Second harmonic generation by focused beams in layer media

A theoretical study is made of second harmonic generation by focused beams in inhomogeneous media where the nonlinear susceptibility changes sign from one layer to another. It is found that the second harmonic power l and the coherence length l_c. A detailed analysis is made of the case l = 2l_c and it is established that under optimal focusing conditions, the maximum second harmonic power in a layer medium is almost the same as that in a homogeneous medium.