T. Bienaimé – 4 décembre

In this talk, I will report on the measurement of the dispersion relation of small amplitude density waves propagating on top of a photon fluid. We find a dispersion relation of Bogoliubov type: linear at small wave vector as expected in the superfluid regime and “particle-like” (quadratic) at larger wave vectors [1]. In the superfluid regime, we characterize the dependence of the sound velocity with intensity (photon density) and compare our results with theoretical predictions.When the perturbation on top of the photon fluid becomes large, it can propagate faster than the local speed of sound. This leads to the generation of dispersive shock waves. I will discuss the peculiar dynamics of these waves and confront our observations to analytical and numerical models.

Finally, I will present an ongoing experiment to test light superfluidity by studying the drag force that a photon fluid exerts on a mobile obstacle. This experiment is based an nanofiber, immersed in an atomic vapor so that the refractive-index mismatch provides a constant potential. A light beam hitting the nanofiber tip at a small angle, will create a flow around the impurity, or in optics language, a radiation pressure force resulting in the opto-mechanical deformation of the obstacle. We expect to observe a cancellation  of this ”optical drag force” at high intensity indicating a superfluid flow of photons around the obstacle. In terms of optics, this leads to a non-intuitive cancellation of the radiation pressure thanks to nonlinear interactions.

[1] Observation of the Bogoliubov Dispersion in a Fluid of Light, Q. Fontaine, T. Bienaimé, S. Pigeon, E. Giacobino, A. Bramati, and Q. Glorieux, Phys. Rev. Lett. 121, 183604 (2018).