Ultraconfined terahertz phonon polaritons in hafnium dichalcogenides

The confinement of electromagnetic radiation to subwavelength scales relies on strong light-matter interactions. In the infrared and terahertz spectral ranges, phonon polaritons are commonly employed to achieve deeply subdiffractional light confinement, with such optical modes offering much lower losses in comparison to plasmon polaritons. Among these, hyperbolic phonon polaritons in anisotropic materials offer a promising platform for light confinement. Here we report on ultraconfined phonon polaritons in hafnium-based dichalcogenides with confinement factors exceeding λ₀/250 in the terahertz spectral range. This extreme light compression within deeply subwavelength thin films is enabled by the large magnitude of the light-matter coupling strength in these compounds and the natural hyperbolicity of HfSe₂. Our findings emphasize the role of light-matter coupling for polariton confinement, which for phonon polaritons in polar dielectrics is dictated by the transverse-longitudinal optical phonon energy splitting. Our results demonstrate transition-metal dichalcogenides as an enabling platform for terahertz nanophotonic applications.