Fig. 2: Confinement and dispersion of thin-film polaritons.
From: Ultraconfined terahertz phonon polaritons in hafnium dichalcogenides
a,b, PhP dispersions of HfS2/SiO2 (d = 68 nm) (a) and HfSe2/SiO2 (d = 68 nm) (b) thin films. Experimental data (black circles) are plotted over the simulated Im(rpp) illustrating the extremely large k of the propagating PhPs. Dashed lines show the in-plane TO and LO phonon frequencies, ωTO and ωLO, respectively, as reported previously27. c, Light confinement of the PhPs in HfSe2 as a function of substrate permittivity εsub. Confinement factors are calculated using the transfer matrix method with the flake thicknesses (d = 47, 68, 85 and 119 nm) used in the experimental s-SNOM imaging. Experimental values (circles) for HfSe2/SiO2 (εsub = 4 and ω0 = 165 cm−1) and HfSe2/Si (εsub = 12 and ω0 = 162 cm−1) are plotted along with the simulated values (solid lines, ω0 = 163.5 cm−1) for different HfSe2 thicknesses (for the colours, see the labels). d, Maximum confinement of thin-film polaritons of a type-II hyperbolic model material as a function of normalized light–matter coupling strength η at three fixed ratios of thickness to the free-space wavelength (d/λ0 = 2 × 10−3, black; 1 × 10−3, blue; and 5 × 10−4, red). See Supplementary Information, section 5 for details. Max., maximum; Min., minimum; Sim., simulated data.
