**2.1 Photonic crystals with a nonchiral nematic liquid crystal**

Nowadays, available LC materials can be classified into two categories: nonchiral and chiral system, according to their different operation functions on their molecule arrangement. The nonchiral LC owns only one stable arrangement state, which is determined by the use of the alignment layer. In addition, nonchiral LC molecule is continuously oriented, when the voltage is applied. On the contrary, two or multistable states are exhibited in the chiral LC system. Moreover, the multistable states can be switched and controlled from one to another based on the condition of applied voltage pulse. Based on the properties of the nonchiral LC system, it can serve as a phase retarder or optical rotator tuned by electrically

controlled birefringence (ECB) effect. Combined with the PC cells from Prof. Ozaki introduced in Section 1.1, the LC defect layers used are nonchiral LCs, and it can be operated like phase retarders. In order to understand the PC/nonchiral LC, operation method of PC/LC devices is most important. The operation method of the PC/LC in nonchiral type as tunable defect can be described in terms of two effects: ordinary and extraordinary refractive indices effects [12, 20, 27]. From **Figure 1** in Section 1.1, the positive dielectric anisotropy of the nonchiral LC is aligned along the x-axis as well as the light propagates along the z-axis. In addition, the PC structure consists of two dielectric materials: high and low refractive index materials, which are stacked alternatively. When the light is incident to the hybrid PC/LC device normally as well as the light polarization direction parallel to the LC molecule along the x-axis, the extraordinary refractive index (ne) contributed the optical path length (OPL) is exhibited. Thus, the appearance of defect modes in the PBG represents extraordinary defect modes. When the electric field is applied across the PC/LC device to make LC molecule along the z-axis, the OPL is only contributed by the sole ordinary refractive index (no). The sole no makes the OPL decreasing, and the ordinary defect modes shift to the shorter wavelength. However, the wavelength of defect modes in PBG remains unchanged, when the field-on and field-off are applied, because the polarization direction of the incoming light is in the y-axis and the same no contribution of the OPL is unchanged. We can conclude that the tunability of the defect modes is attributed to the change in refractive index (ne or no) and its corresponding OPL.

In addition, the other nonchiral type is the twisted-nematic (TN) LC, in which the molecular orientation only exhibits 90° twist, acts like the optical polarization rotator, so that the incoming light passing through the TN LC is characterized by the rotation of polarization. The hybrid PC/TN LC structure was first demonstrated in 2010 [28]. The optical phenomena attributable to the PC/TN LC structure are quite different from the mechanism mentioned in the preceding paragraph. The 90° TN LC modes are divided into three groups based on the polarization angle β between the axis of the first polarizer and the director axis lying in the front substrate. They are classified as the ordinary-mode (O-mode), extraordinary-mode (E-mode), and mixed-mode (M-mode), and then TN satisfies the conditions of β = 90, 0, and 45°, respectively. The M-mode TN (abbreviated as MTN) combines both the polarization-rotation effect and birefringence effect. **Figure 3** shows the phenomenon of the wavelength shift of defect modes in two PC/TN cells impregnated with two different nematic LC materials. The defect modes of PC/TN for the ordinary ray are independent of the applied electric field. However, the wavelengths of defect modes in PBG with both E-mode and O-mode in PC/TN device are also shown as the blueshift, when we increase the applied voltage. This effect is unlike the ECB-based defect modes of PC/LC in the preceding paragraph. Compared with the O-mode and the E-mode in PC/TN, ECB-based PC exhibits more blueshift in wavelength because of the decrease in effective refractive index of defect layer significantly. This PC/TN result can be explained by Mauguin parameters [29]. A perfect adiabatic following in the TN LC makes the linearly polarized light to traverse the LC with the rotation of the LC molecular twist, makes the effective refractive index with the incident light nearly equal to ne in E-mode and no in O-mode. The neff is no longer a constant in the O-mode TN LC cell, but becomes a weak dependence with applied voltage. Thus, the small shifts for the defect modes are demonstrated in the O-mode PC/TN cells. The most important is the integrated effect (M-mode) of defect modes in PC/TN device. The wavelengths of defect peaks in the M-mode are located at the same wavelength positions of the E-mode and O-mode because the defect peaks of the M-mode in PC/TN are contributed by two effects: the adiabatic following and birefringence effects of LC. Moreover, the intensity of the transmittance of

**55**

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

defect mode in E- or O-mode spreads to the other, making the intensity of the transmittance of the defect modes in the M-mode almost the same as both in E- or O-mode. Thus, we can observe that the M-mode is a superposition of both E-mode and O-mode. In addition, we observe carefully the spectra of E- and O-mode, small defect peaks accompanying the main defect modes are observed. This phenomenon is very different comparing with other types of PC/LC cells. Finally, no matter PC/ LC or PC/TN, the PC-based nonchiral LC is the useful tool for optical devices or

*the photonic bandgap with three different modes (adapted from [28]).*

*The blueshift of defect modes for both E- and O-modes in PC/TN system, and transmittance of the PC/TN in* 

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

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

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

photonic applications.

