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 GatesEPJ Web Conf., 307 (2024) 01004DOI: https://doi.org/10.1051/epjconf/202430701004