Ultrafast sampling techniques

As semiconductor devices approach THz operation frequencies and device sizes enter the submicron regime, new tools for investigating their performance are required. For this purpose we developed an ultrafast scanning tunneling microscope (USTM). The technique relies on gating of the signal in the tunneling current circuit with a photoconductive switch. The switch is activated for 1 ps upon illumination with a 100 fs probe laser pulse. By exciting the sample with a pump laser beam from the same source, a short transient is generated. The only requirement for this transient is that it can be picked up by the tunneling tip either through a physical or an electrical change of the tunneling junction. As is common to all ultrafast sampling techniques, the time resolution is achieved by varying the optical delay between pump and probe beams.
Our experiments on transmission lines have clarified the operation principle of the USTM. When measuring a picosecond pulse on a coplanar transmission line, the signal is picked up through the geometrical capacitance formed by the tunneling junction. The spatial resolution is, therefore, determined by the tip radius and is not given by the tunneling region. We also demonstrated the first spatially and temporally resolved measurements with this instrument. Electrical pulses on transmission lines can be mapped out in great detail with a temporal resolution of 2 ps. The ability to measure the field distribution in the 100 GHz to 1 THz range, will be essential for the integration of active devices operating in this frequency range.
The more established techniques like photoconductive sampling and electro-optic sampling complement our USTM-measurements. The temporal resolution limit of electro-optic sampling is 200 fs and the spatial resolution is about 3 µm. We measure response times down to 800 fs and pulse amplitudes up to 3 V.