Abstract

Recently, we have proposed a general technique to wed ultrafast laser spectroscopy and Scanning Probe Microscopies (SPM) to obtain simultaneous picosecond time and atomic space resolution.¹ Demonstration of this concept immediately followed: we have implemented this general technique in a specific device, the Ultrafast Scanning Tunnelng Microscope (USTM).² Ultrafast time response occurs when the SPM probe has a nonlinear response to a short laser pulse. This nonlinearity can be essential to the nature of the probe-sample interaction or can be artificially introduced to the probe. For instance, the intrinsic nonlinearity of the STM I-V characteristic can be used to obtain ultrafast time resolution.¹,³ The tip nonlinearity mixes the high frequency (short time) responses to produce a DC (correlation) signal. Temporal resolution is defined by the temporal width of the nonlinear tip response and the width of the laser pulses. Spatial resolution should be comparable to the resolution of the normal SPM. In the first demonstration, the nonlinear element was a fast photoconductive switch integrated with the STM tip assembly. Conceptually, the switch acts as a several picoseconds long gate.

© 1994 Optical Society of America