Applied field, third-order effects

At time t = 0, a dc field is applied it produces an electric field-induced Pockels effect (EFIPE), which is solely due to a third-order effect Eq) in the case of the copolymer because the molecules are not oriented by the dc field alone at room temperature, but which also contains a part due to the rotation, in a polar manner, of free chromophores in the guest-host system (induced The value of x is measured from the modulations of ATR... 

AH the properties dealt with up until now involve linear interactions between light and polymer. Interaction of li t with polymers in the nonlinear region involves second- and third-order effects as well as the phenomenon of photo refrac-tivity (56,57). An optical nonlinear optical (NIX)) polymer is one that, in response to an externally applied electric field, can either vary the speed of incoming light or alter its fi uency. Var3dng the speed of light involves a change in the reflective index of the material. An optically nonlinear polymer has two components the polymer itself and an optically nonlinear molecule (chromophore), which is either chemically attached to the polymer or dissolved in it. 

The quadratic effect of an externally applied field on the refractive index n is described by the third-order susceptibility (- ) w,0,0) (Kerr susceptibility). The two independent components Yilzz and x ixx can be interpreted in terms of molar polarizabilities. The results for 2 symmetric molecules with only one significant component of the second-order polarizability are expressed in (113) and (114),... 

The quadratic effect of an externally applied field on the absorption coefficient is described by the imaginary part of the third-order susceptibility -o) a),0,0). influences the molar decadic absorption coefficient of the solute. The absorption coefficient in the presence of the field is a quadratic function of the applied field strength (118),... 

Combination with Static Fieids. A common technique, useful for optoelectronic devices, is to combine a monochromatic optical field with a DC or quasistatic field. This combination can lead to refractive index and absorption changes (linear or quadratic electrooptic effects and electroabsorption), or to electric-field induced second-harmonic generation (EFISH or DC-SHG, 2 > = > - - third-order process. In EFISH, the DC field orients the molecular dipole moments to enable or enhance the second-harmonic response of the material to the applied laser frequency. The combination of a DC field component with a single optical field is referred to as the linear electrooptic (Pockels) effect co = co + 0), or the quadratic electrooptic (Kerr) effect ( > = > - - 0 -I- 0). These electrooptic effects are discussed extensively in the article Electrooptical Applications (qv). EFISH is... 

TheEinEqs. 17.9 and 17.10 is an electric field vector. As such, the symmetry properties of the material will be important. In particular, it can be shown that the odd-order terms of these equations are independent of any symmetry considerations, but the even-order terms vanish in a centrosymmetric environment. That is, x is zero for any sample that has a center of inversion. This is always the case for bulk liquids, and it is true for most solids. Thus, symmetry and orientation effects should be crucial for discussions of SHG and related phenomena. We have noted earlier that crystal engineering has significant implications for many types of applications, and NLO is definitely one of them. The ability to rationally design or predict non-centrosymmetric crystals would be very valuable. Similarly, at the molecular level, p will be zero for molecules with a center of symmetry. No such restrictions apply to the third-order terms (x and y). 

In the case of third-order nonlinear phenomena the materials can be centrosymmetric in its molecular structure. One example is the optical Kerr effect reported for the first time by J. Kerr in 1877 and 1878 exposing a material to an electric field the refraction index of an optical medium changes, proportional to the square of the applied field. A double refraction can be generated with the difference between Kerr and Pockels effect being that in the latter case the double refraction is linearly proportional to the electric field. 

The most widely exploited effect in display applications is the influence of low frequency electric fields on the birefringence. Here, due to a distortion of the director, elements of the dielectric tensor change as a function of the applied field. From Eq. (6), the contribution of the third-order susceptibility -or,0,0,co) to the dielectric tensor is given by e=eo I+) U) Ea Eac]. Although the response is non-local, it is possible to obtain a crude estimate of the average susceptibility E. ), where... 

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