FIBER-COUPLED ULTRAFAST SCANNING TUNNELING MICROSCOPE

We report on a scanning microscope with a photoconductive gate in the tunneling current circuit. The tunneling tip is attached to a coplanar transmission line with an integrated photoconductive switch. The swit ch is illuminated through a fiber which is rigidly attached to the swi tch substrate. By using a firmly attached fiber we achieve an excellen t reproducibility and unconstrained positioning of the tip. We observe a transient signal with 2.9 ps pulse width in tunneling mode and 5 ps in contact mode. The instrument is applied to investigating the mode structure on a coplanar waveguide. The measurements show that the prob e works as a transient voltage detector in contact and a capacitively coupled transient field detector in tunneling mode. We do not measure the transient voltage change in the ohmic tunneling current. In this s ense, the spatial resolution for propagating electrical pulses is bett er in contact mode than in tunneling mode. (C) 1997 American Institute of Physics.