From single particle to superfuid excitations in a dissipative polariton   gas
  Using angle-resolved heterodyne four-wave-mixing technique, we probe the low momentum excitation spectrum of a coherent polariton gas. The experimental results are well captured by the Bogoliubov transformation which describes the transition from single particle excitations of a normal fluid to sound-wave-like excitations of a superfluid. In a dense coherent polariton gas, we find all the characteristics of a Bogoliubov transformation, i.e. the positive and negative energy branch with respect to the polariton gas energy at rest, sound-wave-like shapes for the excitations dispersion, intensity and linewidth ratio between the two branches in agreement with the theory. The influence of the non-equilibrium character of the polariton gas is shown by a careful analysis of its dispersion. 
Verena Kohnle, Yoan L\'eger, Michiel Wouters, Maxime Richard, Marcia   T. Portella-Oberli, Benoit Deveaud-Pl\'edran
cond-mat.other cond-mat.quant-gas