The invention of the scanning tunneling microscope (STM) revolutionized the field of surface science, enabling the first images of surface structure on an atomic length scale. In the quest for both atomic spatial and temporal resolution several groups have integrated an ultrafast optoelectronic switch which gates the current from the tip, achieving picosecond time resolution. In this paper, the authors describe a novel STM tip consisting of a cleaved GaAs substrate with a 1-{micro}m thick epilayer of low-temperature-grown GaAs (LT-GaAs) deposited on the face. Since the LT-GaAs has a carrier lifetime of 1 ps, photoexcitation of the tip with an ultrafast, above-bandgap pulse both provides carriers for the tunneling current and photoconductively gates (with ps resolution) the current from the tip. They use this tip to detect picosecond transients on a coplanar stripline and demonstrate a temporal resolution of 1.2 ps in tunneling mode
