Efficient tunable terahertz generation via noncollinear phase matching in the ZnGeP2 crystal

A novel noncollinear phase-matching (PM) scheme by introducing a small tunable angle between two pump beams, was proposed to notably enhance the effective nonlinear coefficient (d(eff)) in difference frequency generating (DFG) tunable terahertz waves in the ZnGeP2 crystal. Compared with the collinear PM condition, the noncollinear geometry transfers the PM angles to be around theta = 90 degrees or theta = 30 degrees for type-II (o(-)e -> o) or type-I (o(-)e -> e) PM to maintain large values of d(eff) in the entire output frequency band, in which tunable bands of 1.90-4.5 THz or 0.47-4.30THz can be achieved, respectively, leading to a high conversion efficiency improved by tens of times. Based on the theory of noncollinear PM, the angle-tuning characteristics were studied and the crystal design was provided for efficient outcoupling. Rigorous theoretical models were built for both types under small-signal approximation to show the affecting factors of noncollinear PM and reveal its superiority compared with collinear PM. The idea presented in this paper not only provides a good solution for efficient terahertz generation in ZnGeP2, but it is also applicable in various optical frequency converters in different nonlinear materials.