Polymorphic phase engineering of flat plasmons in a correlated oxide

Plasmons with nearly dispersionless, long-lived behavior in momentum space have great potential for novel imaging techniques and nonlinear optics, due to their ability to generate localized plasmon wave packets and to enhance giant light fields in real space. However, flat plasmons typically manifest in low-dimensional systems that conventionally originate from intraband free electrons and are usually confined to restricted momentum regions (q < ∼0.7 Å^-1), which limits their applications. Here, we report the emergence and polymorphic phase-engineering of flat plasmons in the strongly correlated oxide Ti₂O₃, characterized by highly anisotropic and long-lived behavior (q > 0.7 Å^-1). The electronic correlation effect, that is, the on-site Coulomb interaction (U), was tuned by polymorphism through epitaxial stabilization. We demonstrate a close relationship between U and the energy fluctuation of plasmons (Δω_p). Specifically, a larger U leads to smaller Δω_p, that is, a stronger electronic correlation effect makes plasmons flatter. This tunability can be attributed to the renormalized bandwidth of Hubbard bands, which contribute to the generation of those flat plasmons. Our work offers a practical strategy for manipulating flat plasmons in strongly correlated systems, thereby promoting the development of novel plasmonic and nonlinear optical devices.