**3.3 Photo-manipulated photonic devices based on tristable chiral-tilted homeotropic nematic liquid crystal**

In recent years, energy saving materials have attracted much attention from scientists. The energy saving materials need to own excellent optical stability and do not require constantly applied energy. Based on the stable state natural properties, bistable LC devices are shown and can be used as e-books or e-papers. Recently, compared with the bistable in LC modes, tristable or multi-stable LCs have been scarcely proposed. Historically, the tristable LC mode was first exhibited in a ferroelectric LC system in 1988, and then the first tristable CLC device was later proposed by Hsiao et al. [22–24]. However, the stability of a LC state is very pressure-sensitive of the LCs to be bistable or tristable. Hsiao et al. demonstrate a new tristable optical composite—dye-doped tristable chiral-tilted homeotropic nematic (TCHN). This TCHN mode is extended from the technique of BHN mode. In comparison with BHN, TCHN adopts a common nematic LC material instead of DFLC material; it possesses an additional stable state and is stress-insensitive in

**65**

**Figure 15.**

0 mW/cm<sup>2</sup>

*Hybrid Liquid-Crystal/Photonic-Crystal Devices: Current Research and Applications*

stable states. Recently, Huang et al. proposed the spectral properties of an optically switchable TCHN incorporated as a tunable defect layer in PC structure. By controlling the polarization angle of the incident light as well as the intensity ratio between UV and green light, the tunable transmission characteristics of defect modes in the PC/TCHN were obtained. The hybrid PC structure realizes photo-tunability of defect mode peaks within the photonic bandgap. The PC/TCHN has much potential for many photonic applications such as a low-power consumption filter and an opti-

A schematic of the hybrid PC/TCHN structure is shown in **Figure 15a**. The optical switching of TCHN is among the tT, fingerprint (FP), and tH states as the new tristable PC device and is displayed in **Figure 15b**. The chiral bis(azobenzene) molecule is photoresponsive and is used in TCHN system. The chiral has two azo linkages to confer two distinct isomeric conformations: the rod like *trans* form and the bent *cis* form. The photo-induced unwinding effect caused by the *tran*-to-*cis* isomerization exposed to UV light and the winding effect due to the *cis*-to-*trans* isomerization under green light illumination. Based on the mechanism, we can optically switch TCHN among the tT, FP, and tH states. The experimental spectra of the PC/TCHN devices were measured under irradiation by controlling both green light and UV light. In Hsiao's paper, the used green light is from LED at wavelength of 524 nm and the UV light is from a UV LED at wavelength of 365 nm. Based on the two mixed irradiation lights, the PC/TCHN can be switched among three stable states (tT, FP, and tH). In addition, **Figure 15** shows the micrographic optical textures of the state-transform process among tristable tT, FP, and tH states under a crossed-polarizing microscope. Moreover, **Figure 15a, c**, and **e** shows the micrographic optical textures of tT, FP, and tH states, respectively. The tH state is completely dark state in the crossed-polarizer scheme because of no birefringence effect. The tT state is a bright optical texture with some rubbing traces, and the FP texture involves the lying helix structure of chiral nematic molecules as shown. The transmission spectra of defect modes within the PBG at various irradiations are also proposed. **Figure 16** demonstrates the transmittance spectra of defect modes that are controlled by the UV light with various intensities. In addition, the

. From **Figure 16f**, we can observe

*DOI: http://dx.doi.org/10.5772/intechopen.82833*

cally controllable intensity modulator device.

intensity of green light is fixed at 2.02 mW/cm<sup>2</sup>

*switching among the tH, FP and tT states (adapted from [34]).*

that the PC/TCHN device is exhibited in the tT state when intensity of UV is

the cis-form dopant molecules will increase when intensity of UV is strengthened gradually. Because the high-pretilt angle of LC and particular d/p conditions,

*(a) Sandwich structure of the PC/TCHN device. The arrows in the device's front view show the transmission axis of the polarizer (P) and rubbing direction (R). (b) Operating mechanisms of photo-induced TCHN* 

because of the azo chiral dopant being in the trans-form. However,

#### *Hybrid Liquid-Crystal/Photonic-Crystal Devices: Current Research and Applications DOI: http://dx.doi.org/10.5772/intechopen.82833*

stable states. Recently, Huang et al. proposed the spectral properties of an optically switchable TCHN incorporated as a tunable defect layer in PC structure. By controlling the polarization angle of the incident light as well as the intensity ratio between UV and green light, the tunable transmission characteristics of defect modes in the PC/TCHN were obtained. The hybrid PC structure realizes photo-tunability of defect mode peaks within the photonic bandgap. The PC/TCHN has much potential for many photonic applications such as a low-power consumption filter and an optically controllable intensity modulator device.

