Figure 3: Femtosecond imaging of ultrafast photocurrents in InP NWs.

(a) Projection image of the same NW as in Fig. 2b) recorded in pulsed fsPPM mode at negative time delays. Photoecxitation by an ultrashort laser pulse leads to a transient, spatially inhomogeneous change of the projected NW diameter (b, normalized difference plot). (Data recorded at 70 eV electron energy; scale bars: 500 nm). Different dynamical behaviour and amplitudes of the transient diameter change Δd_NW are observed for the p- and n-doped segments along the NW (c), where an empirical three-exponential function was fitted to the data. Both segments show a fast initial photo-induced effect with 10–90 rise times in the p- and n-segments of 140 and 230 fs, respectively, followed by multi-exponential decay on the fs-to-few picosecond time scale. As Δd_NW is directly proportional to the transient electric field change, the derivate Δd_NW/dt plotted in the inset in c is a direct measure of the instantaneous photocurrent inside the NW. Surface states cause effective radial doping leading to band bending at the NW surface as sketched in d, where r is the radial coordinate, causing a radial photocurrent of electrons, j_e, and holes, j_h, after photoexcitation. This leads to a pump-induced transient shift Δ_pump of the conduction band edge E_CB and valence band edge E_VB, and hence a shift of the vacuum level E_vac (red shaded area), compared to the reference level E_ref (given by the environment), with the magnitude of the shift depending on the specific band bending and doping level.