Their approach holds promise for exploring new quantum-fluid regimes, including some that could serve as analog models of gravity. Now Ferdinand Claude of the Kastler-Brossel Laboratory (LKB) of Sorbonne University, France, and co-workers have provided an unprecedentedly detailed characterization of a quantum fluid of polaritons-quasiparticles resulting from the strong coupling of photons and excitons in a semiconductor microcavity. Our understanding of these exotic states, however, is hampered by experimental limitations-in particular, the difficulty of probing the collective excitations that are hallmarks of quantum-fluid behavior. Both configurations allow photons to acquire an effective mass and experience an effective mutual interaction-properties that can lead them to collectively behave as a quantum fluid. Two platforms emerged for the study of these “fluids of light”: semiconductor microcavities in which photons are confined and propagating geometries in which photons travel in a bulk medium. Over a decade ago, optics researchers started to take an interest in superfluids and other quantum fluids, driven by the realization that light propagating in a nonlinear medium can exhibit quantum hydrodynamics features. Superfluidity, the ability of a fluid to flow without friction, isn’t restricted to systems described by hydrodynamics. By measuring the cavity reflectivity for different incidence angles of a “probe” pulse, the team obtained the polariton dispersion curve. A “pump” laser pulse (red) generated polaritons via photoexcitation. APS/ Alan Stonebraker Figure 1: Sketch of the “pump-probe” setup used by Claude and co-workers to characterize the fluid of polaritons in a semiconductor cavity.
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