Nonlinear Optical Properties of Liquid Crystals

Simoni F 1997 Nonlinear Optical Properties of Liquid Crystals and Polymer-Dispersed Liquid Crystals (Singapore World Scientific)... 

F. Simoni, Nonlinear optical properties of liquid crystals and polymer dispersed liquid crystals World Scientific, Singapore 1997. 

Laser-induced molecular reorientation is a common cause of optical nonlinearity in a fluid medium. In this respect, liquid crystals are often strongly nonlinear because of their large molecular anisotropy and strong correlation between molecules. The nonlinear optical properties of liquid crystals in the isotropic phase have already been studied quite extensively by a number of researchers in the past decade, This is, however, not true for liquid crystals in the mesophases. 

Linear and nonlinear optical properties of liquid crystals in their mesophases have been studied in several contexts, in both fundamental and application-oriented pursuits. In the context of nonlinear optical processes, they have recently received considerable renewed interests as a result of the newly discovered extraordinarily large optical nonlinearity due to the laser-induced molecular reorientation, and a renewed effort explicitly at the large thermal index effect in liquid crystals. In the last few years, several groups [2]-[10] have looked at the optical nonlinearity in the mesophases of liquid crystals and the associated nonlinear processes. A brief review of some of these nonlinear optical processes and the fundamental mechanisms in both the liquid crystal and the isotropic phases has recently appeared [1]. In this paper, therefore, we will concentrate only on optical wave mixing processes that are relevant to this Special Issue. 

The optical properties of liquid crystals determine their response to high frequency electromagnetic radiation, and encompass the properties of reflection, refraction, optical absorption, optical activity, nonlinear response (harmonic generation), optical waveguiding, and light scattering [1], Most applications of thermotropic liquid crystals rely on their optical properties and how they respond to changes of the electric field, temperature or pressure. The optical properties can be described in terms of refractive indices, and anisotropic materials have up to three independent principal refractive indices defined by a refractive index ellipsoid. 

Liquid ciystals are also optically highly nonlinear materials in that their physical properties (temperature, molecular orientation, density, electronic stmctuie, etc.) are easily perturbed by an applied optical field. Nonlinear optical processes associated with electronic mechanisms will be discussed in Chapter 10. In this and the next chapter, we discuss the principal nonelectronic mechanisms for the nonlinear optical responses of liquid crystals. 

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

Finally, the combination of dendrimers and organometallic entities as fundamental building blocks affords an opportunity to construct an infinite variety of organometallic starburst polymeric superstructures of nanoscopic, microscopic, and even macroscopic dimensions. These may represent a promising class of organometallic materials due to their specific properties, and potential applications as magnetic ceramic precursors, nonlinear optical materials, and liquid crystal devices in nanoscale technology. 

Bugaychuk S, Klimusheva G, Garbovskiy Y et al (2006) Nonlinear optical properties of composites based on conductive metal-alkanoate liquid crystals. Opto-Electron Rev 14 275-279... 

Like solid ferroelectrics, the ferroelectric liquid crystals, particularly the FLCPs, show a pyroelectric effect and a piezoelectric effect and are capable of switching polarization direction (dielectric hysteresis). Moreover, they can switch propagating or reflected polarized light. Finally, the polar symmetry of the phase leads to nonlinear optical properties of the FLCPs such as second-harmonic generation, the Pockels effect, and the Kerr effect. These physical properties of the ferroelectric LC polymers are discussed in the following sections. 

The nonlinear optical properties of nematic liquid crystals have recently been studied by various authors. It has been shown that an intensity-dependent refractive index is due to the optical reorientation of the molecules. 

The liquid crystal mesophases provide various opportunities for thermal reorientation for instance, heating alters the pitch of cholesteric helices. Reference 14 considered nonlinear optical properties of C smectics associated with changes in the molecular orientation angle during heating. In this section we show that thermal orientation effects are also present in nonuniformly oriented nematics. 

In the present volume we discuss a relatively new and rapidly developing branch of the field, namely nonlinear optical effects in liquid crystals. Optical studies have always played a significant role in liquid crystal science. Research of optical nonlinearities in liquid crystals began at the end of the sixties. Since then it became a powerful tool in the investigation of symmetry properties, interfacial phenomena or dynamic behaviour. Furthermore, several new aspects of nonlinear processes were demonstrated and studied extensively in liquid crystals. The subject covered in this book is therefore of importance both for liquid crystal research and for nonlinear optics itself. 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

The Frederiks transition in linear-chain and comb-like polymers may be treated in an analogous way as low-molecular-weight compounds, with some precautions [13], because, in general, the electric and viscoelastic properties of liquid crystal polymers are field dependent and the response of the materials to an external field is essentially nonlinear. Unfortunately, in the major part of electro-optical experiments this nonlinearity is not taken into account and results are interpreted in terms of conventional nema-todynamics and constant material parameters. In addition, in only a few papers (e.g. 

For isotropic media < 2> and < 4> are zero. Hence X333 reduces to the first term of the Langevin function. If the first-order parameters approach unity, the value of is five times larger than for an isotropic medium. This large difference has created interest in the use of liquid crystals, both low-molar-mass and polymeric, for nonlinear optics [24]. The nonlinear optical properties of poled polymers and poled pre-oriented polymers have been analysed thoroughly by the group at AT T Bell Laboratories [25, 26]. 

Among all the experimental investigations performed in anthraquinone-doped nematic liquid crystals, two results are worth being noted here as they highlight some important features of the microscopic intermolecular interactions involved in this phenomenon. In the first experiment, we have found that slight changes in the molecular structure of relatively simple anthraquinone dyes may lead to dramatic modifications of the macroscopic nonlinear optical properties of the liquid-crystalline host [22]. This is shown in Fig. 5.2... 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

The driving force in polymer synthesis is the search for new polymers with improved properties to replace other materials of construction. Polymers are lightweight and can be processed easily and economically into a wide range of shapes and forms. The major synthetic efforts at present are aimed at polymers with high temperature, liquid crystal, conducting, and nonlinear optical properties [Maier et al., 2001 Sillion, 1999]. There is an interrelationship between these efforts as will become apparent. 

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