**7. Recent research trends**

With time, the research on PhC gained enough maturity, thereby yielding exciting results. R&D investigations report the possibilities of exploiting PhCs in many different applications. However, the aim of these remains pivoted to the tailoring of band-gap characteristics in the desired range of frequencies.

As stated in the preceding section, defects introduced in 1D PhC structures modify the propagation of waves, because such modes are governed by the PBG of medium [27, 28]. In this stream, the role of functional materials remains highly demanding as the form of PhC allows the possibility of tuning the spectral characteristics. The external effects, such as electromagnetic fields, temperature generating elastic, and/or shear waves, would be varied to achieve the desirable optical (or electromagnetic, in general) features of PhC. Therefore, hybrid PhCs would be useful in designing tunable optical filters, modulators, pulse compressors, and many others.

Apart from functional materials, metals may also be embedded in 1D PhCs to modify the confinement of modes, thereby altering the band-gap characteristics [29]. Interestingly, PhC structures embedded with metal-matrix arrangements could be used to reduce the low-frequency vibration and noise related issues.

Since PhCs can be exploited to control the light-matter interactions within micro/nano scales, these are advantageous for gas analyzing purpose [30–32]. In particular, the mid-infrared region of electromagnetic spectrum can be utilized for gas sensing applications, which would yield the development of such devices with high sensitivity [33, 34]. Indeed, the variations of optical spectrum and/or measuring the material properties are the techniques to determine the features of sensing.

PhC cavities can also be grown in nano-scaled photonic wire waveguides based on silicon-on-insulator (SOI). Such structures are capable to exhibit high reflectivity, which make them sophisticated candidates for mirroring in PhC structures [35–37]. These have been proved to be useful to realize active tuning—the feature that makes these suitable as basic building blocks for high-density photonic integrated circuits. Apart from this, the efficacy in designing filters and high-speed optical switches for networking applications have also been conceptualized.

The aforementioned features of PhCs describe only a few of the research ventures where these complex structures have been investigated. In reality, however, there are many other novel areas of research pivoted to exploiting varieties of new forms of PhCs to demonstrate fantastic electromagnetic features; all of those scopes remain beyond the coverage of this volume.

### **8. Summary**

In analogy to the propagation of electron waves in periodic lattice structures, waves propagating in a structure that is periodically modulated with refractive

**7**

**Author details**

Pankaj Kumar Choudhury

provided the original work is properly cited.

Malaysia, UKM, Bangi, Selangor, Malaysia

\*Address all correspondence to: pankaj@ukm.edu.my

*Introductory Chapter: Photonic Crystals–Revisited DOI: http://dx.doi.org/10.5772/intechopen.85246*

technological applications.

index also exhibit photonic bands. Such *periodic* structures, comprised of high refractive index difference materials, yield photonic bands separated by gaps, thereby disallowing the propagation of waves. This triggers many novel approaches to manipulate the electromagnetic fields, thereby opening up varieties of possible

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan

*Introductory Chapter: Photonic Crystals–Revisited DOI: http://dx.doi.org/10.5772/intechopen.85246*

*Photonic Crystals - A Glimpse of the Current Research Trends*

**7. Recent research trends**

in dielectric boundaries [24, 25]. Furthermore, it has been found that, in the case of omniguiding fibers, the number of allowed and forbidden bands increases with the increase in the difference between the values of refractive index of different dielectric layers. Furthermore, the widths of the allowed band remain larger in the case of fibers having stacked layers of larger thickness values [26]. In the dispersion characteristics of omniguiding fibers as well, it has been found that the width of allowed range decrease with the increase in *k*/*k*0, *k* and *k*0 being the wave vector in the medium and that in the free-space, respectively. This has been demonstrated through obtaining thick curves [26] that represent the existence of allowed and forbidden bands of wavelengths, instead of simple lined curves shown by conventional optical fibers.

With time, the research on PhC gained enough maturity, thereby yielding exciting results. R&D investigations report the possibilities of exploiting PhCs in many different applications. However, the aim of these remains pivoted to the tailoring of

As stated in the preceding section, defects introduced in 1D PhC structures modify the propagation of waves, because such modes are governed by the PBG of medium [27, 28]. In this stream, the role of functional materials remains highly demanding as the form of PhC allows the possibility of tuning the spectral characteristics. The external effects, such as electromagnetic fields, temperature generating elastic, and/or shear waves, would be varied to achieve the desirable optical (or electromagnetic, in general) features of PhC. Therefore, hybrid PhCs would be useful in designing tunable optical filters, modulators, pulse compressors, and many others. Apart from functional materials, metals may also be embedded in 1D PhCs to modify the confinement of modes, thereby altering the band-gap characteristics [29]. Interestingly, PhC structures embedded with metal-matrix arrangements could be used to reduce the low-frequency vibration and noise related issues. Since PhCs can be exploited to control the light-matter interactions within micro/nano scales, these are advantageous for gas analyzing purpose [30–32]. In particular, the mid-infrared region of electromagnetic spectrum can be utilized for gas sensing applications, which would yield the development of such devices with high sensitivity [33, 34]. Indeed, the variations of optical spectrum and/or measuring the material properties are the techniques to determine the features of sensing. PhC cavities can also be grown in nano-scaled photonic wire waveguides based on silicon-on-insulator (SOI). Such structures are capable to exhibit high reflectivity, which make them sophisticated candidates for mirroring in PhC structures [35–37]. These have been proved to be useful to realize active tuning—the feature that makes these suitable as basic building blocks for high-density photonic integrated circuits. Apart from this, the efficacy in designing filters and high-speed optical switches for networking applications have also been conceptualized.

The aforementioned features of PhCs describe only a few of the research ventures where these complex structures have been investigated. In reality, however, there are many other novel areas of research pivoted to exploiting varieties of new forms of PhCs to demonstrate fantastic electromagnetic features; all of those scopes

In analogy to the propagation of electron waves in periodic lattice structures, waves propagating in a structure that is periodically modulated with refractive

remain beyond the coverage of this volume.

band-gap characteristics in the desired range of frequencies.

**6**

**8. Summary**

index also exhibit photonic bands. Such *periodic* structures, comprised of high refractive index difference materials, yield photonic bands separated by gaps, thereby disallowing the propagation of waves. This triggers many novel approaches to manipulate the electromagnetic fields, thereby opening up varieties of possible technological applications.
