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 (${d_{\rm{eff}}}$) in tunable terahertz wave difference frequency generation in the ${{\rm ZnGeP}_2}$ crystal. Compared with the collinear PM condition, the noncollinear geometry transfers the PM angle to be around $\theta = {90}^\circ$ for type-II (${\rm o} - {\rm e} \to {\rm o}$) 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.

中文翻译：

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ZnGeP ₂晶体中通过非共线相位匹配有效产生可调谐太赫兹

提出了一种新颖的非共线相位匹配（PM）方案，该方案通过在两个抽运光束之间引入一个小的可调角度，以最大化可调谐太赫兹波差频率产生中的有效非线性系数（$ {d _ {\ rm {eff}}} $）。在$ {{\ RM ZnGeP} _2} $结晶。与共线PM条件相比，非共线几何将类型II的PM角转移到$ \ theta = {90} ^ \ circ $左右（$ {\ rm o}-{\ rm e} \到{\ rm o} $）PM以充分利用最大非线性系数$ {d_ {36}} $，因此理论上将转换效率提高了数十倍。基于非共线PM理论，计算了角度调谐特性，并为有效的耦合提供了晶体设计。在小信号逼近下建立了严格的理论模型，以显示非共线PM的影响因素并显示其与共线PM相比的优越性。