A schematic of the hybrid PC/TCHN structure is shown in **Figure 15a**. The optical switching of TCHN is among the tT, fingerprint (FP), and tH states as the new tristable PC device and is displayed in **Figure 15b**. The chiral bis(azobenzene) molecule is photoresponsive and is used in TCHN system. The chiral has two azo linkages to confer two distinct isomeric conformations: the rod like *trans* form and the bent *cis* form. The photo-induced unwinding effect caused by the *tran*-to-*cis* isomerization exposed to UV light and the winding effect due to the *cis*-to-*trans* isomerization under green light illumination. Based on the mechanism, we can optically switch TCHN among the tT, FP, and tH states. The experimental spectra of the PC/TCHN devices were measured under irradiation by controlling both green light and UV light. In Hsiao's paper, the used green light is from LED at wavelength of 524 nm and the UV light is from a UV LED at wavelength of 365 nm. Based on the two mixed irradiation lights, the PC/TCHN can be switched among three stable states (tT, FP, and tH). In addition, **Figure 15** shows the micrographic optical textures of the state-transform process among tristable tT, FP, and tH states under a crossed-polarizing microscope. Moreover, **Figure 15a, c**, and **e** shows the micrographic optical textures of tT, FP, and tH states, respectively. The tH state is completely dark state in the crossed-polarizer scheme because of no birefringence effect. The tT state is a bright optical texture with some rubbing traces, and the FP texture involves the lying helix structure of chiral nematic molecules as shown. The transmission spectra of defect modes within the PBG at various irradiations are also proposed. **Figure 16** demonstrates the transmittance spectra of defect modes that are controlled by the UV light with various intensities. In addition, the intensity of green light is fixed at 2.02 mW/cm<sup>2</sup> . From **Figure 16f**, we can observe that the PC/TCHN device is exhibited in the tT state when intensity of UV is 0 mW/cm<sup>2</sup> because of the azo chiral dopant being in the trans-form. However, the cis-form dopant molecules will increase when intensity of UV is strengthened gradually. Because the high-pretilt angle of LC and particular d/p conditions,

#### **Figure 15.**

*(a) Sandwich structure of the PC/TCHN device. The arrows in the device's front view show the transmission axis of the polarizer (P) and rubbing direction (R). (b) Operating mechanisms of photo-induced TCHN switching among the tH, FP and tT states (adapted from [34]).*

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

making the biologists easy to use the PC/LC device.

*without the PC/LC device (adapted from [38]).*

**homeotropic nematic liquid crystal**

used, the photo damage effect can be easily reduce and lower the excitation power, which is applied to achieve the same signal intensity. We can observe that PC/ LC device can efficiently reduce the both operation power and photo damage. In addition, **Figure 14b** shows the same effect of photo damage reducing in the green fluorescent balls under applied voltage 10 V. Thus, this novel PC/LC device is much more powerful for bio-imaging in photo damage reducing. In addition, this PC/LC device for laser pulse compression does not need any dispersion correction,

*The photo images of (a) the red and (b) the green fluorescent balls at different illumination time with and* 

In conclusion, Dr. Hsiao used a PC/LC to compress the laser pulse, exhibiting a 15-fold enhancement of the fluorescence. Without any dispersion compensator, the PC/LC device can be more convenient for nonphotonic researchers. By using the both pulse compression effect of PCs and the tunability of LCs, the PC/LC device shows a new way to enhance the multiphoton fluorescence microscopy with lower

**3.3 Photo-manipulated photonic devices based on tristable chiral-tilted** 

In recent years, energy saving materials have attracted much attention from scientists. The energy saving materials need to own excellent optical stability and do not require constantly applied energy. Based on the stable state natural properties, bistable LC devices are shown and can be used as e-books or e-papers. Recently, compared with the bistable in LC modes, tristable or multi-stable LCs have been scarcely proposed. Historically, the tristable LC mode was first exhibited in a ferroelectric LC system in 1988, and then the first tristable CLC device was later proposed by Hsiao et al. [22–24]. However, the stability of a LC state is very pressure-sensitive of the LCs to be bistable or tristable. Hsiao et al. demonstrate a new tristable optical composite—dye-doped tristable chiral-tilted homeotropic nematic (TCHN). This TCHN mode is extended from the technique of BHN mode. In comparison with BHN, TCHN adopts a common nematic LC material instead of DFLC material; it possesses an additional stable state and is stress-insensitive in

**64**

photo damage.

