Nano-shaping of chiral photons

Abstract

Localized surface plasmon polaritons can confine the optical field to a single-nanometer-scale area, strongly enhancing the interaction between photons and molecules. Theoretically, the ultimate enhancement might be achieved by reducing the "photon size" to the molecular extinction cross-section. In addition, desired control of electronic transitions in molecules can be realized if the "photon shape" can be manipulated on a single-nanometer scale. By matching the photon shape with that of the molecular electron wavefunction, optically forbidden transitions can be induced efficiently and selectively, enabling various unconventional photoreactions. Here, we demonstrate the possibility of forming single-nanometer-scale, highly intense fields of optical vortices using designed plasmonic nanostructures. The orbital and spin angular momenta provided by a Laguerre-Gaussian beam are selectively transferred to the localized plasmons of a metal multimer structure and then confined into a nanogap. This plasmonic nano-vortex field is expected to fit the molecular electron orbital shape and spin with the corresponding angular momenta.

Keywords: Laguerre–Gaussian mode; localized surface plasmon; nano-vortex; optically forbidden electronic transition; plasmonic nanoantenna.

Figure 1:

Nano-shaping of photons. Conceptual illustration of plasmonic nano-vortex fields formed by metal multimer structures, where the orbital angular momentum (OAM) and spin angular momentum (SAM) are transferred from Laguerre–Gaussian (LG)-mode beams to localized plasmons and then confined into nanometer-sized gaps.

Figure 2:

Optical vortex fields in nano-gaps of plasmonic multimer structures. (A) Designed nanostructures. (a) Dimer, (b) trimer, (c) tetramer, and (d) pentamer are composed of gold triangle blocks with 150-nm sides, 5-nm corner curvatures, and 30-nm thicknesses. The gap size is 10 nm. The surrounding medium is air. (B) Illumination beam profiles (average intensity and instantaneous electric field). (a) Linearly polarized Gaussian beam LP-G (l = 0, s = 0), (b) circularly polarized Gaussian beam CP-G (l = 0, s = 1), (c) linearly polarized Laguerre–Gaussian beam LP-LG (l = 0, s = ±1), and (d) circularly polarized Laguerre–Gaussian beam CP-LG (l = 1, s = 1), that are irradiated on the multimer structures (Aa–Ad), respectively. Red arrows represent electric field vectors, which linearly oscillate and circularly rotate in time for linear and circular polarizations, respectively. In the case of LG beams, the directions of the electric field vectors depend on the azimuthal angle. The grey shadings indicate the electric field intensity distribution. The white bar indicates a scale of 1 µm. (C) Near-field enhancement spectra of the plasmonic multimers (Aa–Ad) excited by the G and LG beams (Ba–Bd), respectively. The near-field spectra are obtained at a 1-nm distance from the top corner of a metallic vertex facing the gap; the intensities are normalized by the incident fields. (D) Instantaneous electric field distributions within the multimer gaps. The wavelengths of the incident beams are set at the resonant peaks of corresponding near-field spectra (Ca–Cd). Yellow arrows represent electric field vectors, and the grey shadings indicate the intensity distribution. The motions of the electric field vector are shown in Supplementary Movies S1–S6. The white bar indicates a scale of 1 nm.

Figure 3:

Electric field model created in a gap surrounded by N triangular metal blocks.

Figure 4:

Interaction between nano-vortex fields and a dimer molecule. (A) Numerical model of the dimer molecule. (B, C) Absorption cross section spectra. (B) Molecule without plasmonic nanostructures with CP-G beam illumination. The broken and solid curves are the spectrum of monomer and dimer molecules, respectively. (C) Dimer molecule that is placed at the center of the nano-gap in the triangle pentamer. The blue and red curves are the spectra of the G beam and LG-mode excitation, respectively.