Research ArticlePHYSICS

Recording interfacial currents on the subnanometer length and femtosecond time scale by terahertz emission

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Science Advances  08 Feb 2019:
Vol. 5, no. 2, eaau0073
DOI: 10.1126/sciadv.aau0073
  • Fig. 1 Recording subnanometer interfacial currents by THz emission.

    (A) Schematic of the type II band alignment in a WS2/MoS2 heterostructure. Photogenerated electrons and holes separate into different layers, giving rise to a net current that flows from the MoS2 to WS2 monolayer. (B) An optically triggered interfacial current Jz from the TMDC heterostructure emits a transient EM pulse at THz frequencies. (C) Emitted THz electric field waveforms from WS2/MoS2 and MoS2/WS2 heterostructures upon excitation with 3.1-eV femtosecond pulses as recorded using EO sampling. Inset: Calibration THz waveform from a bulk InSb crystal under identical conditions.

  • Fig. 2 Scaling of THz emission at low to moderate excitation fluences.

    (A) Dependence of peak THz field on excitation fluence. The dashed line is a linear fit, which applies for low to moderate fluences. The regime of saturated response is shown in Fig. 4. (B) Dependence of peak THz field on excitation polarization. The black curve is the calculated polarization-dependent absorbance of the heterostructure. These results demonstrate that the peak THz field is proportional to the absorbed excitation energy for low to moderate fluences. a.u., arbitrary units.

  • Fig. 3 Determination of the characteristic time scales and efficiency for charge transfer.

    (A) High-bandwidth EO sampling data overlaid with simulations for various combinations of the characteristic material response time τ = τr + τt and transfer efficiency ξ, as discussed in the text, assuming τr << τt (or τt << τr). Inset: Corresponding impulse response of interfacial current η. (B) Same as in (A), but for the case of τr = τt. The impulse functions are shown in the inset with the same scales and values for τ and ξ. (C) Temporal evolution of the current Jz(t) for the best-fit parameters: τr << τt = 50 fs and τr = τt = 30 fs, both with ξ = 70%. The temporal profile of the excitation pulse used in the experiment is shown in purple in arbitrary units.

  • Fig. 4 Saturation of interfacial charge transfer due to buildup of a reverse electric field.

    (A) Schematic of charge transfer saturation due to the buildup of a reverse electric field by the transferred charge that compensates the original band offset. d is the interlayer separation. Net charge transfer ceases to grow when the band alignment crosses over to type I (when the reverse potential completely compensates the smaller of the conduction and valence band offsets). For the purposes of illustration, we fix the MoS2 bands as the reference. (B) Dependence of peak THz field on excitation fluence. The blue shaded area is the fluence accessed with the 0.3-mm beam (Fig. 2A). The dotted line is a linear fit using the low-fluence data points, and the dashed line is the saturation fit. The inset shows the same plots presented with logarithmic axes. See the main text and Materials and Methods for details.

  • Fig. 5 Dependence of the interfacial charge transfer in MoS2/MoSe2 heterostructures on the excitation photon energy.

    The THz waveforms observed when exciting the heterostructure with 1.55-eV (red) and 3.1-eV (violet) pulses, normalized by the number of absorbed photons, showing no appreciable difference within instrument response. Inset: Schematic of the type II band alignment for a MoS2/MoSe2 heterostructure. 3.1-eV photons excite hot carriers in both materials, while 1.55-eV photons excite only band-edge excitons in MoSe2.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaau0073/DC1

    Fig. S1. Schematic of the time-domain TES setup.

    Fig. S2. Reproducibility of the measurements.

    Fig. S3. Comparison between the low- and high-bandwidth data.

    Fig. S4. Typical reflectance contrast and Raman spectra of the heterostructures.

    Fig. S5. Analysis of the optical properties calculation.

    Fig. S6. Determination of THz refractive index of GaP.

    Fig. S7. Total transfer function of the setup.

    Fig. S8. Normalized THz waveforms for representative excitation fluences.

    Fig. S9. Reflected excitation power as a function of excitation fluence.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Schematic of the time-domain TES setup.
    • Fig. S2. Reproducibility of the measurements.
    • Fig. S3. Comparison between the low- and high-bandwidth data.
    • Fig. S4. Typical reflectance contrast and Raman spectra of the heterostructures.
    • Fig. S5. Analysis of the optical properties calculation.
    • Fig. S6. Determination of THz refractive index of GaP.
    • Fig. S7. Total transfer function of the setup.
    • Fig. S8. Normalized THz waveforms for representative excitation fluences.
    • Fig. S9. Reflected excitation power as a function of excitation fluence.

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