# Light Propagation in Confined Nematic Liquid Crystals and Device Applications
## 1 Introduction  
It is well known that the most important application of liquid crystals (LCs) nowadays is flat panel displays (FPD), which is a very mature industry. Recently, other FPD technologies, such as OLED and micro-LED, are competing with LC-FPD in many applications. However, LCs are excellent materials for light phase modulation which cannot be achieved by OLED and micro-LED. Indeed, many photonic devices/systems and applications based on LC modulations are developing. Liquid crystalline materials used for most applications are usually made of mixtures of different compounds to optimize their electro-optical performance. LC molecules, made of ring systems, linking groups, substituents and terminal groups as sketched in Figure 1, are designed in order to optimize LC physical and optical properties. LCs have low optical losses from UV to IR with scattering decreasing as \(\lambda^{-2.34}\)[1] gaining interest for guide wave devices [2; 3]. LC have been demonstrated as effective materials to make both electrically [4] and optically [5] controlled cores for switchable and reconfigurable waveguides to be used at the wavelength of 1.55 \(\upmu\)m in optical communications for their attractive optical properties. The efficient electro-optical effect with the additional advantage of negligible electrical current absorption and the nonlinear optical properties allow very low power consumption without appreciable energy dissipation. Linear optical properties stimulated the study and development of LC-based waveguides [6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. Nonlinear optical properties fostered intense research activity on the development of solitonic propagation in LC with many potential applications [16; 17; 18; 19; 20; 21; 22]. Photonic bandgap structures obtained by infiltrating LC in hollow optical fibres, the so-called photonic LCfibres, were also intensely studied, generating another broad research line [23; 24; 25; 26; 27]. The light characteristics make LC-based photonic devices interesting potential candidates for both datacom and optofluidic applications [28; 29; 30; 31; 32]. LC propagation properties were also interestingly investigated to generate optical vortices [33; 34], which may result in an important breakthrough in the development of optical tweezers, high-order quantum entanglement and novel nonlinear optic effects with competing nonlinearities [35; 36]. Another interesting effect of light propagation in LC is related to all-optical angular control of the molecular alignment in LC films due to light and matter angular momentum conservation [37; 38]. More recently, LC-based optical sources were demonstrated to envision full integration of optical functionalities in a single chip using liquid crystalline materials. OLEDs were reported by employing liquid-crystalline host matrix in the emission layer [39]. Lasers were demonstrated by means of a novel approach to generate actively Q-switched laser pulses based on an integrated waveguide chip and an LC cell in the deformed helix ferroelectric (DHF) mode, able to behave like an active Q-switch modulator under a wide range of repetition frequencies [40]. Vertical cavity surface-emitting laser, including LC as a tuning medium, was also reported [41]. Despite the many interesting optical properties of LCs, it is no small undertaking to make waveguiding devices in which input and output signals can be coupled in and out by using standard optical fibres. In this paper, a review of proposed devices is presented to make optical switches and filters based on propagation properties of nematic LCs in which a signal is transmitted between two standard single mode fibres. Such devices can be integrated in many systems for several applications such as communications and sensor microsystems. In Section 2, materials and main device structures of LCs confined in different waveguide geometries are presented by focusing on fabrication issues. Section 3 reports on modelling for designing guide wave LC devices and simulations of their operation.