Noncentrosymmetric structures, generation

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. 

Certain physical phenomena, such as piezoelectric or pyroelectric effects, or second harmonic generation, are found only for noncentrosymmetric structures [see Table 4.6 (Chapter 4) and Chapter 5]. The statistics of the intensities will usually tell if a crystal structure is centrosymmetric or noncentrosymmetric (see Chapter 8). 

Azobenzene-containing polymers are reported in the literature at a rate of about 150 papers/year for the last five years. There are two main directions of interest the first relates to the donor-acceptor substituted azobenzenes as the structure generating second-order polarizability and - by noncentrosymmetrical alignment - second order nonlinear optical properties in poled films. The second direction of interest is related to the photoinduced isomerization between the trans (more stable) and cis configurations of the azobenzene. This photoisomerization has a wealth of unexpected and useful consequences which were noted only in the last decade. Both these directions appeared because of the availability of lasers, which on the one hand allowed the observation of the nonlinear optical properties and on the other hand... 

In a recent study, Ashwell et al. have observed that centrosymmetric squaraine dyes incorporated into LB monolayers showed second harmonic generation efficiencies, which compare favourably with the highest values hitherto reported for LB monolayers of noncentrosymmetric dyes [126]. Based on comparison of the absorption spectra and SHG characteristics of the LB films of squaraine 45 (Structure 15) as well as the effect of deposition pressure on these properties, it has been suggested that formation of noncentrosymmetric aggregates is responsible for these effects. Nonlinear optical studies have also shown that symmetric squaraines have quite large molecular second hyperpolarizabilities [127-131]. 

Second harmonic generation is an effect of the interaction of electromagnetic fields with materials having high molecular and bulk polarizability. After the interaction, the frequency of the output field is twice that of the incident field. Pure p-nitroaniline microcrystals do not generate second harmonic radiation when excited by a Nd-Yag laser [261]. The phenomenon was displayed by / -CD-p-nitroaniline microcrystals, as shown by the formation of noncentrosymmetric crystal structures. 

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