Fig. 1
Ultrafast mid-IR/THz pump and THz probe spectroscopy for Bi2Se3. a Schematic of the surface and bulk electronic band structures of the Bi2Se3 film with Fermi level (EF) indicated, illustrating mid-IR/THz intraband excitations and THz conductivity detection. b Static THz spectra of σ1(ω) and ε1(ω) (red dots) of the Bi2Se3 film at T = 5 K. Shown together are the theoretical fitting (gray lines) using the composite THz model from Eq. (1). c, d A 2D false-color plot of pump-induced THz field changes ΔEsample as a function of gate time tgate (horizontal axis) and pump-probe delay Δtpp (vertical axis) after 248 meV (5 μm) photoexcitation of the sample at c T = 300 K and d T = 5 K, respectively. Plotted together are the corresponding static THz fields Esample(tgate) (green curves) and their pump-induced changes ΔEsample(tgate) at Δtpp = 1 ps (blue curves, ×5, from the cut positions as marked by the blue-dashed lines), in order to compare their relative amplitude changes and phase shifts at different temperatures
