Field-induced birefringence generation

B. Higher Harmonics Generation in Field-Induced Birefringence by a Suspension of Rigid Dipoles Zero Bias Field... 

In the external electric field the uniaxial crystal becomes biaxial. In addition to the natural birefringence of the uniaxial crystal, a field-induced birefringence is generated, which is approximately proportional to the field strength E [4.56]. The changes of no or n by the electric field depend on the symmetry of the crystal, the direction of the applied field, and on the magnitude of the electro-optic coefficients. For the KDP crystal only one electro-optic coefficient < 35 = —10.7 x 10 [ni/V] (see Sect. 5.8.1) is effective if the field is applied parallel to the optical axis. 

An(t)/n, the transiently induced birefringence. By this means, deterministic equations of motion, without stochastic terms, can be used via computer simulation to produce spectral features. As we have seen, a stochastic equation such as Eq. (1) is based on assumptions which are supported neither by spectral analysis nor by computer simulation of free molecular diffusion. The field-on simulation allows us the direct use of more realistic fimctions for the description of intermolecular interaction than any diffusional equation which uses stochastically generated intermolecular force fields. 

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. 

On the assumption of total symmetry of the tensor of third-order nonlinear polarizability c(— co coi, cog, cog), its non-zero and independent elements are the same as those of Table 12. Direct theoretical calculations of c = c(0 0,0,0) have been performed for the atoms of inert gases and some simple molecules. Values of the tensor elements = c(— cu cu, 0,0) have been determined for numerous molecules from static Kerr effect studies and values of c = c(— cd ot>,coi — col) from measurements of optical birefringence induced by laser li t. Measurements of second-harmonic generation by gases in the presence of a static electric field yield the tensor elements c " = c( — 2co co, to, 0), which can also be obtained from second-harmonic scattering in centro-symmetric liquids. The elements of the tensor c = c(— 3co co, co, co)... 

The role of push-pull molecules in PR systems is to provide refractive index modulation in response to an electric field. In polymers with a low glass transition temperature (Tg), where the molecules are relatively free to rotate due to a higher free volume, the chromophores can reorient in the applied/generated electric field. Because of the anisotropy in polarizability of the chromophores, such a reorientation will induce high refractive index anisotropy termed as orientational birefringence. The ability of a chromophore molecule to induce refractive index change is usually expressed as the figure of merit (FOM) defined by Equation 8.4 [44],... 

---