From Single Particle to Superfluid Excitations in a Dissipative Polariton Gas

Abstract

Using an 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 soundlike 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, soundlike shapes for the excitations dispersion, intensity, and linewidth ratio between the two branches in agreement with the theory. The influence of the nonequilibrium character of the polariton gas is shown by a careful analysis of its dispersion.

Four-wave mixing excitations in a dissipative polariton quantum fluid

“…At this excitation intensity, the Bogoliubov transformation is achieved and the superfluid dispersion features the expected shape with normal and ghost branches. As previously observed, 14 the normal and ghost branches are not symmetric to each other in Fig. 6 and only the NB shows an almost linear behavior.…”

“…21,22 Microcavity polaritons, for which superfluidity has first been demonstrated through frictionless measurements, 3,4 appear as a model system to investigate the Bogoliubov excitation spectrum. 13,14,23,24 These composite bosons show indeed unique properties inherited from both their components: a very small mass from the photonic part and Coulomb mediated repulsive interactions from the excitonic one. A first attempt to measure the Bogoliubov dispersion in a spontaneous polariton condensate has been carried out through photoluminescence experiments.…”

“…14 In the superfluid regime, the FWM response shows two main emission peaks with comparable intensities and linewidths, which are the signatures of the normal and ghost branches of the Bogoliubov spectrum [ Fig. 1(c)].…”

“…The Bogoliubov transformation was already discussed deeply in a previous paper. 14 We focus here on the temporal and wave-vector dependence of the FWM spectrum. We discuss in detail the strong effect of dissipation on the superfluid excitation spectrum resulting in the formation of replicas of the dispersion branches.…”

“…12 Recently, the Bogoliubov dispersion has been investigated in the solid state in a coherent polariton gas, 13 and the ghost dispersion branch has finally been observed. 14 Microcavity polariton fluid is extremely different from other bosonic quantum fluids as polaritons originate from the strong coupling of quantum-well excitons with cavity modes of semiconductor microcavities. The main consequence of this unique nature is the short lifetime of the particles in the 10-ps range.…”