Optical Magnetic Multipolar Resonances in Large Dynamic Metamolecules

Abstract

Dynamic metamolecules (DMMs) are composed of a hydrogel dielectric core surrounded by randomly packed plasmonic nanobeads. The optical properties of DMMs can be tuned by controlling their core diameter using temperature variations. We have recently shown that DMMs display strong optical magnetism, including magnetic dipole and magnetic quadrupole resonances, offering significant potential for novel applications. Here, we use a T-matrix approach to characterize the magnetic multipole resonance modes of model metamolecules and explore their presence in experimental data. We show that high-order multipole resonances become prominent as the nanobead or the overall structure size is increased and when the interbead gap is decreased. In this limit, mode mixing among high-order magnetic multipole modes also becomes significant, particularly in the directional scattering spectra. We discuss trends in magnetic scattering observed in both experiments and simulations and provide suggestions for the experimental design and verification of high-order optical magnetic resonances using forward and backward scattering measurements. In addition, we show that the angular scattering of higher-order magnetic modes can display Fano-like interference patterns, which should also be experimentally detectable.

Heterogeneity in Dynamic Metamolecules

“…The strength of the magnetic dipole resonance compared to the broad electric dipole scattering is consistent with our previous reports in single-component h-MMs and originates from the fact that the electric dipole scattering is mostly due to locally coupled dipolar oscillations, while the magnetic dipole resonance is due to the collective behavior across the entire structure, significantly improving its quality factor. The lack of higher-order modes in mixed size systems is also consistent with our previous studies showing that MMs composed of densely packed large nanobeads exhibit prominent higher-order magnetic modes and their strengths increase with the size of metal beads. , The multiple-peak feature shown in densely packed 45 nm beads (Figure S8) indicates the presence of higher-order modes, which were absent in mixed systems (Figure f).…”

“…A strong magnetic dipole resonance emerges upon increasing the number of 45 nm beads to 25, and it becomes the dominant feature in the overall redshifted scattering and extinction cross-sections (Figure c,d, blue). Magnetic modes are expected to be enhanced by increasing the bead size and the number. ,, However, the distance between 45 nm beads in BMMs presented in Figure is too large to sustain strong magnetic resonances, even for the structure with the largest number of 45 nm beads. Indeed, it is notable that the addition of only a small number of large nanobeads is sufficient to produce a strong magnetic dipole resonance with a prominent far-field extinction that can be observed in ensemble-level experiments in solution.…”

“…Other simulation details were set based on our previous work on metamolecules with a single type of nanobead. 25,27 Basis decomposition of the simulated scattering spectra was used to obtain the contributions of each scattering multipole mode (electric dipole, electric quadrupole, and magnetic dipole) to the total scattering cross-section. More details of the simulation and structural parameters for simulation are provided in the Supporting Information, Figures S1 and 2, and Table S3 (1).…”

“…The lack of higherorder modes in mixed size systems is also consistent with our previous studies showing that MMs composed of densely packed large nanobeads exhibit prominent higher-order magnetic modes and their strengths increase with the size of metal beads. 25,27 The multiple-peak feature shown in densely packed 45 nm beads (Figure S8) indicates the presence of higher-order modes, which were absent in mixed systems (Figure 1f). Effect of Size Heterogeneity on Magnetic Resonances.…”

“…Recently, we have resolved the issue by adopting thermoresponsive poly­( N -isopropylacrylamide) (PNIPAM) hydrogels as the dielectric core . In this approach, dynamic metamolecules (MM) are prepared by attaching negatively charged metal nanoparticles on a positively charged PNIPAM core by electrostatic interactions, and the postsynthetic temperature control induces magnetic resonances. , An important advantage of the dynamic self-assembly approach is that any presynthesized nanoparticles can be used as building blocks to form tightly packed nanoparticle assemblies for strong coupling.…”