Extremely Confined THz Phonon Polaritons in HfS₂ and HfSe₂

Plasmonics and polaritonics have enabled the field of nanophotonics to reduce the wavelength of light to length scales far below the diffraction limit, leading to optoelectronic devices miniaturization and enhanced light-matter interactions. Traditionally, complex heterostructures were necessary to achieve the desired confinement capabilities. Coherently coupled vibrations in polar dielectrics, or phonon polaritons, are an attractive alternative to realize these goals as they use a different mechanism intrinsic to the material, to compress light. Quality phonon polaritonic materials possess a large splitting between the transverse optic (TO) and longitudinal optic (LO) phonons, which results from a strong internal electric dipole. Within the TO-LO splitting, or Reststrahlen band, the dielectric permittivity becomes negative, opening the door for phonon polaritons. Traditionally, the dominant spectral range for phonon polaritons has been the mid-infrared (mid-IR), despite a rich potential for nanophotonic materials at far-infrared (far-IR) and terahertz (THz) frequencies where molecular vibrations and rotations are observed. The transition of plasmonics and polaritonics into the far-IR and THz has been stunted due to a lack of materials that can host phonon polaritons.<br/><br/>Transition metal dichalcogenides (TMDs) are commonly studied in the near-infrared (near-IR) and visible wavelengths, specifically for the excitonic properties in MoS₂, MoSe₂, WS₂, and WSe₂. However, the phonon resonances of these TMDs are not strong and do not possess large TO-LO splitting, making them a poor candidate for infrared nanophotonics. Conversely, Group-IVB TMDs such as HfS₂ and HfSe₂ have exceptionally large Reststrahlen bands (&gt; 100 cm^-1), a priority for phonon polaritonics. They are also two-dimensional (2D) van der Waals crystals, making them even more attractive for nanoscale optics. Furthermore, due to the crystals’ anisotropy, the dielectric permittivity between the in-plane (ε_xy) and out-of-plane (ε_z) directions differs with respect to the incident wavelength, which separates the Reststrahlen band into two dispersion bands, elliptic (ε_xy*ε_z &gt; 0) and hyperbolic (ε_xy*ε_z &lt; 0). In the hyperbolic regions, the isofrequency contour becomes an open hyperbola with extraordinary waves that do not have a momentum (<i>k</i>) limit, facilitating extreme compression of the free-space wavelength (k = 2π/λ).<br/><br/>In this study, we use polarized Fourier transform infrared (FTIR) spectroscopy to extract the anisotropic IR permittivity of HfS(e)₂ and reveal the exceptional TO-LO splitting in the mid- to far-IR. Within these broad bands, we image the near-field optical signal of phonon polaritons and measure confinement of the free-space wavelength by a factor of more than 100. Finally, by varying the substrate permittivity, we demonstrate tunable control of the coupling strengths between the hyperbolic phonon polaritons and epsilon-near-zero polaritons in HfSe₂. By examining an underexplored class of materials, this work directs the search for quality nanophotonic materials, in particular those that facilitate access to far-IR/THz wavelengths, a spectral range that suffers from limited optical materials.