Fig. 5: Energy dispersions of propagating modes.

a Plasmon wavelength λ_p in Bi₂Se₃ as a function of thickness, extracted from the interference pattern measured at 66.7 cm⁻¹ (black dots), at 76.7 cm⁻¹ (gray dots), and at 89.7 cm⁻¹ (light gray dots). b Wavelength λ_p of the photoexcited modes in Bi₂Te_2.2Se_0.8 as a function of flake thickness extracted from the interference pattern measured at 66.7 cm⁻¹ (red dots), at 76.7 cm⁻¹ (light red dots), and 89.7 cm⁻¹ (magenta dots). c Plasmon energy dispersion in Bi₂Se₃ for different flake thicknesses d ranging from 86 to 16 nm (colored dots) together with the predicted dispersion law (solid line). d Energy dispersion of the photoexcited modes in Bi₂Te_2.2Se_0.8 for different flake thicknesses d ranging from 118 to 39 nm (colored dots), compared to the energy dispersion simulated for Dirac plasmons (colored solid lines) and massive bulk plasmons (colored dot lines) reported in Supplementary Fig. 12 and therein-discussed in detail. Error bars indicate the 95% confidence bands from the fit of interference patterns.