Electrical susceptibility, nonlinear light

Note 3 A polymer that exhibits a nonlinear optical effect due to anisotropic electric susceptibilities when subjected to electric field together with light irradiation is called an electro-optical polymer. A polymer that exhibits electro-optical behavior combined with photoconductivity is called a photorefractive polymer. 

The nonlinear interaction of light with matter is useful both as an optical method for generating new radiation fields and as a spectroscopic means for probing the quantum-mechanical structure of molecules [1-5]. Light-matter interactions can be formally classified [5,6] as either active or passive processes and for electric field based interactions with ordinary molecules (electric dipole approximation), both may be described in terms of the familiar nonlinear electrical susceptibilities. The nonlinear electrical susceptibility represents the material response to incident CW radiation and its microscopic quantum-mechanical formalism can be found directly by diagrammatic techniques based on the perturbative density matrix approach including dephasing effects in their fast-modulation limit [7]. Since time-independent (DC) fields can only induce a... 

Electro-optic and magneto-optic phenomena contain terms of nonlinear optics effects (see Eqs. (4.32), (4.33), (4.36), and (4.39)). On the other hand, acousto-optic effects which arise from a periodical density fluctuation of the medium, analogous to the Brillouin scattering phenomenon, do not contain terms of nonlinear optics, as a general rule.5) The perturbation of light propagation by sonic waves differs from that induced by electric and magnetic fields. As the electric susceptibility, xe, is a function of the density of the medium, it will be influenced by the periodical density fluctuation induced in a medium by sound waves. 

So far, in the description of the interaction of light with matter, we have assumed that the response of the material to an applied optical field was independent of its magnitude. This approximation is valid when the electric field amplitude is negligible compared with the internal electric fields in atoms and molecules. However, when lasers are used as light sources, the intensity of the optical field is usually strong and can drive the electronic response of a dielectric into a nonlinear regime. This nonlinear optical response is described by a field-dependent susceptibility that can be written as... 

The importance of the hyperpolarizability and susceptibility values relates to the fact that, provided these values are sufficiently large, a material exposed to a high-intensity laser beam exhibits nonlinear optical (NLO) properties. Remarkably, the optical properties of the material are altered by the light itself, although neither physical nor chemical alterations remain after the light is switched off. The quahty of nonlinear optical effects is cmciaUy determined by symmetry parameters. With respect to the electric field dependence of the vector P given by Eq. (3-4), second- and third-order NLO processes may be discriminated, depending on whether or determines the process. The discrimination between second- and third-order effects stems from the fact that second-order NLO processes are forbidden in centrosymmetric materials, a restriction that does not hold for third-order NLO processes. In the case of centrosymmetric materials, x is equal to zero, and the nonhnear dependence of the vector P is solely determined by Consequently, third-order NLO processes can occur with all materials, whereas second-order optical nonlinearity requires non-centrosymmetric materials. 

Here 12 are the incident light intensities at mi, m2 N is number of scattering molecules iico ) is the projection of the unit electric field vector at frequency m3 along Cartesian axis i, and similarly for Mj coi), etc. and is averaged over the molecular orientational distribution. The intensity is proportional to the square of both the number of scattering centers and the third-order nonlinear susceptibility. 

A monolayer at an interface between air and water can be considered as an infinitely thin dielectric sheet represented by an induced polarization, which is usually a complicated nonlinear function of the electric field (E) of the incident laser beam. In the linear case, the induced polarization is a simple linearized form, P = XgE where Xs is the surface susceptibility xs = Xs (z)f for simplicity here taken as a scalar, i.e. the monolayer is assumed to be isotropic and z is the direction normal to the surface. With the light incident from air, the phase retardation between the reflected S and P waves (A = " p) which is measured by... 

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