Transfer of orbital angular momentum of light to plasmonic excitations in metamaterials

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Science Advances  12 Jun 2020:
Vol. 6, no. 24, eaay1977
DOI: 10.1126/sciadv.aay1977
  • Fig. 1 Metamaterial structure for OAM transfer.

    (A) Schematic view with the following structural parameters: inner radius (r), outer radius (R), periodicity (d), groove width (a), and number of grooves (N). The refractive indices inside the groove and outside the disk are given by ng and nout, respectively. (B) Optical image of the sample made of gold (r = 70 μm, R = 100 μm, N = 30, and a/d = 0.4). The thickness is around 100 nm. Chromium (10 nm thick) is deposited under the gold as an adhesion layer.

  • Fig. 2 Selective excitation of multipole spoof LSPs.

    Selected snapshots of the near-field evolution around the sample excited by (A) Gaussian beam, (C) vortex beam (OAM +ħ), and (E) vortex beam (OAM −2ħ). The double circle represents the position of the sample (inner and outer radius). The time origin (0 ps) is the time when the first positive peak of the incident pulse comes. The color scales are optimized at each frame for the sake of clarity. (B, D, and F) The electric field taken along the outer circle of the sample as a function of the azimuthal angle φ (red curves). The error bars are almost the same as the thickness of the traces. The dashed cosine curves are expected electric field patterns when the modes depicted on the right are excited. The solid arrows schematically represent the quasi-static electric field around each mode. The cosine functions are obtained by projecting the quasi-static field onto the polarization axis (e0, dashed up arrow) detected in the experiment. er and eφ are cylindrical unit vectors introduced to calculate quasi-static fields (see Materials and Methods). a.u., arbitrary units.

  • Fig. 3 Mode decomposition of near-field distributions.

    Frequency spectra of the dipole [E(±2, f)], quadrupole [E(±3, f)], and hexapole [E(±4, f)] modes excited in the sample illuminated by (A) Gaussian beam, (B) vortex beam (+ħ), and (C) vortex beam (−2ħ). (D) Dispersion relation of the spoof LSP. The red dots represent the resonance frequencies determined in (A) to (C). The blue curve is a theoretical fitting.

  • Table 1 Selection rules.

    The orbital (first row), spin (second row), and total (third row) angular momentums of the excitation beams are summarized. The last row shows the angular momentums of the spoof LSPs observed in the experiment.

    Vortex beamVortex beam
    SAM+ħ and −ħ+ħ and −ħ+ħ and −ħ
    TAM+ħ and −ħ+2ħ0−3ħħ
    (+ħ and −ħ)

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