Non-Linear Optical Liquid Crystalline Polymers

The liquid crystalline polymer s potential application in non-linear optics has been the interest of research since the material appeared, as reviewed by Dubois et al. (1993). 

As an infrared YAG laser beam of wavelength 1.06 p,m impacts a nonlinear optical (NLO) medium, a green light of A = 0.53 pm is observed. This is the second harmonic generation (SHG) effect. The medium is the NLO sample. In addition to the second harmonic generation, the NLO effects include the Pockel effect, the Kerr effect, the third harmonic generation and the four-wave mixing. 

In the presence of an electric or optical field, the component of polarization in the sample can be expressed as (the convention summation is applied) 

In order to achieve high NLO coefficients and x 3 the medium must follow two criteria the constituent blocks must have high NLO polarizabilities, depending on the chemical structure and the medium must have the appropriate symmetry. For those possessing the center-symmetry, vanishes. Among them are amorphous and isotropic materials. However, these media may have non-zero x -3 - In the presence of an electric field, some of these materials may change from center-symmetrical to non-center symmetrical. 

The high molecular NLO polarizability is a basic requirement. For an independent molecule under an electric field the molecular electric moment (the i-th component) is as follows 

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