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Published May 11, 2017 | Supplemental Material + Published
Journal Article Open

Super-diffusion of excited carriers in semiconductors


The ultrafast spatial and temporal dynamics of excited carriers are important to understanding the response of materials to laser pulses. Here we use scanning ultrafast electron microscopy to image the dynamics of electrons and holes in silicon after excitation with a short laser pulse. We find that the carriers exhibit a diffusive dynamics at times shorter than 200 ps, with a transient diffusivity up to 1,000 times higher than the room temperature value, D_0≈30 cm^2 s^(−1). The diffusivity then decreases rapidly, reaching a value of D_0 roughly 500 ps after the excitation pulse. We attribute the transient super-diffusive behaviour to the rapid expansion of the excited carrier gas, which equilibrates with the environment in 100−150 ps. Numerical solution of the diffusion equation, as well as ab initio calculations, support our interpretation. Our findings provide new insight into the ultrafast spatial dynamics of excited carriers in materials.

Additional Information

© 2017 The Author(s). This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Received: 01 July 2016; Accepted: 03 March 2017; Published online: 11 May 2017. This work was supported by NSF grant DMR-0964886 and Air Force Office of Scientific Research grant FA9550-11-1-0055 in the Physical Biology Center for Ultrafast Science and Technology at California Institute of Technology, which is supported by the Gordon and Betty Moore Foundation. M.B. thanks the California Institute of Technology for a start-up fund. V.I. and M.B. gratefully acknowledge support by the Caltech-GIST program. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Author Contributions: E.N. and A.Z. conceived and designed the experiments. E.N. carried out the experiments and analysed the data. V.I. and M.B. developed the theory and carried out numerical simulations and ab initio calculations. E.N. and M.B. wrote the manuscript. All authors reviewed the manuscript. The authors declare no competing financial interests.

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