[Paper Review] Multipolar analysis of electric and magnetic modes excited by vector beams in core-satellite nano-structures

J. A. Parker, Stephen K. Gray+1 more|arXiv (Cornell University)|Nov 18, 2017

Plasmonic and Surface Plasmon Research25 references3 citations

TL;DR

This paper introduces a finite-difference time-domain (FDTD) multipolar analysis (ED-MA) method to characterize electric and magnetic multipolar modes in silver nanoparticle-decorated dielectric core-satellite nanostructures under vector beam excitation. It demonstrates that azimuthally polarized beams selectively excite and enhance magnetic dipole and quadrupole modes with high contrast over electric modes, while radially and shear-polarized beams selectively excite electric and higher-order quadrupolar modes, revealing new selection rules for metamaterial design.

ABSTRACT

Core-satellite structures are known to exhibit magnetic modes at optical frequencies and their characterization is important for the development of metamaterials and metafluids. We develop a finite-difference time-domain electrodynamics simulation and multipolar analysis approach and apply it to identify the electric and magnetic multipolar nature of modes excited in core-satellite structures composed of silver nanoparticles decorated on dielectric spheres. In addition to excitation with linearly polarized scalar beams, we investigate the scattering and multipolar properties induced by cylindrical vector beams. In contrast to linearly polarized beams, the nature of the polarization state in these beams (radial, azimuthal, or "shear") can selectively excite, enhance, and rotate a family of multipolar modes. Displacement currents induced in the nanoparticle gaps are investigated to better understand the nature of these excitations. We show that the efficiency of driving these modes depends on nanoparticle density and placement. We propose that selective magnetic and electric excitations can be codified as "selection rules" associated with the symmetries of the beams and particles.

Motivation & Objective

• To develop a rigorous electrodynamics-multipolar analysis (ED-MA) method combining FDTD simulations with multipolar field decomposition for characterizing optical modes in complex nanostructures.
• To investigate how cylindrical vector beams (azimuthal, radial, shear) selectively excite electric and magnetic multipolar modes in core-satellite Ag/SiO2 nanostructures.
• To identify and quantify the role of nanoparticle density and spatial arrangement in tuning scattering spectra and multipolar mode excitation.
• To establish new 'selection rules' linking beam symmetry and particle geometry to selective excitation of specific multipolar modes.
• To analyze angular scattering patterns and interference effects between dipolar and quadrupolar modes, particularly in backward and forward directions.

Proposed method

• Employing a finite-difference time-domain (FDTD) simulation framework with a spherical monitor centered on the nanostructure to collect scattered fields for multipolar decomposition.
• Applying a spherical multipole expansion up to 16 poles to decompose the scattered field into electric and magnetic multipoles (dipole, quadrupole, etc.) with high fidelity.
• Using the multipole decomposition to compute angular scattering intensity distributions and identify dominant modes under different beam polarizations.
• Analyzing displacement current distributions in nanoparticle gaps to link collective mode formation to electromagnetic coupling and retardation effects.
• Comparing excitation efficiency and mode contrast under linearly polarized, azimuthally polarized, radially polarized, and shear-polarized beams to identify symmetry-dependent selection rules.
• Investigating interference patterns between magnetic dipole and quadrupole modes, particularly the destructive interference in backward scattering for certain beam types.

Experimental results

Research questions

• RQ1 How do different cylindrical vector beams (azimuthal, radial, shear) affect the selective excitation of electric and magnetic multipolar modes in core-satellite nanostructures?
• RQ2 What is the role of nanoparticle density and spatial arrangement in determining the spectral position and intensity of multipolar resonances?
• RQ3 How do displacement currents in nanoparticle gaps contribute to the formation of collective magnetic modes?
• RQ4 What angular scattering patterns emerge from interference between magnetic dipole and quadrupole modes under various beam excitations?
• RQ5 Can symmetry-based selection rules be derived to predict which multipolar modes are preferentially excited by specific vector beam polarizations?

Key findings

• Azimuthally polarized beams selectively excite magnetic dipole and quadrupole modes with high contrast over electric modes, independent of retardation effects.
• Radially polarized beams selectively excite and enhance electric modes compared to linearly polarized excitation.
• Shear-polarized beams selectively excite and enhance electric and magnetic quadrupolar modes, with distinct m-state contributions (e.g., m = -2 and m = 2 for quadrupoles).
• Magnetic dipole modes driven by linearly polarized light arise from retardation and field circulation in the xz plane, while those from azimuthally polarized light result from the intrinsic polarization state and circulate in the xy plane.
• A strong destructive interference in backward scattering is observed due to spatial interference between magnetic dipole and quadrupole modes, particularly under linear and azimuthally polarized excitation.
• The multipolar analysis reveals that the magnetic quadrupole mode excited by azimuthally polarized light contains only the m = 0 component, while linearly polarized beams excite equal m = -1 and m = 1 components, and shear beams excite m = -2 and m = 2 components, confirming mode-specific excitation.