Electric-field induced birefringent materials

Electro-Optic and Electric-Field-Induced Birefringent Materials. As can be seen from the analysis of The Photorefractive Process section, one material parameter to be optimized is the electro-optic and orientational response. This can be accomplished either by finding compatible materials with large linear electro-optic coefficients or by optimizing the orientational effects using materials with large birefringence (polarization anisotropy). One major difference in these approaches is the response speed of the photorefractive effect in that the orientational approach is expected to be slower, but submillisecond responses of low Tg materials have been demonstrated (93-96). 

An optically isotropic liquid crystal (LC) refers to a composite material system whose refractive index is isotropic macroscopically, yet its dielectric constant remains anisotropic microscopically [1]. When such a material is subject to an external electric field, induced birefringence takes place along the electric field direction if the employed LC host has a positive dielectric anisotropy (Ae). This optically isotropic medium is different from a polar Uquid crystal in an isotropic state, such as 5CB (clearing point = 35.4°C) at 50 C. The latter is not switchable because its dielectric anisotropy and optical anisotropy (birefringence) both vanish in the isotropic phase. Blue phase, which exists between cholesteric and isotropic phases, is an example of optically isotropic media. 

Electro-optic effects refer to the changes in the refractive index of a material induced by the application of an external electric field, which modulates their optical properties [61, 62], Application of an applied external field induces in an optically isotropic material, like liquids, isotropic thin films, an optical birefringence. The size of this effect is represented by a coefficient B, called Kerr constant. The electric field induced refractive index difference is given by... 

A number of optical effects arise out of both the first and second hyperpolarizibilities. However, only some among them have been systematically studied for practical applications. In this chapter, we will discuss the electric field-induced optical birefringence in second-order NLO materials and the light-induced optical nonlinearities including optical Kerr effect and two-photon absorption (TPA) in third-order NLO materials. Molecular design for... 

The first and third order terms in odd powers of the applied electric field are present for all materials. In the second order term, a polarization is induced proportional to the square of the applied electric field, and the. nonlinear second order optical susceptibility must, therefore, vanish in crystals that possess a center of symmetry. In addition to the noncentrosymmetric structure, efficient second harmonic generation requires crystals to possess propagation directions where the crystal birefringence cancels the natural dispersion leading to phase matching. 

When an electric field is applied to a material, the induced birefringence is the difference between the index of refraction for directions parallel and perpendicular to the applied field [35], as measured by the Kerr constant. 

Since 1875, thanks to Kerr s discovery [ 1], it is known that a static electric field can induce a modification of the optical properties of a liquid. Many years later researchers found out that also an optical electromagnetic field was capable of producing a measurable modification of the dielectric properties, inducing a hirefiringence effect the first experimental observation of the optical Ken-effect (OKE) was reported in 1963 [2]. After few years, with the introduction of the first pulsed lasers, spectroscopists discovered the chance to induce in a material a transient birefringence and to measure its relaxation toward the equiUhrium [3]. They also realized that this could be a relevant new spectroscopic tool able to collect new information on the dynamical processes present in the material. The spectroscopic research, worked out in the following years, confirmed this forecast beyond the expectations. Two important experimental improvements of this spectroscopic technique have been made. On one hand, the pulsed laser sources have become able to produce very short pulses of high... 

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