The Fermi polaron is a quasi-particle born when an impurity is interacting coherently with a Fermi sea. During the years, it was found to be widely applicable in a variety of physical systems, including semiconductors, high-Tc superconductors, and insulators. One of the important open questions is under what conditions the quasi-particle description holds. We experimentally study this with an ultracold Fermi gas, taking advantage of the unparalleled tunability over interactions and spin-imbalance. To this end, we have developed a new spectroscopic method based on Raman transitions and our recently developed high-sensitivity fluorescence detection [1,2] on a highly imbalanced strongly-interacting Fermi gas. Our probing technique enables us to distinguish between the spectral response of quasi-particles and molecules. The momentum-resolved character of Raman transitions directly reveals the zero-momentum polaron energy, which we find to be in good agreement with theoretical predictions. We also infer the quasi-particle residue and find that it gradually decreases as the binding energy increases, until it vanishes above a certain interaction strength. None of the observables exhibit any sudden change of slope, as would be expected in the predicted first-order polaron-to-molecule phase transition. Instead, the emerging physical picture is that of polarons and pairs coexistence.

[1] Constantine Shkedrov, Yanay Florshaim, Gal Ness, Andrey Gandman, and Yoav Sagi, Phys. Rev. Lett. 121, 093402 (2018).
[2] Constantine Shkedrov, Gal Ness, Yanay Florshaim, and Yoav Sagi, Phys. Rev. A 101, 013609 (2020).