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

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

#### **Figure 9.**

*Photographs and micrographs of the PC/PSCT device placed between crossed polarizers in P, FC, and H states at zero voltage (adapted from [22–24]).*

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

**58**

**Figure 8.**

**Figure 7.**

*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* 

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** 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/

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

*defect modes in both P and H states (adapted from [22–24]).*

#### **Figure 10.**

*(a) Transmission spectra of the empty PC cell and the PC/PSCT structure in three different states P, FC, and H states. In addition, (b) transmittance of the defect modes from H to FC in the PBG induced by a 100-kHz various voltage amplitudes. (c) Transmittance of the defect modes from P to FC in PC/PSCT induced by various voltage at a fixed frequency of 1 kHz (adapted from [22–24]).*

intense defect modes in the empty PC cell because of the transparent air defect. With a PSCT defect layer embedded in the PC device, the PC/PSCT device initially in the H state and the more spectral defect windows in the PBG due to the higher (ordinary) refractive index no in the LC layer. And the FC state is demonstrated when a 30-kHz voltage pulse is applied. The lower transmission of the defect modes in the PBG is also shown in **Figure 10a**. We can employ the defect modes of FC to switch off the PC device by the light scattering property. When the frequency still increases to 100 kHz, the PC/PSCT will be in the P state. The redshifted defect modes is shown and the increasing defect mode number is exhibited (**Figure 10a**). Moreover, **Figure 10b** illustrates the spectra of the PC/PSCT device in the H and FC states induced by various voltage amplitudes at a fixed high frequency of 100 kHz. We can observe that the H state of the cell is the initial state. The intensity strength of the defect modes can be tuned by increasing the voltage. **Figure 10c** demonstrates the transmission spectra of the defect modes by applying various voltage of 0, 10, 25, 40 Vrms at a low frequency of 1 kHz. We can easily modulate the strength of defect modes between FC and P states. This powerful photonic device has the potential to expand optics applications, making it use as an electrically tunable device and optically tristable filter based on these special properties.

To conclude, the electrically tunable PC/PSCT devices have been investigated. In addition, the tunability is caused by the incorporation of a PSCT material as a new defect layer in PC structure. This hybrid PC/PSCT owns three stable P, FC, and H states. The electrically tunable PC device has been investigated, and it can be directly switched from one to another stable state by just applying a voltage pulse. Due to the tristability, the optical defect modes of PC/PSCT remain at zero voltages. This PC/PSCT composite device exhibits many different defect mode transmission spectra when we switch among P, FC, and H states. In addition, the intensity of the defect modes can be tuned by the amplitude of voltage as well as the wavelengths can be switched by the frequency in the H and P states. Based on the properties of tristable switching, wavelength controlling, and intensity tunability in the defect modes, the novel PC/PSCT device can be used as a low-power consumption optical filter, light shutter or an electrically intensity modulator without any polarizers, which let the PC/PSCT device more potential for applications.

**61**

**Figure 11.**

*multilayers device (adapted from [32]).*

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

**3. Applications in liquid crystal-based photonic crystals**

**3.1 Electrically switchable liquid crystal-based photonic crystals** 

Laser source is the most unique light source with many special optical properties such as coherence and collimation. The laser emission needs both the elements: stimulating source and the gain media. Today, various solid and gas materials have been employed as gain media for lasing. However, white light lasers that span the visible spectrum (red, green, and blue colors) are important for lighting, imaging, and communication applications. Recently, the organic white light laser source was successfully demonstrated [32]. Recently, an inorganic semiconductor laser source has also been proposed with a monolithic multi-segment semiconductor nanostructure [33]. Huang et al. also shows that PC/CLC hybrid structure (**Figure 11a**) is a new way to achieve white light laser [34]. In addition, the complex stacking PC/CLC structure is designed (see **Figure 11a**) and can be simply coded as [GI(HL)4HH(LH)3]−P(D)P−[(HL)3HH(LH)4IG], where D means the dye-doped CLC (DDCLC); P is the polyimide alignment layer; H and L are the high and low refractive indices of dielectrics; G represents the glass substrate; and I is the ITO. In

addition, the high and low refractive indices of dielectric materials are Ta2O5 (nH = 2.18) and SiO2 (nL = 1.47). The configurations of the CLCs in three states (P, FC, and H states) are shown in **Figure 11b**. Note that the voltage V1 leads to the FC state exhibiting an optical scattering property, and a larger voltage V2 induces the H state. The transmission spectra of CLC and a PC substrate are also displayed in **Figure 12a**. In addition, the PBG is divided by a defect mode peak at the 640 nm of PBG. The Bragg reflection of DDCLC is located at right half of PBG in hybrid PC cell. The dye composition (C540A, PM580, and LD688) in the PC/DDCLC device was adjusted to fluoresce in three wavelengths (red, green, and blue lasing emissions). However, the artificial defect mode peak in the PC is at 446nm, which allowed the pumping light to penetrate the PC cell. An organo-inorganic white light laser from PC/DDCLC composed of three colors red, green, and blue lasing emissions is therefore achieved, as displayed in **Figure 12b**. A genuine photo of the PC/ DDCLC laser is shown in **Figure 12c**, which is accompanied by the CIE1931 chromaticity diagram. In addition, the color of red, green, and blue are mixed as the discrete

*Schematics of (a) the hybrid phonic structure and (b) the configurations of the three CLC states in the* 

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

**for a white light laser**
