  The electronic exciton polaron is a hypothetical many-body quasiparticle
formed by an exciton dressed with a polarized electron-hole cloud in the Fermi
sea (FS). It is predicted to display rich many-body physics and unusual
roton-like dispersion. Exciton polarons were recently evoked to explain the
excitonic spectra of doped monolayer transition metal dichalcogenides (TMDs),
but these studies are limited to the ground state. Excited-state exciton
polarons can exhibit richer many-body physics due to their larger spatial
extent, but detection is challenging due to their inherently weak signals. Here
we observe gate-tunable exciton polarons for the 1s - 3s excitonic Rydberg
series in ultraclean monolayer MoSe$_2$ devices by optical spectroscopy. When
the FS expands, we observe increasingly severe suppression and steep energy
shift from low to high Rydberg states. Their gate-dependent energy shifts go
beyond the trion description but match our exciton-polaron theory. Notably, the
exciton-polaron absorption and emission bands are separated with an energy gap,
which increases from ground to excited state. Such peculiar characteristics are
attributed to the roton-like exciton-polaron dispersion, where energy minima
occur at finite momenta. The roton effect increases from ground to excited
state. Such exciton-polaron Rydberg series with progressively significant
many-body and roton effect shall provide a new platform to explore complex
many-body phenomena.
