Abstract
The confinement of electromagnetic radiation to subwavelength scales relies
on strong light-matter interactions. In the infrared (IR) and terahertz (THz)
spectral ranges, phonon polaritons are commonly employed to achieve extremely
subdiffractional light confinement, with much lower losses as compared to
plasmon polaritons. Among these, hyperbolic phonon polaritons in anisotropic
materials offer a highly promising platform for light confinement, which,
however, typically plateaus at values of {\lambda}0/100, with {\lambda}0 being
the free-space incident wavelength. In this study, we report on ultraconfined
phonon polaritons in hafnium-based dichalcogenides with confinement factors
exceeding {\lambda}0/250 in the terahertz spectral range. This extreme light
compression within deeply sub-wavelength thin films is enabled by the
unprecedented magnitude of the light-matter coupling strength in these
compounds, and the natural hyperbolicity of HfSe2 in particular. Our findings
emphasize the critical role of light-matter coupling for polariton confinement,
which for phonon polaritons in polar dielectrics is dictated by the
transverse-longitudinal optic phonon energy splitting. Our results demonstrate
transition metal dichalcogenides as an enabling platform for THz nanophotonic
applications that push the limits of light control.