Field-induced refractive index changes

Field-Induced Refractive Index Change and Phase Shift... 

Nematic liquid crystals, on the other hand, possess centrosymmetiy. The field-induced refractive index change is quadratically related to the electric field, that... 

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... 

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],... 

In these equations, eq and a are respectively the vacuum permittivity and the linear absorption coefficient. E a>) represents the apphed electric field at the frequeney co [ (complex conjugate of ( )], whilea>, —m) and/( >,< , —< ), which are fourth-order tensors, are respectively the third-order susceptibUity and hyperpolarisabUity. The real part of these tensors represents the induced refractive index change and is a function of the laser intensity (m) oc (imaginary part describes the 2PA process [40]. 

A change in the surface plasmon propagation constant induced by a surface refractive index change occurring within a layer with a thickness h can be calculated by substituting the perturbation of the permittivity profile Eq. 55 and the field distribution of the surface plasmon Eq. 48 into Eq. 53. After a straightforward mathematical manipulation we obtain ... 

Refractive index change induced by the non-uiuform electric field. 

Optical Kerr Effect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an intensity-dependent refractive index change. The sample is placed between crossed polarizers and a weak, typically tunable, continuous wave (cw) probe laser (usually at a different wavelength and polarized at 45° to the pump pulse) overlaps the pumped region. The increased transmission of the probe beam when the pump pulse arrives is proportional to (Xeff), a combination of elements of the tensor. Many... 

A fast linear electrooptic light modulatkm effect, which can be employed in waveguiding structures, b the Pockek effect. In contrast to ferroelectric switching in SSFLC structures, the Pockek effect b basically a pure NLO effect, whereby a small refractive index change An, b induced along the opti axb by an applied electric field E, which acb along the polar axis [83] ... 

Figure 5.9. Typical laser-nematic director axis interaction geometry in a wave mixing experiment to measure the nonlinear refractive index change induced in the nematic liquid crystal. Note that an external dc or ac voltage can be applied in conjunction with the optical field. Also the liquid crystal could be doped with photosensitive molecules or nanoparticulates to enhance their nonlinear scattering efficiency.

In general, the distortions on the electronic wave function of liquid crystal molecules caused by an applied field do not cause appreciable change to its contribution to the refractive indices (see Chapter 10). However, the orientation of the molecules can be dramatically altered by the apphed field. This process alters the overall optical properties of the medium and is the principal mechanism used in liquid-crystal-based electro-optical devices. As noted in Section 6.2.2, the electrically induced orientational refractive index changes could be Pockel or Kerr effect. In this and the next sections, we shall focus on nematic liquid crystals in which the director axis reorientation is a Kerr-like effect that is, the process is quadratic in the applied field. 

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