Until this very point, we mostly discussed the statically-casued Friedel oscillations, however we mostly do encounter dynamical systems. Thus, Poyli et al. in 2018 investigated dynamical response to an external oscillating dipole (of the dipole mo-ment µ = 1 e • nm and emitting the radiation of the wavelength λ = 35 µm) in thin (i.e. of thickness d = 10 nm) topological insulators (TIs) . They modelled such a TI via a sandwich model, where we have a thin slab of insulator (with relative electric permittivity ϵr = 25) with two 2D Fermi liquids (i.e. 2D interacting electron gases describing the typical state of the majority of metals at low enough temperatures) on both sides. They obtained two interesting things (in terms of this review): (1) when the dipole oscillates really close to the TI (i.e. at a distance 5 nm), then the charge density oscillations (of wavelength ≈ 100 nm) on those two sides are in counter phase; however with the dipole is instead far from the TI (i.e. at about 150 nm, then we can notice the oscillations (now of wavelength ≈ 1000 nm) to be in phase instead, and (2) they also obtained the charge-and spin-density 3D plots, showing the oscillatory decaying patterns, which are also presented in the Fig. (34). Plots of surface charge density distributions are visualized in the Fig. ) for both cases. Moreover, we may notice that in the former case (i.e. of the dipole close by), we obtain almost zero average charge density, yet the spin-density oscillations do not vanish, whereas in the latter case (i.e. of the dipole far away), we get the reverse situation -the average spindensity is almost non-existent, while the average charge-density is quite apparent. Furthermore, we may relate those quantities by the following simple formula