29 April 2025 Tue 4 pm

Photons confined in optical cavities or propagating in paraxial geometries acquire an effective mass and behave like matter particles. Moreover, an effective photon-photon interaction can be engineered when the photons propagate in a nonlinear medium, resulting in collective fluid-like behaviors of light, such as superfluidity. The characterization of the elementary excitations in such quantum fluids of light [1] is essential to study their collective effects. I will present a novel coherent probe spectroscopy technique to study the Bogoliubov dispersion of these excitation modes in a fluid of microcavity exciton-polaritons under resonant pumping [2]. Thanks to the unprecedented spectral and spatial resolution, we observe directly the low-energy sonic behaviour and detect the negative-energy modes, i.e. the ghost branch, of the dispersion relation. This technique provides the missing tool for the quantitative study of quantum hydrodynamics in polariton fluids. In the second part of the talk, I will show how this technique allows to observe spontaneous symmetry breaking in a parametrically driven microcavity, revealing the appearance of a diffusive Goldstone mode; a new phenomenology stemming from the out of equilibrium nature of the system [3]. Finally, I will show how the properties of polariton quantum fluids can be used to simulate astrophysical objects like Black Holes and I will briefly discuss our progresses on the experimental investigation of the Hawking effect in a quantum fluid of polaritons [4]. [1] I. Carusotto and C. Ciuti, Quantum Fluids of Light, Rev. Mod. Phys. 85, 299 (2013). [2] F. Claude, M. Jacquet, R. Usciati, I. Carusotto, E. Giacobino, A. Bramati, Q. Glorieux, Phys. Rev. Lett. 129, 103601 (2022), High-Resolution Coherent Probe Spectroscopy of a Polariton Quantum Fluid [3] F. Claude et al., Observation of the diffusive Nambu-Goldstone mode of a non-equilibrium phase transition, To be published, Nature Physics [4] K. Falque et al, Polariton Fluids as Quantum Field Theory Simulators on Tailored Curved Spacetimes, ArXiv: 2311.01392v2 (2024)

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