Fig. 5. Relative propagation length L/λ_p of 2.52 THz polaritons as a function of wavevector and group velocity.

a Polariton amplitude decay time is $\tau$ = 0.48 ps. Black numbers indicate the relative propagation lengths. White symbol shows experimental relative propagation length for THz polaritons in the 25-nm-thick Bi₂Se₃. b Polariton amplitude decay time is $\tau$ = 0.6 ps. Black numbers indicate the relative propagation length. Black symbol shows the experimental relative propagation length of acoustic graphene plasmon (AGP) of an amplitude decay time of 0.6 ps in high-quality graphene encapsulated in h-BN (data taken from Alonso-Gonzalez et al.³¹).