**2.2 Chiral-tilted homeotropic nematic liquid crystal-based photonic crystal devices**

We introduce one of the chiral-type LC called bistable homeotropic nematic LC (BHN). Recently, the green energy concept is concerned with not only how to generate clean energy, but also how to save energy. Following this trend, PC devices with low energy consumption are highly desired. The novel device: a PC infiltrated with a BHN to achieve both the tunability of defect modes and the low energy consumption. The BHN bistable switching mechanisms involve the backflow and the frequency revertible dielectric anisotropy effect [30]. The PC/BHN can perform in two stable states, the tilted homeotropic (tH) and tilted twist (tT) states with nonvoltage. In addition, the two voltage-sustained states: the biased homeotropic (bH) and biased twist (bT) states at frequency 1 and 100 kHz, respectively, are proposed. **Figure 4** shows the LC configurations of the PC/BHN device in both tH state and tT state at 0 V; bH state and bT state at 10 V and 1 kHz. In addition, the switching between the bistable tH and tT states can be achieved by applying short voltage pulses to permit the BHN to pass through the intermediate states (bH and bT) [30]. In this BHN, the voltage-sustained states are necessary pathways for bistable operation served as the transient states. However, no voltage has to be applied to sustain the bistable tH and tT states, making PC device with green energy. The spectra of defect modes in the PC/BHN are interesting. The four states (tT, tH, bT, and bH) have different spectral profiles. The bH state at 10 Vrms exhibits the defect modes attributed to the ordinary refractive index, all the other states tT, tH, and bT have more peaks spectra caused by the effective refractive index. In addition, the intensity of the extraordinary defect modes in the tH state can be tuned by switching between the tH and bH states as intensity modulator. In the condition, the defect modes will diminish when the stable tH state transforms to the bH state at high voltages. This finding makes PC/BHN device with light-on and light-off states without any polarizers. In addition, **Figure 5** demonstrates the experimental spectra

**Figure 4.**

*LC configurations of the PC/BHN device. The tH state and tT state are at 0 V; bH and bT state are at 10 V and 1 kHz (adapted from [30]).*

of a PC/BHN cell in the four bH, tH, bT, and tT states under the parallel polarizer. We can observe that the different spectra of bH, tH, bT, and tT are shown under the parallel polarizer.

Therefore, the PC/BHN device placed between a pair of linear polarizers has been proposed recently [30], and the axes of the two polarizers were parallel or perpendicular to each other in the new PC/BHN system. **Figure 5** demonstrates the bH state exhibited the defect modes corresponding to the sole ordinary refractive index no. From **Figure 5**, we can see that the defect modes of the bH state did not change with ϕ because of the LC molecules oriented vertically. In addition, the stable tT state was obtained from the bT state by turning off the high applied frequency. Comparing with the bT state, the tT state possessed a higher tilt angle, implying that the birefringence effect became more significant. Thus, both the ordinary and extraordinary components were conspicuous. Moreover, from **Figure 5**, the complementary in terms of defect mode wavelengths between the conditions of ϕ = 0 and 90° in the tT state was clearly shown. Furthermore, **Figure 6** shows the simulated spectra for the two tH and bH states under the parallel polarizer scheme with various polarization angles. We can observe that the differently distributed defect modes are shown. The calculation of the optical responses for both the bH and tH states by the transfer matrix method is also proposed [30]. The perfect agreement is satisfied between the simulated spectra and the experimental data (**Figure 6**). The profile width at half-maximum (FWHM) of the simulated defect mode peaks is narrower than the experimental one. This is attributable to minor experimental uncertainties like interface roughness and imperfect dielectric materials. Based on tunable optical properties of defect modes in PC/BHN, many photonic applications can be achieved.

