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Lattices of planar semiconductor micropillars have shown to be an ideal environment to study condensed-matter phenomena by the manipulation of exciton-polaritons, quasiparticles with light-matter behavior [1] . Two great assets of this platform are the ability of engineering lattices with at-will nearest-neighbors hoppings, and the possibility of directly measuring the wavefunctions and spectrum associated to the lattice hamiltonian in photolu-minescence experiments.

Thanks to their versatility, photonic systems are very promising platforms for exploring topological physics beyond what is physically reachable in solid-state crystals [1] . In this sense, lattices of semiconductor microcavities confining exciton-polaritons, an hybrid quasi-particle formed from the strong coupling of optical photons and quantum well excitons, offer many advantages. For example, their intrinsic non-hermitian nature allows for probing specific topological properties efficiently and without relying on complex schemes.

Exploring topological physics in photonic systems is a promising avenue for developing novel generation of optical devices that are robust against external perturbations and fabrication defects [1]. It is however still challenging to implement topological lattices in media exhibiting strong optical nonlinearities, which is a necessary requirement for exploring the confluence of nonlinear optics and topological robustness. Hence, cavity polaritons formed from the strong coupling between quantum well excitons and cavity photons are particularly appealing: their photonic part allows for engineering topological properties in lattices of coupled micropillars [2], while their excitonic part gives rise to strong Kerr-like nonlinearities and to lasing [3].

Elementary excitations of a two-dimensional electron system (2DES) can be described in terms of exciton-polarons, i.e. excitons dressed by Fermi-sea electron-hole pairs due to interactions between excitons and the surrounding electrons [1,2]. An interesting open problem is the modification of this description in the quantum Hall regime, where electrons occupy flat-bands and electron-electron interactions become important. Here, we perform cavity spectroscopy, in the strong-coupling regime, of a two-dimensional electron system embedded in a semiconductor microcavity [3,4]. We observe exciton-polaron-polariton resonances in the regime of integer and fractional quantum Hall effect (FQHE).