Short-range surface plasmonics: Localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface

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Science Advances  12 Jul 2017:
Vol. 3, no. 7, e1700721
DOI: 10.1126/sciadv.1700721
  • Fig. 1 Concept of SR-SPP imaging in 2PPE PEEM.

    (A) Illustration of the SPP dispersion relations at gold-vacuum and gold-silicon interfaces. The dispersion relations are calculated from a 20-nm-thick gold film [with optical properties discussed by Johnson and Christy (38)] on a silicon substrate with a native SiO2 layer. λ0 = 800 nm (dashed black line) is the excitation wavelength for LR-SPPs (blue line) and SR-SPPs (red curve). Image (B) represents cross sections through the sample, together with the x component of the SPP electric field. In (i), LR-SPPs are illustrated, which are predominantly located at the gold-vacuum interface. In (ii), SR-SPPs are described, which mainly exist at the gold-silicon interface. They can extend to the gold-vacuum interface in the z direction if the gold thickness is small enough. (C) Illustration of the experimental concept: Single-crystalline gold platelets are deposited on silicon substrates. This allows SPP excitation at the material interfaces due to normal incident ultrashort laser pulses (<15 fs) of 800-nm wavelength with linear transversal magnetic (TM) polarization. LR-SPPs are indicated at the gold-vacuum interface (blue), and SR-SPPs are indicated at the gold-silicon interface (red). (D) SPP excitation and imaging are realized using two-photon photoemission microscopy at normal incidence. After SPP excitation, electrons are emitted from the sample and imaged with a nanometer resolution using electron optics. As recently demonstrated (41), the Ex component of the surface plasmon field along the incident polarization is responsible for the electron emission.

  • Fig. 2 Normal-incidence 2PPE PEEM results.

    (A) Long-range plasmon wave pattern, excited with zero time delay pulses on a 120-nm-thick hexagonal platelet. (B) The platelet is 37 nm high and permits long- and short-range surface plasmon excitation simultaneously. (B) Snapshot at t0 + 4.52 fs time delay, taken from a time-dependent series. (C and D) The PEEM results are modeled, taking all specifications from images (A) and (B) into account. (E) Emission profiles from platelets (A) and (B). From profile (A), the LR-SPP wavelength is determined as λLR-SPP = 785 nm. Profile (B) illustrates the superposition of LR-SPPs and SR-SPPs, where the SR-SPP wavelength is determined as λSR-SPP = 185 nm. (F) The emission profiles derived from the theoretical PEEM images.

  • Fig. 3 Nanofocus formation.

    (A) SEM image of a 22-nm-high single-crystalline gold platelet, taken at an inclined angle of 54°, patterned with a circular grating of 150 nm period and a central disk with a diameter of 2 μm. (B) 2PPE PEEM at an 800-nm laser wavelength excites both long- and short-range surface plasmons on the structured platelet. Long-range surface plasmons of about 800-nm wavelength are hardly visible, whereas the short-range plasmons are predominant. Because of the surrounding grating, the short wavelength plasmon can couple into the central disk, which acts as a focusing device. At the focal spot, the highest electron emission is observed. From these time-integrated data, the emission profile is characterized perpendicular (C) and parallel (D) to occurring wavefronts. The focus size in profile (C) is determined by the full width at half maximum (FWHM) of the central peak and is 60 nm. (D) The focus width is 120 nm. The images and cuts present raw data without any postprocessing, which is characteristic of the extremely high sample quality and the noninvasive imaging process. They share the same y scale.

  • Fig. 4 Time-resolved PEEM measurements illustrate the dynamic SPP focus in the center of the 2-μm disk.

    (A) The plasmon focus is at a relative maximum at delay time Δt = t0 between pump and probe pulse. (B) At delay time Δt = t0 + 1.33 fs, the PEEM image shows minimum electron emission at the central spot. Images (C) and (D) show the simulated PEEM images for the same delay times. Images (E) to (H) show the emission profiles from the PEEM measurements and simulations above. Profiles (E) and (G) illustrate the maximum case, where the FWHM of the central peak is 60 nm as well. The complete experimental and simulated movies are available in the Supplementary Materials.

  • Fig. 5 Sequence of emission profiles through the nanofocus (see inset) for increasing delay times up to 50 fs.

    The plasmon waves counterpropagate from the edges into the central position 0 and superpose constructively and destructively with each other and the electric field of the pump laser pulse. At the focal spot, within an optical cycle, one emission maximum and one minimum occur. From the slope of the dashed yellow line, the phase velocity of SR-SPPs can be calculated, which is 0.22c.

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Determination of phase and group velocity of SR-SPPs.
    • Legends for movies S1 and S2

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.avi format). Nanofocusing movie experiment.
    • movie S2 (.mp4 format). Nanofocusing movie theory.

    movie files S1 and S2

    Files in this Data Supplement:

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