**Figure 5.**

*Transmission spectra within the PBG of a PC/BHN device in four different states (bH, tH, bT, and tT) (adapted from [30]).*

**57**

**Figure 6.**

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

In conclusion, a novel photonic structure PC/BHN with stopband, defect mode

tunability and optical bistability have been shown. The BHN as a defect layer infiltrated within a PC was investigated recently. With the rich optical properties in the PC/BHN system and its association with the polarization effect, PC/BHN device further opens up new possible applications for the low-power consumption photonic devices in tunable spectral bandwidth and optical multichannel technologies.

*Simulations of the transmission spectra of a PC/BHN device under the parallel polarizer at various* 

*polarization angles in (a) bH and (b) tH states (adapted from [30]).*

**2.3 Chiral nematic and cholesteric liquid crystal with photonic crystal devices**

Comparing with the nematic LCs, the configuration of a cholesteric LC (CLC) or chiral LC with stacked layers shows a periodic helix of LC molecule. CLC characterized by a specific pitch length makes the structure regarded as 1D PC material by itself. The optical Bragg reflection or photonic band is the most important property in CLCs. Utilizing the special properties of periodic helix-induced photonic band in CLCs, many optic applications such as low-threshold single-mode laser with band edge excitation has been proposed. In addition, the special type of CLC is a dual frequency CLC (DFCLC). And DFCLC owns many special properties such as fast switching. Typical CLCs own bistable states, namely, the planar (P) and focal conic (FC) states. And then CLCs cannot directly switch from the FC state to the P state. Typically, this transition must be passing through an intermediate state: homeotropic (H) state or the transient P state [22–24]. However, the DFCLCs made of a DF nematic LC mixed with a chiral dopant could achieve fast and direct FC-to-P switching (∼10 ms) [30]. In DFCLC, the dielectric anisotropy is positive and the LC director tends to be paralleled to the electrical field direction (tend to H state) when applied frequency below the crossover frequency. In contrast, the dielectric anisotropy is negative when the applying frequency is higher than the crossover frequency. And the DFLC director tends to be vertical to the field direction (tend to FC and P states). Therefore, we can use frequency-modulated voltage to switch between the bistable P and FC states reversibly, making the PC/CLC device with more tunable and switchable properties. Based on PC/CLC device, many applications such as intensity tunable and fast switching in the defect mode PC device. The detail structure of the PC/CLC device has been depicted in **Figure 7**. In addition,

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

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

#### **Figure 6.**

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

**56**

**Figure 5.**

*(adapted from [30]).*

parallel polarizer.

*1 kHz (adapted from [30]).*

**Figure 4.**

*Transmission spectra within the PBG of a PC/BHN device in four different states (bH, tH, bT, and tT)* 

PC/BHN, many photonic applications can be achieved.

of a PC/BHN cell in the four bH, tH, bT, and tT states under the parallel polarizer. We can observe that the different spectra of bH, tH, bT, and tT are shown under the

*LC configurations of the PC/BHN device. The tH state and tT state are at 0 V; bH and bT state are at 10 V and* 

Therefore, the PC/BHN device placed between a pair of linear polarizers has been proposed recently [30], and the axes of the two polarizers were parallel or perpendicular to each other in the new PC/BHN system. **Figure 5** demonstrates the bH state exhibited the defect modes corresponding to the sole ordinary refractive index no. From **Figure 5**, we can see that the defect modes of the bH state did not change with ϕ because of the LC molecules oriented vertically. In addition, the stable tT state was obtained from the bT state by turning off the high applied frequency. Comparing with the bT state, the tT state possessed a higher tilt angle, implying that the birefringence effect became more significant. Thus, both the ordinary and extraordinary components were conspicuous. Moreover, from **Figure 5**, the complementary in terms of defect mode wavelengths between the conditions of ϕ = 0 and 90° in the tT state was clearly shown. Furthermore, **Figure 6** shows the simulated spectra for the two tH and bH states under the parallel polarizer scheme with various polarization angles. We can observe that the differently distributed defect modes are shown. The calculation of the optical responses for both the bH and tH states by the transfer matrix method is also proposed [30]. The perfect agreement is satisfied between the simulated spectra and the experimental data (**Figure 6**). The profile width at half-maximum (FWHM) of the simulated defect mode peaks is narrower than the experimental one. This is attributable to minor experimental uncertainties like interface roughness and imperfect dielectric materials. Based on tunable optical properties of defect modes in

