Fig. 4: Light–matter coupling strengths of mid-IR and terahertz polar dielectrics.
From: Ultraconfined terahertz phonon polaritons in hafnium dichalcogenides
a, RBs (ωTO to ωLO) of several polar dielectric materials in the terahertz to mid-IR range. Anisotropic materials are represented by different colour bands, with isotropic materials by one band (blue), uniaxial materials by two bands (blue and red) and biaxial materials by three bands (blue, red and yellow). b, Normalized coupling strength η calculated with equation (1) for each material in a.
