Wang Lu, Fallahi Arya, Ravi Koustuban, Kärtner Franz
Opt Express. 2018 Nov 12;26(23):29744-29768. doi: 10.1364/OE.26.029744.
We describe a robust system for laser-driven narrowband terahertz generation with high conversion efficiency in periodically poled Lithium Niobate (PPLN). In the multi-stage terahertz generation system, the pump pulse is recycled after each PPLN stage for further terahertz generation. By out-coupling the terahertz radiation generated in each stage, extra absorption is circumvented and effective interaction length is increased. The separation of the terahertz and optical pulses at each stage is accomplished by an appropriately designed out-coupler. To evaluate the proposed architecture, the governing 2-D coupled wave equations in a cylindrically symmetric geometry are numerically solved using the finite difference method. Compared to the 1-D calculation which cannot capture the self-focusing and diffraction effects, our 2-D numerical method captures the effects of difference frequency generation, self-phase modulation, self-focusing, beam diffraction, dispersion and terahertz absorption. We found that the terahertz generation efficiency can be greatly enhanced by compensating the dispersion of the pump pulse after each stage. With a two-stage system, we predict the generation of a 17.6 mJ terahertz pulse with total conversion efficiency η = 1.6% at 0.3 THz using a 1.1 J pump laser with a two-lines spectrum centered at 1 μm. The generation efficiency of each stage is above 0.8% with the out-coupling efficiencies above 93.0%.
我们描述了一种在周期性极化铌酸锂（PPLN）中用于激光驱动窄带太赫兹产生的强大系统，该系统具有高转换效率。在多级太赫兹产生系统中，泵浦脉冲在每个PPLN阶段之后被循环利用，以进一步产生太赫兹波。通过将每个阶段产生的太赫兹辐射外耦合出去，避免了额外的吸收，并增加了有效相互作用长度。每个阶段太赫兹脉冲与光脉冲的分离是通过一个精心设计的外耦合器实现的。为了评估所提出的架构，使用有限差分法对圆柱对称几何结构中的二维耦合波动方程进行了数值求解。与无法捕捉自聚焦和衍射效应的一维计算相比，我们的二维数值方法捕捉了差频产生、自相位调制、自聚焦、光束衍射、色散和太赫兹吸收等效应。我们发现，通过在每个阶段之后补偿泵浦脉冲的色散，可以大大提高太赫兹产生效率。对于一个两级系统，我们预测使用中心波长为1μm的双线光谱的1.1J泵浦激光，在0.3THz频率下可产生能量为17.6mJ的太赫兹脉冲，总转换效率η = 1.6%。每个阶段的产生效率均高于0.8%，外耦合效率高于93.0%。
