Double resonant processes in $\chi^{(2)}$ nonlinear periodic media

In a one-dimensional periodic nonlinear $\chi^{(2)}$ medium, by choosing a proper material and geometrical parameters of the structure, it is possible to obtain two matching conditions for simultaneous generation of second and third harmonics. This leads to new diversity of the processes of the resonant three-wave interactions, which are discussed within the framework of slowly varying envelope approach. In particular, we concentrate on the fractional conversion of the frequency $\omega \to (2/3) \omega$. This phenomenon occurs by means of intermediate energy transfer to the first harmonic at the frequency $\omega/3$ and can be controlled by this mode. By analogy the same medium allows "nondirect" second harmonic generation, controlled by the cubic harmonic. Propagation of localized pulses in the form two coupled bright solitons on first and third harmonics and a dark soliton on the second harmonic is possible.