**Acknowledgements**

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

the FP texture will appear. Then, we can see that the intensities of defect mode vanish because of strong scattering. However, the defect mode shifts to distinct wavelengths and emerges again with enhanced intensity of UV. When the azo chiral molecules become fully converted to the trans-form at 0.44 mW/cm<sup>2</sup>

*The transition process among three stable states by increasing irradiation ratios between UV and green light. (a) The tT state; (b) the coexistence of tT and FP states; (c) the FP state; (d) the coexistence of FP and tH states; (e) the pure tH state of a TCHN. The arrows indicate the transmission axes of the polarizer (P) and analyzer (A). In addition, photo-manipulated transmittance of the defect modes under irradiation by various UV powers. (f) From the tT to the FP state and (g) from the FP to tH state* 

PC/TCHN comes to the tH state and the transmittance of defect modes reaches the highest intensity as shown in **Figure 16g**. Based on these optical properties, the

In conclusion, a novel concept of photo-switchable PC/TCHN devices is proposed. In comparison with its bistable counterpart BHN, the TCHN can be prepared using a regular nematic host LC material (e.g., E7 in experimental from Huang et al.). The PC/TCHN device owns optical tristability and tunability in wavelength and intensities of the defect modes by photo manipulating. By adjusting the ratio of the UV and green light intensity, the defect modes of PC/TCHN not only show a variation in spectral amplitude through the stable FP state, but also can control the wavelengths between the stable tT and tH states. This novel PC/TCHN mode and the special properties of TCHN material should be fully developed for next poten-

In this chapter, PC/NLC and PC/chiral LC devices, which exhibit special photonic applications and several fascinating features have been reviewed. In addition, the optical properties of defect modes of PC switching among each state are reported. In Section 2.1, "Photonic crystals with a nonchiral nematic liquid crystal," the mechanism of defect modes in nonchiral PC is well-described by OPL. The shifting can be understood from a change in neff under applied voltage. Section 2.2 "Chiral-tilted homeotropic nematic liquid crystal-based

PC/TCHN device can be used as tunable optical filters.

tial photonic applications [39–72].

**4. Conclusions**

**Figure 16.**

*(adapted from [34]).*

, the

**66**

This work was financially supported by the Ministry of Science and Technology, Taiwan, under grant No. 107-2218-E-038-007-MY, and Taipei Medical University, Taiwan, under grant No. TMU106-AE1-B49.
