Efficient tunable terahertz generation via noncollinear phase matching in the ZnGeP 2 crystal

A novel noncollinear phase-matching (PM) scheme was proposed by introducing a small tunable angle between two pump beams to maximize the effective nonlinear coefficient ( d e f f ) in tunable terahertz wave difference frequency generation in the Z n G e P 2 crystal. Compared with the collinear PM condition, the noncollinear geometry transfers the PM angle to be around θ = 90 ∘ for type-II ( o − e → o ) PM to fully use the maximum nonlinear coefficient d 36 , leading to a high conversion efficiency improved by tens of times in theory. Based on the theory of noncollinear PM, the angle-tuning characteristics were calculated and the crystal design was provided for efficient outcoupling. A rigorous theoretical model was built under small-signal approximation to show the affecting factors of noncollinear PM and reveal its superiority compared with collinear PM.