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Transient exciton-polariton dynamics in WSe2 by ultrafast near-field imaging

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Science Advances  01 Feb 2019:
Vol. 5, no. 2, eaat9618
DOI: 10.1126/sciadv.aat9618
  • Fig. 1 Near-field ultrafast and broadband pump-probe of EP in WSe2.

    (A) Schematic of the ultrafast pump-probe EP excitation and detection. The pump pulse impinges on the tip apex, which localizes the pulse energy, and excites an EP wave packet, which propagates on the WSe2 slab. The wave packet is reflected at the boundaries and within the thickness of the flake and is ultimately scattered back by the tip. (B) Schematic of the experimental near-field pump-probe apparatus. The ultrafast sub–10-fs pulse Ti:sapphire source, spanning a broad spectrum of 650 to 1050 nm, is split by a dichroic mirror (DM) into two pulses: the pump encompassing the higher energies of 650 to 700 nm and the probe from 700 to 1050 nm. The probe is delayed with respect to the pump by steps of 66 ± 3.3 fs. The two pulses are recombined after the delay and focused onto a scattering SNOM tip. The tip is scanned across the WSe2 slab while maintained in the vicinity of the sample’s surface with a closed-loop feedback based on a tapping mode operation. Det, detector; PB, parabolic mirror; BS, 90/10 beamsplitter; RM, reference mirror; a.u., arbitrary units. (C) Spectrally resolved steady-state (probe only) SNOM imaging of a 60-nm-thick WSe2 slab at several wavelengths around the exciton A bandgap. The signals are normalized with respect to the background on the SiO2 far from the flake. We reveal an enhanced response at 760 nm, around the exciton A transition. AFM, atomic force microscopy.

  • Fig. 2 Pump-probe near-field images showing the evolution of the EP wave packet in WSe2.

    (A) Selected snapshots of the SNOM images collected at different time delays for a probe wavelength of 760 ± 5 nm. The SNOM signal depicted is |S3(ω)|2, the intensity of the scattered signal demodulated at the third harmonic of the tip resonance frequency ω. A clear emergence of the interference fringes is observed across the flake as a function of the time delay. The thickness of the WSe2 flake is measured to be 60 nm (~100 layers); discussion and SNOM data for flakes ranging from monolayer to more than 90 nm can be found in the main text and in the Supplementary Materials. White scale bars, 2 μm. (B) Two-dimensional (2D) map of the SNOM signal as a function of the position across the flake along the white dashed line (right inset) and as a function of the pump-probe time delay. We observe the consecutive appearance of interference fringes as the position goes inward inside the flake and as the time delay increases. As we show further in this article, this is the expression of the EP wave packet excited by the tip that propagates and bounces back from the nearby flakes’ boundaries at a group velocity of vg. Therefore, vg can be retrieved by analyzing the time elapsed until the appearance of a particular fringe with respect to its position relative to the boundary. The gray dashed line is a guide for the eye to show the experimental retrieval of vg, which we found to be equal to 4.7 ± 0.5 × 106 m/s.

  • Fig. 3 Modeling of the near-field EP spatiotemporal propagation.

    (A) Schematic of the different optical paths that contribute to the SNOM signal at different time delays according to the velocity of the wave packet excited at the tip location. At t = 0, the EP wave packet (depicted as yellow dotted circles) is launched at the tip by the incoming field Ein and starts to propagate along the flake. At this point, only direct scattering by the tip E(0)scat contributes to the near-field signal collected at the detector (Det.). Further on, as the EP wave packet propagates along the flake, it bounces back from a nearby boundary toward the tip and, at the adequate time delay that corresponds to the round trip from the tip to the boundary (2l1), reaches it and contributes an additional component, E(1)scat, to the scattered near-field amplitude. Further secondary (third panel) and tertiary (not depicted) back reflections are also taken into account in accordance with previous models. An additional contribution, E(2)scat, accounts for the light trapped in the thickness of the flake and that bounces back and forth n times until it exits the flake toward the detector (depicted as a light cyan dotted line in the last panel). All these contributions add up with different phases and at different times, if at all, according to the time delay and the relative position of the tip and the flakes’ boundaries. (B) Numerical results of the described model depicted as the total near-field scattered intensity. We observe that the model predicts the consecutive appearance of one to four fringes in about 420 fs corresponding to a group velocity of vg (λ = 760 nm) = 5.2 ± 0.5 × 106 m/s, in good quantitative agreement with the experimentally retrieved vg. Furthermore, this quantitative agreement yields a propagation loss of γ ~ 2.6 μm−1. This relatively high loss figure suggests that the pump pulse heavily dopes the WSe2, causing damping of the EP via scattering with optically injected electrons. White scale bar, 1 μm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaat9618/DC1

    Fig. S1. Typical WSe2 flake under the optical microscope with the SNOM tip.

    Fig. S2. Pump and probe synchronization.

    Fig. S3. Illumination geometry in our SNOM setup.

    Fig. S4. Guided modes as a function of relative position between the flake and the incoming beam and polarization.

    Fig. S5. SNOM images and line profile (at 3 W) under different illumination and collection conditions relative to the slab apex.

    Fig. S6. SNOM snapshots at different time delays.

    Fig. S7. Calculated dispersion [E(k)] map for the multilayer stack with 60-nm-thick WSe2.

    Fig. S8. Extended view of the calculated dispersion [E(k)] map.

    Fig. S9. Dispersion curve for an EP.

    Fig. S10. Group velocity of EP in WSe2 around the A exciton transition for different longitudinal-transverse splitting.

    References (44, 45)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Typical WSe2 flake under the optical microscope with the SNOM tip.
    • Fig. S2. Pump and probe synchronization.
    • Fig. S3. Illumination geometry in our SNOM setup.
    • Fig. S4. Guided modes as a function of relative position between the flake and the incoming beam and polarization.
    • Fig. S5. SNOM images and line profile (at 3 W) under different illumination and collection conditions relative to the slab apex.
    • Fig. S6. SNOM snapshots at different time delays.
    • Fig. S7. Calculated dispersion E(k) map for the multilayer stack with 60-nm-thick WSe2.
    • Fig. S8. Extended view of the calculated dispersion E(k) map.
    • Fig. S9. Dispersion curve for an EP.
    • Fig. S10. Group velocity of EP in WSe2 around the A exciton transition for different longitudinal-transverse splitting.
    • References (44, 45)

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