30 years of PPLN-what is old, what is new, and where next for PPLN and QPM material research? Peter G.R. Smith, Goronwy Tawy, Rex H. Bannerman, Corin B.E. Gawith, Paolo Mennea, Noelia Palomar Davidson and James C Gates EPJ Web Conf., 307 (2024) 01004 Published online: 18 October 2024 DOI: 10.1051/epjconf/202430701004