Floquet polaron and molecule in a highly polarized Fermi gas with periodically modulated interactions

We investigate the impurity problem of a highly polarized Fermi gas with periodically modulated atom-atom interactions. We find that generalized time-periodic Floquet polaron and molecule emerge as quasistationary quasiparticle states of the system, which exhibit peculiar features distinct from those in a static system. Even in the high-frequency limit, the periodic driving can renormalize the scattering length of the static (effective) Hamiltonian and thus modify the polaron-to-molecule transition. Remarkably, due to the folding of the quasienergy of the Floquet states in the Floquet Brillouin zone, unusual energy-level crossings beyond the conventional polaron-to-molecule transition occur. In particular, when the driving frequency is in resonance with the quasienergy difference, both branches of the Floquet polaron can intersect with the molecular state or even themselves under weak driving. The unique physics of these Floquet impurity states is further characterized and elucidated by the experimentally measurable spectral functions and the radio-frequency spectra. Our results pave a way to explore the intriguing nonequililbrium impurity physics beyond a static system via “Floquet engineering.”