His research began with the confirmation of the existence of biophotons, which had earlier been studied by Terence Quickenden in Australia. International projects led to cooperation with scientists such as Walter Nagl, Ilya Prigogine and David Bohm. He became an Invited Member of the New York Academy of Sciences and an Invited Foreign Member of the Russian Academy of Natural Sciences (RANS). For a time, he worked at Princeton University. Popp was the founder of the International Institute of Biophysics in Neuss, Germany.