Claude and co-workers focus on the quantitative measurement of these collective excitations, also known as Bogoliubov waves. This propagation is described by a Bogoliubov-like dispersion relation, one that has a sound-like region (a linear energy-momentum relation on large length scales) and a free-particle-like region (a parabolic relation on small length scales). One hallmark of quantum-fluid behavior is the existence of collective excitations-in particular, small density perturbations that propagate on the surface of the fluid at rest. The similarity between cavity polaritons and two-dimensional quantum fluids goes beyond this qualitative description, as the time evolution of both systems is ruled by the same mathematical formalism: the so-called Gross-Pitaevskii equation. In the last decade, polaritonic systems have indeed been shown to display quantum-fluid behaviors ranging from Bose-Einstein condensation to superfluidity. They can thus behave collectively as a flow of massive, interacting particles-that is, as a quantum fluid. These polaritons have a mass determined by the exciton effective mass and the photon effective mass and they interact via exciton–exciton coupling. The coupling between photons and excitons in the cavity gives rise to quasiparticles called polaritons, which inherit the properties of both photons and excitons. Simultaneously, the laser illumination creates bound hole–electron states, called excitons. As a result, the relation between this wave vector component and the photon frequency exhibits a quadratic dependence that confers an effective mass to the photons. When such a cavity is illuminated by electromagnetic waves whose frequency matches the cavity resonance, the component of the wave vector perpendicular to the plane of the cavity becomes quantized. Semiconductor microcavities offer a powerful platform to observe photon-hydrodynamical effects. 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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