Optical nonlinearities in organic multilayers

Consider now one of the examples which shows how a tensor of nonlinear polarizability Xije is formed in a superlattice. Suppose this tensor, the same as in cubic crystals of the group 7, /, takes the form 

An analogous relation may easily be obtained also for Dy which leads to X213 = Xi23- However, the expression for %3i2 is quite different. One can easily show that 

The difference between %3i2 and X123 is especially noticeable near the frequencies where eT1 ) 0, i.e. when 

In this case X123 has a pole whereas X312 has no such peculiarity. 

In the case of quadratic nonlinearity we shall consider such a simple example of the interaction of two waves with frequencies uj and 2w, so that in each layer there are two material relations 

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