Electric-field-switchable exciton polarons involving Γ-valley or K-valley Fermi sea in trilayer WSe₂

The exciton polaron, a quasiparticle arising from many-particle interactions, is a well-explored phenomenon in optical spectra of charged monolayer transition metal dichalcogenides (TMDs). However, the exciton-polaron properties in multilayer TMDs are still not well understood. In this study, we investigate optical spectra of exciton polarons in 2H-stacked trilayer WSe₂ dual-gate devices encapsulated by boron nitride. We find that the application of electric field can induce an abrupt change in the exciton-polaron spectra, which is attributed to the switching of the valence band maximum (VBM) between the Γ and K valleys. At weak electric fields, the VBM resides in the Γ valley, leading to the formation of exciton polarons involving the Fermi sea in the Γ valley. When the electric field exceeds a critical value, the VBM switches to the K valley, leading to the formation of exciton polarons involving the Fermi sea in the K valley. Our study elucidates the influence of band structure on the exciton polarons in trilayer WSe₂, shedding light on the intriguing nature of exciton polarons in multilayer TMDs.