**1. Introduction**

## **1.1 Liquid crystal-based photonic crystal devices**

Photonic crystals (PCs) are structural materials with periodically varying dielectric permittivity. The term PC was coined three decades ago, and fascinating properties of PCs have attracted numerous scientists to put in a great deal of effort. PCs consist of dielectric materials. In PCs, the index of refraction varies periodically in space. PCs were invented since 1987, when both Yablonovitch and John published their results independently [1, 2]. The most important characteristic of PCs is the photonic band gap (PBG). Furthermore, the PBG of PCs is an optical analog to the electronic bandgap in semiconductor materials, which means photons will be localized or forbidden in the PCs. Based on this characteristic, PCs can be used as various photonic applications [3–7]. However, if a defect layer is introduced to a PC structure, disrupting its periodicity, the transmission of photons at specific wavelengths will be induced within the PBG; these narrow transmission bands within PBG are called defect modes. Based on this special design of PCs with defect layers, many photonic device applications were proposed: PC lasers [8], PC optical fibers [9], and other optical devices [10, 11]. In addition, the spectral properties of the PCs can be controlled if the defect layer is tunable, for instance, liquid crystal (LC).

LCs are anisotropic materials. The optic axis (or molecular orientation) can be controlled by applying electric field, magnetic field, or temperature. Based on optical anisotropy, LC can be used as a phase retarder or an optical polarization rotator, of which the refractive indices can be tuned with external fields. LCs are also used in many other applications such as displays, smart windows, and optical fibers. Furthermore, by inserting a LC layer as a defect layer in PCs, the tunable defect modes can be achieved. The first tunable LC-based PC hybrid structure was developed by Ozaki et al. They employed a planar-aligned nematic LC as a central defect layer sandwiched between two one-dimensional (1D) PCs [12]. **Figure 1** shows the setup of the designed hybrid PC/LC cell. This idea was then extended to PC/cholesteric LC (CLC) structures for tunable laser applications [13–17]. CLCs are self-organized PCs, in which the molecular chirality forms helical structures with the optic axis continuously twisted along the helical axis. The effects of angle of incidence [18], temperature [19], and magnetic field [20] on the optical properties of PC/LC devices are investigated.

Zyryanov et al. investigated the hybrid PC/LC device between the crossed polarizers. Their experimental results show attractive features of the tunable defect modes within the PBG. The wavelengths of defect modes are shifted by the change in effective refractive index (neff) [21]. Larger number of defect modes can be induced by using high refractive index LC or by increasing the thickness of the LC defect layer. Moreover, they experimentally and theoretically demonstrated that the interference of defect modes can be achieved by placing the cell between crossed polarizers. The orthogonal polarization components through vector sum in the projection direction along the axis of the analyzer lead to shift of defect modes. As a result, the transmittance is increased when the defect mode wavelength of an extraordinary component overlaps that of an ordinary one in PCs [20].

In addition, the next milestone is electrically tunable photonic device based on PC/CLC and PC/polymer-stabilized CLC (PSCLC) hybrid structures [22–24]. With the PC/PSCLC structure, not only the wavelength of defect mode is switchable among multistable states by voltage pulses, but also the optical intensity of defect modes can be electrically tuned through switching among different metastable states. **Figure 2** shows the sandwiched structure of the PC/PSCLCs in the three stable states. It is wavelength switchable and intensity tunable of defect modes among three stable states. The unique optical tristability in the defect modes reduces power consumption and enhances flexibility. From then on, the PC and CLC combined

**53**

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

devices became a hot topic. Many types of PC/chiral LC hybrid devices are invented such as PC/BHN, PC/THN, and PC/PSCT. Recently, many scientists employ hybrid

