Orientational optical-field-induced

A well-known nonlinear process taking place in the liquid state of anisotropic molecules is the optical-field induced birefringence (optical Kerr effect ). This nonlinearity results from the reorientation of the molecules in the electric field of a light beam. In the isotropic phase the optical field perturbs the orientational distribution of the molecules. In the perturbed state more molecules are aligned parallel to the electric field than perpendicularly to it and as a consequence the medium becomes birefringent. On the other hand in liquid crystals the orientational distribution of the molecules is inherently anisotropic. The optical field, just as a d.c. electric or magnetic field, induces a collective rotation of the molecules. This process can be described as a reorientation of the director. 

Liquid crystals are generally characterized by the strong correlation between molecules, which respond cooperatively to external perturbations. That strong molecular reorientation (or director reorientation) can be easily induced by a static electric or magnetic field is a well-known phenomenon. The same effect induced by optical fields was, however, only studied recently. " Unusually large nonlinear optical effects based on the optical-field-induced molecular reorientation have been observed in nematic liquid-crystal films under the illumination of one or more cw laser beams. In these cases, both the static and dynamical properties of this field-induced molecular motion are found to obey the Ericksen-Leslie continuum theory, which describe the collective molecular reorientation by the rotation of a director (average molecular orientation). 

In studying the intense laser interactions with the CS2 molecules, and observing the behaviour of the orientational distribution and pair correlation functions [g(S)J through the time-evolution of the optical field-induced anisotropy, we are emphasizing external field-induced phenomena rather than interaction-induced or collision-induced events. 

Optical field-induced director reorientation is responsible for the largest nonlinear optical susceptibility observed in liquid crystals, the largest in any known material. Although the process is slow, the nonlinearity is about 10 times greater than that of CS2. Because of its magnitude, the orientational nonlinearity of liquid crystals has been termed giant optical nonlinearity (GON). 

E. G. Hanson, Y. R. Shen, G. K. L. Wong, Optical-field-induced refractive indices and orientational relaxation times in a homologues series of isotropic nematic substances, Phys. Rev. A 1976, 14, 1281. 

Deuling HJ, Helfrich W (1974) Hysteresis in the deformation of nematic liquid crystal layers with homeotropic orientation. Appl Phys Lett 25 129-130 Durbin SD, Arakelian SM, Shen YR (1981) Optical-field-induced birefringence and Freedericksz transition in a nematic liquid crystal. Phys Rev Lett 47(19) 1411-1414 Freedericksz V, Zolina V (1933) Forces causing the orientation of an anisotropic liquid. Trans Faraday Soc 29 919-930... 

On the other hand, if the molecules are not strongly anchored to the boundary, that is, the so-called soft-boundary condition (Fig. 3.3), an applied field will perturb the orientation of the molecules at the cell boundaries. In this case a quantitative description of the dynamics of the field-induced effects must account for these surface energy terms. A good account of surface energy interaction may be found in the work of Barbero et al.," which treats the case of optical field-induced effects in a hybrid aligned nematic liquid crystal cell. 

There are mote dye molecules oriented along the optical field inducing order... 

Three synthetic approaches to donor-acceptor-substituted conjugated molecules with enhanced orientability in electric fields, potentially applicable to the preparation of electro-optic polymers via electric field poling, are summarized. The three approaches are parallel attachment of chromophores to a common framework, embedding the chromophore in a zwitterion, and head-to-tail oligomerization of chromophores. The oligomerization method as well as the use of dyes as curing agents are briefly discussed in relation to the stability of electric field-induced polar order in polymer matrices. 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

The contribution of photo-induced nonpolar orientation through even order parameters (see Equation 8A.9 in the appendix, page 286) to the effects observed in the EFISH decrease is negligible. The same decrease is observed for a TE- or TM-polarized probe regardless of the pump polarization (not shown). The photochemically induced molecular shape change of the NLO dye blows out the strong optical field driven anharmonic movement of the... 

The first common method for molecular first hyperpolarizability determination is the electric field-induced second harmonic generation (EFISH) technique in solution [6-10]. This technique can be applied only to dipolar molecules. Under an applied external electric field, molecules in solution orient approximately in the direction of the field giving rise to second harmonic generation. The measured third-order nonlinear optical susceptibility is given by the following expression ... 

Dumont and coworkers [136, 155, 156] have observed that shining doped (or functionalized) polymer thin hlms with noncentrosymmetric dipolar chromo-phores, induces a significant increase of electro-optic coefficient in the chromo-phore absorption band, corresponding to a better, polar orientation of chromo-phores. The measurements have been done by using the attenuated total reflection technique, and the optical field polarization was perpendicular to the applied low-frequency external electric field to the thin film (Fig. 33). A better stability of induced orientation was observed in the case of functionalized polymers than in guest-host system, as is usually the case with the static field poled polymers. The chromophores orient with dipolar moments perpendicular to the optical field (and parallel to the applied static (or low frequency) field. As will be discussed later, the chromophore orientation undergoes a trans-cis isomerization process (Fig. 34). 

Fig. 34. Light-induced reorientation of the DR 1 molecule in the presence of the static field and photoisomerization process. From almost parallel orientation to the exciting optical field the molecule dipole moments reorient to an almost perpendicular direction...

In tune with the above introductory remarks, we have arranged this review in the following way Section II deals with the oriented gas model that employs simple local field factors to relate the microscopic to the macroscopic nonlinear optical responses. The supermolecule and cluster methods are presented in Section III as a means of incorporating the various types of specific interactions between the entities forming the crystals. The field-induced and permanent mutual (hyper)polarization of the different entities then account for the differences between the macroscopic and local fields as well as for part of the effects of the surroundings. Other methods for their inclusion into the nonlinear susceptibility calculations are reviewed in Section IV. In Section V, the specifics of successive generations of crystal orbital approaches for determining the nonlinear responses of periodic infinite systems are presented. Finally,... 

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