CNR NANOTEC;
Istituto di Nanotecnologia;
Via Monteroni;
73100 Lecce;
Italy;
Institute of Theoretical Physics and Astrophysics;
University of Gdańsk;
ul.Wita Stwosza 57;
80-952 Gdańsk;
Poland;
Institute of Physics;
Polish Academy of Sciences;
Al.Lotników 32/46;
02-668 Warsaw;
Poland;
Faculty of Physics;
University of Warsaw;
Pasteura 5;
02-093 Warsaw;
Poland;
INFN;
sezione di Lecce;
73100 Lecce;
Italy;
negative mass; nonlinearity; polaritons; superluminal; X-waves;