Femtosecond point-projection microscopy (fs-PPM) is a technique capable of achieving nanometer spatial resolution in combination with femtosecond temporal resolution. Using electrons as an imaging probe, the combination of a nanotip electron source and lens-less imaging allows for a temporal resolution comparable to ultrafast optical microscopy and a spatial resolution that can, in principle, reach the single nm level.

Due to the use of low-energy electrons, fs-PPM is highly sensitive to local electric fields, which makes it especially well suited to visualizing charge carrier separation and dynamics. The trajectories of low-energy electrons are very sensitive to electric fields in the vicinity of nano-objects. Ultrafast carrier propagation in nanoobjects result in transient changes of the electric potential. The visualization of changes of the electric fields therefore reveals information of the photo-currents inside the nano-object.

Photo-emission currents in silver nanowires

In this benchmark experiment we visualize the ultrafast dynamics upon photo-emission, illustrating the potential of fs-PPM in combining nanoscale spatial and femtosecond temporal resolution in imaging charge carrier dynamics. The multiphoton ionization and subsequent space-charge driven dynamics of photoelectrons emitted from silver nanowires is captured on timescales as low as 33 fs.

InP nanowires

Semiconductor nanowires (NWs) and, in particular, heterostructured NWs are promising candidates for future nanoscale electronic and optoelectronic devices, as well as ideal model systems for exploring fundamental semiconductor physics on nanometer length scales. Within the last years there has been vast progress in controlling the doping level in both radial and axial direction during NW growth. In order to understand the physics of carrier transport in these structures, it is of major importance to study the dynamics of charge carriers upon photoexcitation. So far, typical studies such as time-resolved photoluminescence and photoemission electron microscopy provide either spatially- or time-averaged information, respectively. In this regard, it is most appealing to combine nanometer spatial with femtosecond temporal resolution to directly measure the spatio-temporal evolution of photoexcited charge distributions in such nanoobjects.

We investigated ultrafast photocurrents in heterostructured InP-NWs with femtosecond temporal and nanometer spatial resolution, employing the fs-PPM technique. Low-energy electrons are very sensitive to detect transient fields in the near-surface region of nanoobjects generated by ultrafast photocurrents on nanometer dimensions.

Related publications:

• F. Krecinic, J. Malter, A. Paarmann, M. Müller and R. Ernstorfer:
  Point-projection microscopy of nano-localized photoemission currents at sub-40 femtosecond time scales. [arXiv:1803.01766]
• M. Müller, V. Kravtsov, A. Paarmann, M.B. Raschke, and R. Ernstorfer:
  Nanofocused plasmon-driven sub-10 fs electron point source.
  ACS Photonics 3, 611 (2016), [doi: 10.1021/acsphotonics.5b00710].
• M. Müller, A. Paarmann, and R. Ernstorfer:
  Femtosecond electrons probing currents and atomic structure in nanomaterials.
  Nature Communications 5, 5292 (2014), [doi: 10.1038/ncomms6292].
  open access: arXiv:1405.4992
• S. Lüneburg, M. Müller, A. Paarmann, and R. Ernstorfer:
  Microelectrode for energy and current control of nanotip field electron emitters.
  Appl. Phys. Lett. 103, 213506 (2013), [doi: 10.1063/1.4831735].
• M. Müller, A. Paarmann, C. Xu, and R. Ernstorfer:
  Coherent Electron Source for Ultrafast Electron Diffraction and Imaging.
  EPJ Web of Conferences 41, 10007 (2013).
• A. Paarmann, M. Gulde, M. Müller, S. Schäfer, S. Schweda, M. Maiti, C. Xu, T. Hohage, F. Schenk, C. Ropers, and R. Ernstorfer:
  Coherent femtosecond low-energy single-electron pulses for time-resolved diffraction and imaging: A numerical study.
  J. Appl. Phys. 112, 113109 (2012), [doi: 10.1063/1.4768204].