Scaling THz sources of radiation to higher energies is an important goal for THz science. In particular, the prospects of mJ-class energy narrowband, multi-cycle sources via difference frequency generation have gained in importance in the last decades, with a myriad of potential applications in accelerator science, coherent control and spectroscopy.  Many new techniques have been presented in the last few years aiming at this goal, but a comprehensive comparison and an evaluation of performance and future scaling prospects of the different techniques were missing in the literature. In this paper, Matlis et al. present an exceptionally thorough investigation comparing the scaling performance of narrowband THz generation using periodically poled lithium niobate (PPLN) either under a voltage-poled bulk form or as stacks of wafers with opposite polarities, and comparing the latter method with an alternative material, KTP.  In optimal conditions, they reach THz energies exceeding 100 µJ using novel large-aperture PPLN devices, setting record-holding values in terms of yield for this technique. Furthermore, they achieve significant steps forward in the wafer stacking method by implementing cryogenic cooling for the first time, thus allowing them to increase the number of wafers significantly, and by testing an alternative material, KTP, for the first time, showing exceptionally promising performance for further scaling.  The results shown re-define the state-of-the-art for this generation technique, as well as provide guidelines for future work in this field, that promises to have large impact in the Terahertz user community.

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