*Simulations of the transmission spectra of a PC/BHN device under the parallel polarizer at various polarization angles in (a) bH and (b) tH states (adapted from [30]).*

In conclusion, a novel photonic structure PC/BHN with stopband, defect mode tunability and optical bistability have been shown. The BHN as a defect layer infiltrated within a PC was investigated recently. With the rich optical properties in the PC/BHN system and its association with the polarization effect, PC/BHN device further opens up new possible applications for the low-power consumption photonic devices in tunable spectral bandwidth and optical multichannel technologies.

#### **2.3 Chiral nematic and cholesteric liquid crystal with photonic crystal devices**

Comparing with the nematic LCs, the configuration of a cholesteric LC (CLC) or chiral LC with stacked layers shows a periodic helix of LC molecule. CLC characterized by a specific pitch length makes the structure regarded as 1D PC material by itself. The optical Bragg reflection or photonic band is the most important property in CLCs. Utilizing the special properties of periodic helix-induced photonic band in CLCs, many optic applications such as low-threshold single-mode laser with band edge excitation has been proposed. In addition, the special type of CLC is a dual frequency CLC (DFCLC). And DFCLC owns many special properties such as fast switching. Typical CLCs own bistable states, namely, the planar (P) and focal conic (FC) states. And then CLCs cannot directly switch from the FC state to the P state. Typically, this transition must be passing through an intermediate state: homeotropic (H) state or the transient P state [22–24]. However, the DFCLCs made of a DF nematic LC mixed with a chiral dopant could achieve fast and direct FC-to-P switching (∼10 ms) [30]. In DFCLC, the dielectric anisotropy is positive and the LC director tends to be paralleled to the electrical field direction (tend to H state) when applied frequency below the crossover frequency. In contrast, the dielectric anisotropy is negative when the applying frequency is higher than the crossover frequency. And the DFLC director tends to be vertical to the field direction (tend to FC and P states). Therefore, we can use frequency-modulated voltage to switch between the bistable P and FC states reversibly, making the PC/CLC device with more tunable and switchable properties. Based on PC/CLC device, many applications such as intensity tunable and fast switching in the defect mode PC device. The detail structure of the PC/CLC device has been depicted in **Figure 7**. In addition,

**Figure 7.** *The sandwich structure of the 1D PC/DFCLC device (adapted from [22–24]).*

**Figure 8** shows the transmission spectra of the PC/DFCLC device in three distinctive states (P, FC, and H states) at a various voltages. Among the three states, the P and FC states are optical stable states except the H state. Moreover, the stable P state can be achieved from the unstable H state by fast turning off the applied voltage or from the stable FC state by applying high frequency pulse [22–24]. In addition, **Figure 8** also shows that the hybrid PC/CLC device in the P state, which demonstrates a number of defect modes. Furthermore, the FC state of the hybrid PC device is exhibited when we apply voltage pulse of 20Vrms. The optical intensity of the defect modes is very low in the FC state, and the spectra of defect modes in FC are shown in **Figure 8**. The light scattering properties of FC state make all defect modes turn off. This optical effect has the potential to expand as a fast switching light shutter application. Furthermore, the PC/CLC device will be in the H state when the voltage increases to 35Vrms. And the most intense defect modes of H state are generated. **Figure 8** also shows the comparison of the spectra of defect modes between the P and H states in the PC/CLC device. We can observe that the blueshift of the defect modes of H state is shown and caused by the reduced effective index of refraction in the PC defect layer. It is interesting to observe the special phenomenon "complementary" in wavelengths of defect modes. This property can make the PC/ CLC device as a tunable shutter in specific wavelengths of defect modes.

