Improved temporal and spatial resolution in junction-mixing ultrafast scanning tunneling microscopy

Abstract

The growing field of nanotechnology requires special tools capable of probing ultrafast dynamics of nanostructures on atomic scales. Recently, two possible practical realizations of such technique were proposed, which combine the high spatial resolution of scanning tunneling microscopy (STM) and femtosecond temporal resolution of the nonlinear optical spectroscopy [1,2]. One of them, photoconductively-gated STM is believed to be limited in its spatial resolution to several tens of nanometers due to capacitive coupling between the tip and the sample under study [3]. On the other hand, junction-mixing STM (JM-STM) relies only on the intrinsic nonlinearity of the tunneling junction current-voltage characteristic and should provide the atomic resolution of a conventional STM. In order to confirm this assumption, experiments have been performed, which demonstrated the spatial resolution of JM-STM to be of the order of 1 nm and the temporal resolution to be more than 35 ps (full width at half maximum (FWHM) of the detected transient current pulses) [4]. Unfortunately, this temporal resolution is not yet sufficient for the ultrafast dynamics studies. Here we present recent results for JM-STM with a greatly improved temporal resolution of less than 13 ps FWHM and with a spatial resolution of 1 nm.