Self-Organization of Metal Nanoparticles in Light: Electrodynamics–Molecular Dynamics Simulations and Optical Binding Experiments

Abstract

Light-driven self-organization of metal nanoparticles (NPs) can lead to unique optical matter systems, yet simulation of such self-organization (i.e., optical binding) is a complex computational problem that increases nonlinearly with system size. Here we show that a combined electrodynamics-molecular dynamics simulation technique can simulate the trajectories and predict stable configurations of silver NPs in optical fields. The simulated dynamic equilibrium of a two-NP system matches the probability density of oscillations for two optically bound NPs obtained experimentally. The predicted stable configurations for up to eight NPs are further compared to experimental observations of silver NP clusters formed by optical binding in a Bessel beam. All configurations are confirmed to form in real systems, including pentagonal clusters with five-fold symmetry. Our combined simulations and experiments have revealed a diverse optical matter system formed by anisotropic optical binding interactions, providing a new strategy to discover artificial materials.

Silver‐Nanowire‐Based Interferometric Optical Tweezers for Enhanced Optical Trapping and Binding of Nanoparticles

“…Different from the near‐field hotspots generated in a plasmonic nanogap or rough surface, what we observed here clearly reveal that the electromagnetic hotspots created by plasmonic scattering can generate strong spatial confinement hundreds of nanometers away from the metallic structure, which may enable new types of noncontact plasmon‐enhanced spectroscopies. These hotspots are similar to the ones generated by 1D optical matter chains, where localized electromagnetic hotspots via coherent scattering from Ag NPs allow cotrapping of less polarizable particles (e.g., semiconductor quantum dots) in an optical line trap.…”

“…Figure b shows a single, two, and three Ag NPs that are trapped by the hotspots when the light polarization is parallel to the nanowire (Video S3, Supporting Information). Note that optically bound NP dimers or trimers prefer to align perpendicular to the light polarization direction due to the anisotropic optical binding interactions, but here they align parallel to the polarization due to the plasmonic confinement by the nanowire. The corresponding probability density distributions of the NPs along ( x ) and perpendicular ( y ) to the nanowire are shown in Figure c,d, respectively.…”

“…Optical binding forces are interparticle forces arising from electrodynamic interactions between two or more polarizable particles illuminated by light, which lead to self‐arrangement of the particles into ordered structures (i.e., optical matter) . For example, plasmonic NPs can self‐assemble into optical matter chains and arrays with sub‐micrometer interparticle separations and high precision, but generally only a few NPs can form stable structures while an assembly of many NPs becomes unstable. Very recently, we found that a large number of Ag NPs illuminated by a linearly polarized laser beam could self‐assemble into partially ordered arrays, but they exhibited frequent structure transition among dimer chains, hexagonal NP lattice, and disordered structures .…”

“…This dynamic behavior of light‐induced self‐assembly limits the fabrication of large‐scale optical matter with NPs. So far stable NP chains or arrays have only been made with fewer than 20 NPs by optical binding …”

Light‐Driven Self‐Healing of Nanoparticle‐Based Metamolecules

“…Light‐induced self‐organization of colloidal nanoparticles can lead to various equilibrium configurations with the same number ( N ) of nanoparticles . Typical isomers of optical‐matter structures are shown in Figure , but only one structure for a certain N value has self‐healing ability (marked by stars).…”