**Figure 3.**

**crystal devices**

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

**Figure 3.**

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

refractive index (ne or no) and its corresponding OPL.

controlled birefringence (ECB) effect. Combined with the PC cells from Prof. Ozaki introduced in Section 1.1, the LC defect layers used are nonchiral LCs, and it can be operated like phase retarders. In order to understand the PC/nonchiral LC, operation method of PC/LC devices is most important. The operation method of the PC/LC in nonchiral type as tunable defect can be described in terms of two effects: ordinary and extraordinary refractive indices effects [12, 20, 27]. From **Figure 1** in Section 1.1, the positive dielectric anisotropy of the nonchiral LC is aligned along the x-axis as well as the light propagates along the z-axis. In addition, the PC structure consists of two dielectric materials: high and low refractive index materials, which are stacked alternatively. When the light is incident to the hybrid PC/LC device normally as well as the light polarization direction parallel to the LC molecule along the x-axis, the extraordinary refractive index (ne) contributed the optical path length (OPL) is exhibited. Thus, the appearance of defect modes in the PBG represents extraordinary defect modes. When the electric field is applied across the PC/LC device to make LC molecule along the z-axis, the OPL is only contributed by the sole ordinary refractive index (no). The sole no makes the OPL decreasing, and the ordinary defect modes shift to the shorter wavelength. However, the wavelength of defect modes in PBG remains unchanged, when the field-on and field-off are applied, because the polarization direction of the incoming light is in the y-axis and the same no contribution of the OPL is unchanged. We can conclude that the tunability of the defect modes is attributed to the change in

In addition, the other nonchiral type is the twisted-nematic (TN) LC, in which the molecular orientation only exhibits 90° twist, acts like the optical polarization rotator, so that the incoming light passing through the TN LC is characterized by the rotation of polarization. The hybrid PC/TN LC structure was first demonstrated in 2010 [28]. The optical phenomena attributable to the PC/TN LC structure are quite different from the mechanism mentioned in the preceding paragraph. The 90° TN LC modes are divided into three groups based on the polarization angle β between the axis of the first polarizer and the director axis lying in the front substrate. They are classified as the ordinary-mode (O-mode), extraordinary-mode (E-mode), and mixed-mode (M-mode), and then TN satisfies the conditions of β = 90, 0, and 45°, respectively. The M-mode TN (abbreviated as MTN) combines both the polarization-rotation effect and birefringence effect. **Figure 3** shows the phenomenon of the wavelength shift of defect modes in two PC/TN cells impregnated with two different nematic LC materials. The defect modes of PC/TN for the ordinary ray are independent of the applied electric field. However, the wavelengths of defect modes in PBG with both E-mode and O-mode in PC/TN device are also shown as the blueshift, when we increase the applied voltage. This effect is unlike the ECB-based defect modes of PC/LC in the preceding paragraph. Compared with the O-mode and the E-mode in PC/TN, ECB-based PC exhibits more blueshift in wavelength because of the decrease in effective refractive index of defect layer significantly. This PC/TN result can be explained by Mauguin parameters [29]. A perfect adiabatic following in the TN LC makes the linearly polarized light to traverse the LC with the rotation of the LC molecular twist, makes the effective refractive index with the incident light nearly equal to ne in E-mode and no in O-mode. The neff is no longer a constant in the O-mode TN LC cell, but becomes a weak dependence with applied voltage. Thus, the small shifts for the defect modes are demonstrated in the O-mode PC/TN cells. The most important is the integrated effect (M-mode) of defect modes in PC/TN device. The wavelengths of defect peaks in the M-mode are located at the same wavelength positions of the E-mode and O-mode because the defect peaks of the M-mode in PC/TN are contributed by two effects: the adiabatic following and birefringence effects of LC. Moreover, the intensity of the transmittance of

**54**

*The blueshift of defect modes for both E- and O-modes in PC/TN system, and transmittance of the PC/TN in the photonic bandgap with three different modes (adapted from [28]).*

defect mode in E- or O-mode spreads to the other, making the intensity of the transmittance of the defect modes in the M-mode almost the same as both in E- or O-mode. Thus, we can observe that the M-mode is a superposition of both E-mode and O-mode. In addition, we observe carefully the spectra of E- and O-mode, small defect peaks accompanying the main defect modes are observed. This phenomenon is very different comparing with other types of PC/LC cells. Finally, no matter PC/ LC or PC/TN, the PC-based nonchiral LC is the useful tool for optical devices or photonic applications.
