Electrooptic media

Nonlinear second order optical properties such as second harmonic generation and the linear electrooptic effect arise from the first non-linear term in the constitutive relation for the polarization P(t) of a medium in an applied electric field E(t) = E cos ot. 

Inhomogeneity of the field-induced change in the characteristics of the medium, the complex dielectric permittivity esc = ei + >n particular (here e1>2 are real quantities), is a distinguishing feature of electrooptic effects in the space-charge region. The ranges of such inhomogeneities (10 4-10 5 cm)... 

For the sake of illustration, the determination of the electrooptic coefficient for a uniaxial crystal is described below. Considering the nonlinear uniaxial medium of Figure 11, a D.C. electric field is applied in the z direction. The effect of the electric field is to modify the refractive index in the z direction by an amount proportional to the electric field, the modified ellipsoid is given as... 

The linear electrooptic effect is tlie change in the index of refraction of a medium due to the presence of a dc or low-frequency electric field, in such a manner that the change in the index of refraction depends linearly in the strength of the low-frequency electric field. The linear electrooptic effect is tlie mechanism behind optical intensity modulators that are used in optical switching and fiber-optics communications, where the optical signal is modulated at high frequencies (out to 110 GHz) [7-9],... 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Multiplexed twisted nematic liquid crystal devices (TN-LCDs) are the mainstay of low power, medium information content displays. At present, the voltage threshold and the nonlinearity of the electrooptic response intrinsic to the twisted nematic structure are used to achieve multiplexing. The multiplexing level achievable using this technology is limited by the sharpness of the electrooptic response and by the variations of this response with viewing angle. ... 

Electrooptic and magnetooptic effects involve changes in the refractive index of the medium caused by external electric or magnetic fields resulting in changes in the phase of the optical wave or in its state of polarization. 

Electrooptic and magnetooptic effects involve changes in the refractive index of a medium caused by an external electric or magnetic field. These are not normally thought of as nonlinear optical effects but are technically nonlinear optical processes in which the frequency of one of the fields is equal to zero. Various electrooptic and magnetooptic effects can occur depending on the situation. Some of these were listed in Table I. 

The basis of the majority of specific liquid crystal electrooptical effects is found in the reorientation of the director (the axis of preferred orientation of the molecules) in the macroscopic volume of the material under the influence of an externally applied field or the fiow of the liquid. Anisotropy of the electrical properties of the medium (of the dielectric susceptibility and the electrical conductivity) is the origin for reorientation, whereas the dynamics of the process also depend on the viscoelastic properties and the initial orientation of the director of the mesophase relative to the field. The optical properties of the medium, its local optical anisotropy, are changed as a result of this reorientation of the director (either occurring locally or throughout the whole of the sample) and underlies all the known electrooptical effects. 

Polymeric materials are playing an increasingly important role in electronic and photonic applications ". This includes application in active devices utilizing optical effects attributable to the nonlinear polarization of the medium, A number of applications such as frequency mixing, second harmonic generation, optical bistability, optical parametric amplification and oscillation, electrooptic and all optical switching and modulation etc. have been proposed. 

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