Long–lived femtosecond photon echo in thin films on localized exciton states at room temperature N.S. Vashurin, V.O. Kompanets, I.I. Popov, S.E. Putilin and S.V. Chekalin EPJ Web Conf., 161 (2017) 03007 Published online: 31 October 2017 DOI: 10.1051/epjconf/201716103007