T he growing field of nanotechnology requires special tools capable of probing ultrafast surface dynamics on atomic scales in order to unveil the fundamental relationships between material structure and its properties. Ultrafast optical pump-probe techniques offer temporal resolution limited only by the pulse duration. However, the spatial resolution is restricted by diffraction ~200 nm. Alternatively, scanning tunneling microscopy (STM) reveals sub-angstrom spatial resolution, while the

more »

... e of the STM's electronics limits the temporal resolution to milliseconds. Recently, several approaches were proposed to combine the high spatial resolution of the STM and fs temporal resolution of the nonlinear optical spectroscopy into one instrument. 1-3 One technique, Junction-Mixing Scanning Tunneling Microscopy (JM-STM), relies on the intrinsic nonlinearity of the tunneling junction to provide an ultrafast correlation signal with atomic resolution of the conventional STM. 1-3 Preliminary experiments demonstrated the spatial resolution of JM-STM to be <1 nm with a temporal resolution >35 ps, insufficient for ultrafast dynamics studies. 4 Our recent results for JM-STM 5 have yielded a greatly improved temporal resolution of 8 ps and with a spatial resolution of 1 nm. Our experimental setup is described elsewhere. 3,5 Ultrafast voltage pulses are generated on a Pt transmission line, photolithographically deposited on a lowtemperature-grown GaAs sample with a 4-ps carrier lifetime. For spatial definition, Ti dots with 2.5 m diameter and 20 nm height are deposited on the Pt transmission line. The time-dependent I-V relation for the STM is: (t)] is the tunneling current [voltage], ␤ the linear tunneling conductance and ␥ the nonlinear tunneling coefficient. Because of a large difference in the value of the nonlinear parameter ␥ between Ti and Pt, the amplitude of the transient tunneling current induced by voltage pulses is strongly dependent on the position of the tip. The nonlinearity in V(t) yields an ultrafast temporal correlation signal.