Preface

Photonic crystals (PhCs) are, in general, periodic structures, wherein the propagation of waves can be modulated with the variations in the refractive index properties of the medium. The bang-gap characteristics of these have been very attractive, and could be exploited to develop multitudes of integrated optic (IO) devices. The present volume is primarily aimed at the discussions of some of the features of PhCs of a few different types that can be used in certain areas of application.

Within the context, apart from the introductory chapter on the fundamentals of these specialized mediums, Aly and Mehaney present a comprehensive study of one-dimensional PhCs, taking into account their perfect forms as well as those with defects. They perform numerical simulations to investigate the angular effects of the incidence wave, putting the emphasis on the role of ambient temperature to determine the band-gap characteristics. The use of such mediums suggests prominence in temperature sensing. Furthermore, PhCs may also be embedded with engineered metallic structures to achieve certain band-gap characteristics, and Li and Dou discuss such microstructures to manipulate the allowed and forbidden regions so that wide complete band gaps may be achieved. Their results indicate potentials of the developed structures in low-frequency vibration reduction-related issues.

Certain PhC structures infiltrated with liquid crystals need special mention. Many IO devices can be conceptualized exploiting these because liquid crystals are functional materials, which alter their properties under the influence of external parameters, such as electromagnetic fields and temperature. Hsiao touches on the relevant features of liquid crystal-infiltrated PhCs to emphasize the structure as prudent in realizing varieties of tuneable optical devices.

In yet another discussion, Hemati and Weng elaborate the gas-sensing technologies in the mid-infrared region exploiting certain forms of PhC structures. This remains important because the mid-infrared gas-sensing systems can exhibit significantly high sensitivity. The authors review the advancements of research in this arena and emphasize the potentials of the technique in the currently growing Internet of Things technology.

Zain and De La Rue present finite difference time domain simulation results of engineered PhC-based waveguides, wherein the PhC cavities are embedded in narrow photonic wire silicon-on-insulator-type waveguides. The achieved high reflectivity and possibilities of active tuning capability leave the investigators to conceptualize multifunctional integrated circuits on a single chip.

The aforementioned cursory view of the themes encapsulated in *Photonic Crystals—A Glimpse of the Current Research Trends* highlights some of the presentday research in the PhC arena that remains of interest. Apart from these, many other directions of PhC-based research have been reported in the literature, and the editor hopes to take up these in a future volume. Finally, the editor is assured that

the current volume incorporating chapters by a number of leading scientists will be significantly useful for the PhC research community.