The interesting optical characteristics of PC/CLC devices have been investigated. By using the electrically controllable DFCLC materials as defect layer in the

#### **Figure 8.**

*Spectra of the PC/DFCLC device in the photonic bandgap in P, FC, and H states. The PC/DFCLC device is driven by various voltages. In addition, the PC/DFCLC in the photonic bandgap with two different sets of defect modes in both P and H states (adapted from [22–24]).*

**59**

**Figure 9.**

*at zero voltage (adapted from [22–24]).*

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

applications such as filter, light shutter, and optical modulator.

**2.4 Tristable photonic crystal devices with polymer-stabilized** 

PC structure, PC/CLC device owns more powerful properties. Based on the three distinctive states of the CLC defect layer (stable P and FC states, and the voltagesustained H state), the PC/CLC device exhibits different spectra in different LC states. In addition, the strength and wavelengths of the defect modes can be tuned by applying voltage and frequency. Moreover, the novel PC/CLC device is characterized by its fast switching between P and FC states. In the past research, the FC-to-P transition time is as short as 10ms [22–24]. The wavelength and intensity tunability in the defect modes are more obvious comparing with other PC device. In addition, it requires no polarizers and is of low-power consumption because of bistability in P and FC states. This PC/CLC device is useful tool for photonic

In comparison with the typical CLC materials, with inclusion of a photo-polymerizable monomer into CLCs, which make CLC more powerful. The CLC/monomer composites own polymer networks to stabilize the CLC molecule, and we call the composite material as polymer-stabilized cholesteric texture (PSCT). The PSCTs can be employed in green energy devices due to the new stable state in the polymerstabilized H state. This allows the bistable switching between the FC and P states in CLC become tristable P, FC, and H states potentially [22–24]. In the past, bistable PSCT shutters can also be switched between the H and FC states [31]. However, PSCTs are possible to own more than two stable modes. Recently, Hsiao et al. proposed the first tristable PSCT as a new PC device. **Figure 9** shows three photographs of P, FC, and H states and the corresponding micrographs of the PC/PSCT devices. In addition, the PC/PSCT is placed between two crossed polarizers in the tristable P, FC, and H states. We can discover that the colors are distinctive in the three different stable states. Firstly, the P state shows that the purple color due to the transmittance of defect modes are higher in red wavelength range. In addition, the light scattering

FC state shows the multidomains of the PSCT and is presented in **Figure 9**. Moreover, the stable H state with the light leakage under crossed polarizers is also demonstrated in **Figure 9**. In addition, **Figure 10** demonstrates the spectra of defect modes in PC/PSCT device in three distinctive states (P, FC, and H states) at null voltage. The number of defect modes will increase with the increasing defect layer thickness [22–24]. Haiso et al. apply a fixed voltage (50 Vrms) at various frequencies to show the tristable states in PC/PSCT. From **Figure 10a**, we can observe the most

*Photographs and micrographs of the PC/PSCT device placed between crossed polarizers in P, FC, and H states* 

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

**cholesteric textures**

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

PC structure, PC/CLC device owns more powerful properties. Based on the three distinctive states of the CLC defect layer (stable P and FC states, and the voltagesustained H state), the PC/CLC device exhibits different spectra in different LC states. In addition, the strength and wavelengths of the defect modes can be tuned by applying voltage and frequency. Moreover, the novel PC/CLC device is characterized by its fast switching between P and FC states. In the past research, the FC-to-P transition time is as short as 10ms [22–24]. The wavelength and intensity tunability in the defect modes are more obvious comparing with other PC device. In addition, it requires no polarizers and is of low-power consumption because of bistability in P and FC states. This PC/CLC device is useful tool for photonic applications such as filter, light shutter, and optical modulator.
