Exciton-polaritons in semiconductor microcavities constitute the archetypal realization of a quantum fluid of light. Under coherent optical drive, remarkable effects such as superfluidity, dark solitons or the nucleation of vortices have been observed, and can be all understood as specific manifestations of the condensate collective excitations. In this work, we perform a Brillouin scattering experiment to measure their dispersion relation \(\omega ({\bf{k}})\) directly. The results, such as a speed of sound which is apparently twice too low, cannot be explained upon considering the polariton condensate alone. In a combined theoretical and experimental analysis, we demonstrate that the presence of an excitonic reservoir alongside the polariton condensate has a dramatic influence on the characteristics of the quantum fluid, and explains our measurement quantitatively. This work clarifies the role of such a reservoir in polariton quantum hydrodynamics. It also provides an unambiguous tool to determine the condensate-to-reservoir fraction in the quantum fluid, and sets an accurate framework to approach ideas for polariton-based quantum-optical applications.

半导体微腔中的激子极化子构成了光量子流体的原型实现。在相干光驱下，已经观察到了显着的影响，例如超流，暗孤子或涡旋核，这些都可以理解为冷凝物集体激发的特定表现。在这项工作中，我们进行了布里渊散射实验以测量其色散关系\（\ omega（{\ bf {k}}）\）直接地。仅考虑极化子凝结物就无法解释结果，例如声速显然太低了两倍。在理论和实验相结合的分析中，我们证明了激子储层与极化子冷凝物并存对量子流体的特性产生了巨大影响，并定量地解释了我们的测量结果。这项工作阐明了这种储层在极化子量子流体动力学中的作用。它还提供了明确的工具来确定量子流体中的凝结层到储层的比例，并为基于极化子的量子光学应用中的思想设定了准确的框架。