Fig. 2
Frequency-dependent dynamics of THz spectra at low temperature. Pump-induced THz spectra of a, b Δσ1(ω) and c, d Δε1(ω) after 248 meV (5 μm) photoexcitation with fluence 12 μJ cm−2 at T = 5 K as a function of pump-probe delay Δtpp. a and c show the THz spectra from three cut positions from the corresponding 2D plots of b and d, respectively, at Δtpp = 1, 5.4, and 15.4 ps, as indicated by the white-dashed lines. e THz conductivity at 2 meV, i.e., Δσ1(2 meV) (black dots, left axis), as a function of Δtpp, from the frequency-cut position in b as indicated by the red-dashed line. The scattering rate of the surface state γSS (red curve, right axis) obtained from theoretical fitting using Eq. 1 is plotted together to compare relaxation dynamics. f Similar to e, THz conductivity at 7 meV, Δσ1(7 meV) (black dots, left axis) and the scattering rate of bulk state γBS (green curve, right axis) are plotted. Δσ1(7 meV) is from the frequency-cut position in b as indicated by the green-dashed line. As shown clearly, the relaxation dynamics of the surface (bulk) scattering rate matches with that of THz conductivity at 2 meV (7 meV) very well. This frequency-dependent THz relaxation dynamics indicates surface (bulk) state is more sensitive to low (high) THz frequency
