An ultrafast terahertz scanning tunnelling microscope

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Ultrafast studies1,2 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 techniques4,5,6,7,8,9,10,11,12,13,14, but the basic ability to directly modulate the STM junction bias while maintaining nanometre spatial resolution has been limited 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.

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Figure 1: Coupling terahertz pulses to a scanning tunnelling microscope (THz-STM).
Figure 2: Autocorrelation of terahertz pulses at an STM tunnel junction.
Figure 3: THz-STM imaging as a function of terahertz pulse autocorrelation overlap time.
Figure 4: Ultrafast photoexcitation of an InAs nanodot (optical-pump/THz-STM-probe).


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The authors thank S. Xu (Alberta Centre for Surface Engineering and Science, University of Alberta) for scanning electron microscope imaging, Auger electron spectroscopy measurements and analysis of the InAs nanodot sample. The authors also acknowledge technical support from G. Popowich, D. Fortin and D. Mullin. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), the Alberta Science and Research Investments Program (ASRIP), the Informatics Circle of Research Excellence (iCORE) and the NSERC Nano Innovation Platform (NanoIP).

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T.L.C. designed and built the THz-STM set-up, and T.L.C. and F.A.H. wrote the manuscript with contributions from all authors. T.L.C. and V.J. developed the THz-STM and carried out experiments with support from J.A.J.B., G.D.L.R., L.V.T. and F.A.H. S.J.M. contributed to the initial design phase of the THz-STM. M.G. and Y.Y.T. provided the InAs nanodot sample and assisted in sample characterization. T.L.C. analysed the data, and V.J., T.L.C. and M.R.F. developed the Simmons model simulation and fits to the data. F.A.H. initiated and developed the THz-STM concept with contributions from M.R.F., helped design the THz-STM set-up, and supervised the project. All authors contributed to discussions.

Correspondence to Tyler L. Cocker or Frank A. Hegmann.

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Cocker, T., Jelic, V., Gupta, M. et al. An ultrafast terahertz scanning tunnelling microscope. Nature Photon 7, 620–625 (2013) doi:10.1038/nphoton.2013.151

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