Long–lived femtosecond photon echo in thin films on localized exciton states at room temperatureN.S. Vashurin, V.O. Kompanets, I.I. Popov, S.E. Putilin and S.V. ChekalinEPJ Web Conf., 161 (2017) 03007DOI: https://doi.org/10.1051/epjconf/201716103007