Research ArticleAPPLIED PHYSICS

Pulling apart photoexcited electrons by photoinducing an in-plane surface electric field

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Science Advances  07 Sep 2018:
Vol. 4, no. 9, eaat9722
DOI: 10.1126/sciadv.aat9722
  • Fig. 1 Schematic of TR-PEEM and the ultrafast separation of the photoexcited electrons within the optical spot.

    We excite p-doped GaAs with a 1.55-eV pump and photoemit the photoexcited electrons with a 4.6-eV probe. The photoemitted electrons are imaged in a PEEM with high spatial resolution at different pump-probe delays. Assembling the images sequentially provides a movie of our ability to control the redistribution of the photoexcited electrons via optically induced, spatially varying lateral electric fields within the photoexcitation spot.

  • Fig. 2 Pulling apart photoexcited electrons by optically inducing spatially varying electric fields within the photoexcitation spot at high photoexcited carrier densities.

    We show snapshots of the normalized spatial distribution of the photoexcited electrons at three different time delays after photoexcitation (0, 200, and 500 ps) for both (A) low and (B) high photoexcited carrier densities. (A) At low carrier density (1.4 × 1018 cm−3), the photoexcited electrons exhibit well-known diffusion phenomena while continuing to retain a Gaussian distribution. (B) At high carrier density (2.1 × 1019 cm−3), the initial Gaussian profile of the photoexcited electrons at 0 ps starts to separate at +200 ps and eventually splits into two distinct distributions with the separation between the two fitted Gaussian peaks greater than the FWHM of the distributions. White elliptical lines in the XY plane demarcate the FWHM of the distributions.

  • Fig. 3 Control of rate of separation of the photoexcited electron clouds.

    (A) Distribution profile at +500 ps for three different photoexcited carrier densities ranging from a flat-top Gaussian to two overlapping Gaussian distributions with varying amount of separations. For quantitative analysis, the time-delayed distribution profiles are fitted with two Gaussian distributions of the same width and amplitude, leaving the peak positions as free parameters for fitting. The solid black lines show the distribution profiles that arise from the two fitted overlapping Gaussian distributions (solid gray lines). (B) Fitted peak separations as a ratio of the FWHM of their respective profiles at +500 ps for the three different carrier densities. The degree and rate of separation of the quasi-equilibrium distributions can be controlled by tuning the photoexcitation intensities. a.u., arbitrary units.

  • Fig. 4 The inhomogeneous screening of the built-in fields of a doped semiconductor induces lateral potential differences that pull apart the photoexcited electrons into two distinct distributions.

    (A) The spatially varying intensity of the Gaussian photoexcitation beam inhomogeneously screens the built-in surface fields of p-doped GaAs. The screening leads to a complete flattening of the bands at the center of the photoexcitation spot (denoted by the origin in Fig. 4A), but only partial flattening away from the center (see section S1 for details). This creates lateral potential differences or local electric fields within the photoexcitation spot that drive spatially varying currents. The dark yellow and blue symbols represent the dipoles and their polarities due to the charges in the depletion layer. The black arrows represent the electric fields from these dipoles. (B) Spatially varying electric field calculated from the evolving distribution of surface dipoles. (C) The calculated (solid lines) evolution of the density of photoexcited carriers closely reproduces the experimental data (blue lines and gray planes), showing the separation of photoexcited electrons into two separate distributions.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/9/eaat9722/DC1

    Fig. S1. 2D images showing the separation of the photoexcited electrons at the photoexcited carrier density of 2.1 × 1019 cm−3.

    Fig. S2. Distribution of dipoles before and after photoexcitation.

    Fig. S3. Partial screening of the built-in surface field.

    Fig. S4. Origin of the initial fast drop in the photoemission intensity.

    Fig. S5. Formation of the in-plane electric field.

    Fig. S6. Relative extents of the optical pulse penetration depths and the depletion width of the surface space charge region.

    Section S1. Partial screening of the built-in surface space charge field.

    Section S2. Formation of lateral electric field.

    Movie S1. Gaussian electron distribution profile at low carrier density of 1.4 × 1018 cm−3.

    Movie S2. Redistribution of the photoexcited electrons at 2.1 × 1019 cm−3.

    Reference (36)

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. 2D images showing the separation of the photoexcited electrons at the photoexcited carrier density of 2.1 × 1019 cm−3.
    • Fig. S2. Distribution of dipoles before and after photoexcitation.
    • Fig. S3. Partial screening of the built-in surface field.
    • Fig. S4. Origin of the initial fast drop in the photoemission intensity.
    • Fig. S5. Formation of the in-plane electric field.
    • Fig. S6. Relative extents of the optical pulse penetration depths and the depletion width of the surface space charge region.
    • Section S1. Partial screening of the built-in surface space charge field.
    • Section S2. Formation of lateral electric field.
    • Legend for Movies S1 to S2
    • Reference (36)

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Gaussian electron distribution profile at low carrier density of 1.4 × 1018 cm−3.
    • Movie S2 (.mp4 format). Redistribution of the photoexcited electrons at 2.1 × 1019 cm−3.

    Files in this Data Supplement:

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