Raspberry-like metamolecules (RMMs), clusters of closely packed noble metal nanobeads self-assembled on a dielectric core, exhibit emergent optical properties not available in simple nanoparticles. Examples include broad band far-field extinction and artificial optical magnetism. An important feature of these clusters is that their magnetic plasmon resonance and the breadth of their extinction spectra can be tuned via simple synthetic routes, such as by changing the bead size, core size, or the average interbead distance. However, the effect of each of these variables on the final magnetic resonance frequency and strength has not been studied in depth. Understanding how to tune the electric and magnetic resonance modes in these clusters can help improve the design of novel metamaterials for various applications. In this article, we combine theoretical analyses using numerical finite-different time-domain modeling and analytical dipole–dipole coupling theory to study the role of these variables in the global electric and global magnetic dipole modes of RMMs. We also demonstrate that these variables can be readily controlled experimentally using surfactants with varying lengths or changing synthetic conditions and show that the experimental results are consistent with theoretical predictions. The results provide a guideline for synthesizing plasmonic nanoparticle assemblies when specific resonant frequencies and bandwidths are desired.

Here, we report plasmonic metamolecules with dynamically controllable optical magnetism. A dynamic metamolecule (DMM) is constructed by decorating gold or silver nanobeads on a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel sphere, which generates uniform coresatellite-type assembly structures with an interbead distance, allowing for strong interparticle coupling. Experimental and simulation results revealed strong magnetic dipole and quadrupole modes observable in the far field both for gold and silver DMMs when the temperature was set above the lower critical solution temperature (LCST) of PNIPAM. Interestingly, gold DMMs showed stronger and more pronounced magnetic resonances than silver DMMs, despite the general notion that silver nanostructures possess superior plasmonic properties. The strong magnetic coupling and structural uniformity along with the ability to dynamically control the assembly structure allowed us to probe distinct optical magnetism in gold and silver and experimentally observe magnetic quadrupole in solution-phase metamolecules for the first time.

Interfacial interactions can play an important role in the glass transition temperature (Tg) and relaxation dynamics of ultra-thin glass polymer films. We have recently shown that similar to the polymeric systems in ultra-thin molecular glass films of N, N′-Bis(3-methylphenyl)-N, N′-diphenylbenzidine (TPD), the Tg is reduced and dynamics are enhanced. Furthermore, in molecular glass systems, as the film thickness is reduced below ∼30 nm, the dynamics at the two interfaces correlate such that the range of the gradients in the dynamics induced by the free surface narrows compared to thicker films. These observations indicate that the dynamics of the glassy thin films are strongly correlated and cannot be explained by a simple two-layer model consisting of a bulk and a thin interfacial layer with a constant thickness and constant range of dynamical gradients. Here, we investigate the effect of film/substrate interactions on the film dynamics by varying the TPD/substrate interfacial interactions. We show that thin TPD films with thicknesses below ∼60 nm show a smaller extent of Tg reduction and enhanced dynamics when supported on a near-neutral substrate (wetting) compared to a weakly interacting (dewetting) substrate. However, the ∼30 nm length scale, where the activation energy significantly reduces from its bulk value as measured by the onset of the glass transition remains unchanged. Coarse-grained molecular dynamics simulation also shows a narrowing in the range of relaxation times once the thickness is sufficiently reduced for the two interfaces to dynamically correlate, consistent with previous work. These results suggest that the length-scale for the correlated dynamics is independent of interfacial interactions and the polymeric nature of the film and may originate from the bulk glass properties.

Stable glasses are formed during physical vapor deposition (PVD), through the surface-mediated equilibration process. Understanding surface relaxation dynamics is important in understanding the details of this process. Direct measurements of the surface relaxation times in molecular glass systems are challenging. As such, surface diffusion measurements have been used in the past as a proxy for the surface relaxation process. In this study, we show that the absence of enhanced surface diffusion is not a reliable predictor of reduced ability to produce stable glasses. To demonstrate, we have prepared stable glasses (SGs) from two structurally similar organic molecules, 1,3-bis­(1-naphthyl)-5-(2-naphthyl)­benzene (TNB) and 9-(3,5-di­(naphthalen-1-yl)­phenyl)­anthracene (α,α-A), with similar density increase and improved kinetic stability as compared to their liquid-quenched (LQ) counterparts. The surface diffusion values of these glasses were measured both in the LQ and SG states below their glass transition temperatures (T gs) using gold nanorod probes. While TNB shows enhanced surface diffusion in both SG and LQ states, no significant surface T g diffusion is observed on the surface of α,α-A within our experimental time scales. However, isothermal dewetting experiments on ultrathin films of both molecules below Tg indicate the existence of enhanced dynamics in ultrathin films for both molecules, indirectly showing the existence of an enhanced mobile surface layer. Both films produce stable glasses, which is another indication for the existence of the mobile surface layer. Our results suggest that lateral surface diffusion may not be a good proxy for enhanced surface relaxation dynamics required to produce stable glasses, and thus, other types of measurements to directly probe the surface relaxation times may be necessary.

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.