Graphene based frequency doubling has been a thing, though. (Terahertz). VUV 2HG ditto, but as you said nonlinear crystals, and the intensity is not as great as

Graphene based frequency doubling has been a thing, though. (Terahertz). VUV 2HG ditto, but as you said nonlinear crystals, and the intensity is not as great as claimed? https://ieeexplore.ieee.org/document/6259401 https://ieeexplore.ieee.org/document/6259401 https://pubs.aip.org/aip/rsi/article-abstract/84/6/063104/355331/High-intensity-coherent-vacuum-ultraviolet-source?redirectedFrom=fulltext https://pubs.aip.org/aip/rsi/article-abstract/84/6/063104/35... Lots of intertwining uh avenues of research! (Without full fledged devices, it's easy to lapse into pareidolia on spectras & motivations :( )