Ultrafast scanning tunneling microscopy using a photoexcited low-temperature-grown GaAs tip

We describe a novel STM tip consisting of a cleaved GaAs substrate with a 1 /spl mu/m-thick epilayer of low temperature-grown GaAs (LT-GaAs) deposited on the face. Because 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. We use this tip to detect picosecond transients on a coplanar stripline and demonstrate a temporal resolution of 1.2 ps in tunneling mode.