**Figure 14.**

#### **Figure 16.**

*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 (adapted from [34]).*

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

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 potential photonic applications [39–72].

### **4. Conclusions**

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

**67**

provided the original work is properly cited.

© 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,

Graduate Institute of Biomedical Optomechatronics, College of Biomedical

Engineering, Taipei Medical University, Taipei, Taiwan

\*Address all correspondence to: ychsiao@tmu.edu.tw

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,

*Hybrid Liquid-Crystal/Photonic-Crystal Devices: Current Research and Applications*

photonic crystal devices," propose the new type of chiral nematic LC (BHN) in PC device. The optical properties and switching mechanism of four tH, bH, bT, and tT states of the PC/BHN device are proposed. In addition, Section 2.3 "Chiral nematic and cholesteric liquid crystal with photonic crystal devices," and Section 2.4 "Tristable photonic-crystal devices with polymer-stabilized cholesteric textures." are the typical chiral nematic or CLC structure within PC devices. The three states of P, FC, and H states from CLCs are used for defect modes controlling in PCs. Thus, the configuration of many types of PC/CLC cell applications, including the design of PC multilayers for applications is schematically depicted. I introduce the three PC applications; Session 3.1 introduces "Electrically switchable liquid crystal-based photonic crystals for a white-light laser." The first PC/LC white light laser is invented, and the mechanism of the laser chip is shown. Session 3.2 shows "Liquid crystal-based photonic crystals for pulse compression and signal enhancement in fluorescence applications," and it is the first PC/LC device to achieve the pulse compression. From now on, the PC/LC can be as fluorescence enhancement applications. Session 3.3 demonstrates "Photo-manipulated photonic devices based on tristable chiral-tilted homeotropic nematic liquid crystal." In addition to this, many PC/LC applications, such as optical devices, tunable optical filters, tunable optical modulators, and so on, are also developed in recent year. In conclusion, PC/LC devices are new tools to understand and modulate light. It holds good potential to become a

*DOI: http://dx.doi.org/10.5772/intechopen.82833*

useful tool in our daily life.

Taiwan, under grant No. TMU106-AE1-B49.

**Acknowledgements**

**Author details**

Yu-Cheng Hsiao

*Hybrid Liquid-Crystal/Photonic-Crystal Devices: Current Research and Applications DOI: http://dx.doi.org/10.5772/intechopen.82833*

photonic crystal devices," propose the new type of chiral nematic LC (BHN) in PC device. The optical properties and switching mechanism of four tH, bH, bT, and tT states of the PC/BHN device are proposed. In addition, Section 2.3 "Chiral nematic and cholesteric liquid crystal with photonic crystal devices," and Section 2.4 "Tristable photonic-crystal devices with polymer-stabilized cholesteric textures." are the typical chiral nematic or CLC structure within PC devices. The three states of P, FC, and H states from CLCs are used for defect modes controlling in PCs. Thus, the configuration of many types of PC/CLC cell applications, including the design of PC multilayers for applications is schematically depicted. I introduce the three PC applications; Session 3.1 introduces "Electrically switchable liquid crystal-based photonic crystals for a white-light laser." The first PC/LC white light laser is invented, and the mechanism of the laser chip is shown. Session 3.2 shows "Liquid crystal-based photonic crystals for pulse compression and signal enhancement in fluorescence applications," and it is the first PC/LC device to achieve the pulse compression. From now on, the PC/LC can be as fluorescence enhancement applications. Session 3.3 demonstrates "Photo-manipulated photonic devices based on tristable chiral-tilted homeotropic nematic liquid crystal." In addition to this, many PC/LC applications, such as optical devices, tunable optical filters, tunable optical modulators, and so on, are also developed in recent year. In conclusion, PC/LC devices are new tools to understand and modulate light. It holds good potential to become a useful tool in our daily life.
