Efficient broadband sum and difference frequency generation with a   single chirped quasi-phase-matching crystal
  We propose an efficient broadband frequency generation technique for two collinear optical parametric processes $\omega_3=\omega_1+\omega_2$ and $\omega_4=\omega_1-\omega_2$. It exploits chirped quasi-phase-matched gratings, which in the undepleted pump approximation regime perform population transfer that is analogous to adiabatic population transfer in a three-state ``vee'' quantum system. The energy of the input fields is transferred adiabatically either into $\omega_3$ or $\omega_4$ field, depending on which of the two phase matchings occurs first by the local modulation period in the crystal. One can switch the output between $\omega_3$ and $\omega_4$ by inverting the direction of the local modulation sweep, which corresponds to a rotation of the crystal by angle $\pi$ 
Andon A. Rangelov
physics.optics