Speaker
Prof.
Frank Hegmann
(University of Alberta)
Description
The ability to spatially resolve ultrafast phenomena over nanometer length scales is essential for understanding nanoscale excitation dynamics in materials as well as for measuring switching speeds of nanodevices. The scanning tunneling microscope (STM) uses quantum tunneling of electrons between a scanning tip and a sample to achieve nanoscale imaging of surfaces with atomic resolution. However, the time resolution of an STM is typically limited by the bandwidth of the amplifier electronics used to measure the small tunnel currents. This has spurred the development of various ultrafast STM techniques that combine STMs with femtosecond laser sources. We present a new technique that couples terahertz (THz) pulses to the tip of a scanning tunneling microscope (THz-STM) that allows for direct imaging of sub-picosecond dynamics on surfaces with nanometer spatial resolution [1]. The THz pulses act like fast voltage transients across the tunnel junction, resulting in a rectified tunnel current signal. THz pulse autocorrelations performed on gold-on-graphite samples show THz-STM response times of less than 400 fs with simultaneous 2 nm spatial resolution under ambient conditions. Imaging of sub-picosecond carrier capture dynamics into a single InAs nanodot is also demonstrated. The THz-STM accesses an ultrafast tunneling field emission regime that we are currently exploring. The potential of THz-STM for imaging ultrafast phenomena with atomic resolution is discussed.
[1] T. L. Cocker, et al., “An ultrafast terahertz scanning tunnelling microscope,” Nature Photonics 7, 620 (2013).
Primary author
Prof.
Frank Hegmann
(University of Alberta)
