Noncentrosymmetric materials

For the establishment of a preparation method for LB films with well-defined noncentrosymmetric structure, quantitative evaluation of noncentrosymmetric molecular orientation is essential. The linear Stark effect, which is observed only in noncentrosymmetric materials, is expected to be helpful for the characterization of noncentrosymmetric molecular orientation in LB films [5], In this section, we describe the quantitative evaluation of noncentrosymmetric molecular orientation in LB films by the linear Stark effect measurement. 

Second-order optical nonlinearities result from introduction of a cubic term in the potential function for the electron, and third-order optical nonlinearities result from introduction of a quartic term (Figure 18). Two important points relate to the symmetry of this perturbation. First, while negative and positive p both give rise to the same potential and therefore the same physical effects (the only difference being the orientation of the coordinate system), a negative y will lead to a different electron potential than will a positive y. Second, the quartic perturbation has mirror symmetry with respect to a distortion coordinate as a result, both centrosymmetric and noncentrosymmetric materials will exhibit third-order optical nonlinearities. If we reconsider equation 23 for the expansion of polarization of a molecule as a function of electric field and assume that the even-order terms are zero (i.e., that the molecule is centrosymmetric), we see that polarization at a given point in space is ... 

One of the important applications of second-order NLO materials is obtaining of tunable laser sources. Second harmonic generation or sum frequency generation systems lead to monochromatic sources. The optical parametric oscillators are based on the parametric generation of two waves with frequencies co (signal) and CO (idler). In noncentrosymmetric materials an incident photon with frequency co creates two photons satisfying the energy... 

Another application of noncentrosymmetric materials is generation of T Hz electric pulses, impossible to obtain by using classical electric circuitry. This can be done either by linear optical rectification effect with susceptibility or... 

Figure 1 Second harmonic generation. An input light source of frequency produces an output beam of frequency 2w. This frequency doubling originates from the molecular p and occurs only in noncentrosymmetric materials.

The Two-Level Model for y and Its Limitations. Generally, essential-states-model calculations have been very successful in explaining and predicting the second-order response of noncentrosymmetric materials, but considerable work remains for understanding third-order materials. As a starting point, consider again the two-level model, which can be expressed for third-harmonic generation as(139)... 

Figure 1.32 The well-known triangle used to describe the pathways between mechanical, electrical, and thermal energies in a class of noncentrosymmetric materials exhibiting piezoelectric effect, pyroelectric effect (converse of electrocaloric effect), piezocaloric effect (converse ofthermal expansion), and so on. For...

Centrosymmetric vs Noncentrosymmetric Bulk Media. The distinction between second- and third-order nonlinearities is particularly important for the design of NLO materials for different applications. The terms with even powers in the series expansion of the polarization (eq. 2) are identically zero for centrosymmetric materials. Only noncentrosymmetric materials can exhibit second-order... 

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