An overview of the development of PC/LC is presented. The optical properties, operation principles, and applications of liquid crystal-based photonic crystal devices are discussed. In Section 1, I introduce the basic knowledge of LC and PC physics to make the reader understand the next section. Section 2 details the operation principles of nonchiral and chiral LC modes to help the understanding of switching mechanisms described in two following sections. In addition, the optical properties of defect modes in different LC states are reported in Section 2. Section 2.1 is "Photonic crystals with a nonchiral nematic liquid crystal," Section 2.2 is "Chiral-tilted homeotropic nematic liquid crystal-based photonic crystal devices," Section 2.3 is "Chiral nematic and cholesteric liquid crystal with photonic crystal devices," and Section 2.4 is "Tristable photonic crystal devices with polymer-stabilized cholesteric textures." The configuration of many types of PC/ LC cell applications, including the design of PC multilayers for applications is schematically depicted in Session 3. To realize the PC applications; Session 3.1 introduces "Electrically switchable liquid crystal-based photonic crystals for a white light laser," Session 3.2 shows "Liquid crystal-based photonic crystals for pulse compression and signal enhancement in fluorescence applications," and Session 3.3 demonstrates "Photo-manipulated photonic devices based on tristable chiraltilted homeotropic nematic liquid crystal." Finally, I will summarize the results and

**2. Operation method of liquid crystal-based photonic crystal devices**

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

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

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

PC/CLC to achieve the lasing applications [25, 26].

*Schematic of the PC/CLC hybrid device in three stable states.*

**1.2 Aim of this chapter**

**Figure 2.**

conclude about this chapter.

**Figure 1.** *Schematic of the PC/LC hybrid device in an electro-optical setup.*

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

**Figure 2.**

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

of PC/LC devices are investigated.

LCs are anisotropic materials. The optic axis (or molecular orientation) can be controlled by applying electric field, magnetic field, or temperature. Based on optical anisotropy, LC can be used as a phase retarder or an optical polarization rotator, of which the refractive indices can be tuned with external fields. LCs are also used in many other applications such as displays, smart windows, and optical fibers. Furthermore, by inserting a LC layer as a defect layer in PCs, the tunable defect modes can be achieved. The first tunable LC-based PC hybrid structure was developed by Ozaki et al. They employed a planar-aligned nematic LC as a central defect layer sandwiched between two one-dimensional (1D) PCs [12]. **Figure 1** shows the setup of the designed hybrid PC/LC cell. This idea was then extended to PC/cholesteric LC (CLC) structures for tunable laser applications [13–17]. CLCs are self-organized PCs, in which the molecular chirality forms helical structures with the optic axis continuously twisted along the helical axis. The effects of angle of incidence [18], temperature [19], and magnetic field [20] on the optical properties

Zyryanov et al. investigated the hybrid PC/LC device between the crossed polarizers. Their experimental results show attractive features of the tunable defect modes within the PBG. The wavelengths of defect modes are shifted by the change in effective refractive index (neff) [21]. Larger number of defect modes can be induced by using high refractive index LC or by increasing the thickness of the LC defect layer. Moreover, they experimentally and theoretically demonstrated that the interference of defect modes can be achieved by placing the cell between crossed polarizers. The orthogonal polarization components through vector sum in the projection direction along the axis of the analyzer lead to shift of defect modes. As a result, the transmittance is increased when the defect mode wavelength of an

extraordinary component overlaps that of an ordinary one in PCs [20].

In addition, the next milestone is electrically tunable photonic device based on PC/CLC and PC/polymer-stabilized CLC (PSCLC) hybrid structures [22–24]. With the PC/PSCLC structure, not only the wavelength of defect mode is switchable among multistable states by voltage pulses, but also the optical intensity of defect modes can be electrically tuned through switching among different metastable states. **Figure 2** shows the sandwiched structure of the PC/PSCLCs in the three stable states. It is wavelength switchable and intensity tunable of defect modes among three stable states. The unique optical tristability in the defect modes reduces power consumption and enhances flexibility. From then on, the PC and CLC combined

**52**

**Figure 1.**

*Schematic of the PC/LC hybrid device in an electro-optical setup.*

*Schematic of the PC/CLC hybrid device in three stable states.*

devices became a hot topic. Many types of PC/chiral LC hybrid devices are invented such as PC/BHN, PC/THN, and PC/PSCT. Recently, many scientists employ hybrid PC/CLC to achieve the lasing applications [25, 26].
