Polaritons in van der Waals materials

Polaritons in van der Waals (vdW) materials.Polaritons-a hybrid of light-matter oscillations-can originate in different physical phenomena: conduction electrons in graphene and topological insulators (surface plasmon polaritons), infrared-active phonons in boron nitride (phonon polaritons), excitons in dichalcogenide materials (exciton polaritons), superfluidity in FeSe-and Cu-based superconductors with high critical temperature T c (Cooper-pair polaritons), and magnetic resonances (magnon polaritons).The family of vdW materials supports all of these polaritons.The matter oscillation component results in negative permittivity (e B < 0) of the polaritonic material, giving rise to optical-field confinement at the interface with a positive-permittivity (e A > 0) environment.vdW polaritons exhibit strong confinement, as defined by the ratio of incident light wavelength l 0 to polariton wavelength l p .