metallic guides can be used in the IR regime. Nevertheless, these have been proved to be inefficient for operations in the optical regime owing to the absorption loss in metals. As such, the use of conventional dielectric waveguides remains a preferred option for the transmission

Within the context of confinement of light waves in a cavity, the invention of photonic crystals (PhCs) opens up many possibilities of controlling the propagation of light. This made PhCs as the objects of intensive theoretical and experimental research. Properties of optical fibers having high-index core region surrounded by silica or air fall into the class of PhC; various forms of these have been vastly discussed in the literature [1]. It is interesting to note that PhCs having high-index cores possess many features of conventional optical fibers. However, PhCs exhibit photonic band-gap (PBG) effect, which relates to the forbidden regions in dispersion characteristics and transmission spectra—the feature that is distinct from the

As PhCs have been a research topic in the frontline for quite some time, emphasizing the avenues of such specialized microstructures, the present chapter aims at throwing a glimpse of a few different forms of guides falling in the class of PhCs. Some of the novel applications of PhC-based structures and the current research

PhCs can be classified to be one-, two-, or three-dimensional (1D, 2D, and 3D) periodic structures (depending on the kinds of periodic variation), comprised of materials with different refractive indices and having the periods comparable with the wavelengths of operation [2, 3]. Based on the configurations, there would be varieties of structures that can be regarded to be in the class of PhCs. For example, materials having periodic structures can exhibit the effect of PBG, provided the periodicity

Crystals–Revisited

*Pankaj Kumar Choudhury*

Professor, Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, (The National University of Malaysia), Bangi, Selangor, Malaysia
