Photon pair production by STIRAP in ultrastrongly coupled matter-radiation systemsA. Ridolfo, G. Falci, F.M.D. Pellegrino, G.D. Maccarrone and E. PaladinoEur. Phys. J. Special Topics, 227 15-16 (2019) 2183-2188DOI: https://doi.org/10.1140/epjst/e2018-800076-1