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

Abstract

The emergence of the vortex beam with orbital angular momentum (OAM) has provided intriguing possibilities to induce optical transitions beyond the framework of the electric dipole interaction. The uniqueness stems from the OAM transfer from light to material, as demonstrated in electronic transitions in atomic systems. In this study, we report on the OAM transfer to electrons in solid-state systems, which has been elusive to date. Using metamaterials (periodically textured metallic disks), we show that multipolar modes of the surface electromagnetic excitations (so-called spoof localized surface plasmons) are selectively induced by the terahertz vortex beam. Our results reveal selection rules governed by the conservation of the total angular momentum, which is confirmed by numerical simulations. The efficient transfer of light's OAM to elementary excitations in solid-state systems at room temperature opens up new possibilities of OAM manipulation.

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 n_g and n_out, 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. e_r and e_φ 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.