An ultrafast terahertz scanning tunnelling microscope

Ultrafast studies(1,2) of excitations on the nanometre scale are essential for guiding applications in nanotechnology. Efforts to integrate femtosecond lasers with scanning tunnelling microscopes (STMs)(3) have yielded a number of ultrafast STM techniques(4-14), but the basic ability to directly modulate the STM junction bias while maintaining nanometre spatial resolution has been limited to similar to 10 ps (refs 7,8) and has required specialized probe or sample structures. Here, without any modification to the STM design, we modulate the STM junction bias by coupling terahertz pulses to the scanning probe tip of an STM and demonstrate terahertz-pulse- induced tunnelling in an STM. The terahertz STM (THz-STM) provides simultaneous subpicosecond (<500 fs) time resolution and nanometre (2 nm) imaging resolution under ambient laboratory conditions, and can directly image ultrafast carrier capture into a single InAs nanodot. The THz-STM accesses an ultrafast tunnelling regime that opens the door to subpicosecond scanning probe microscopy of materials with atomic resolution.