Rotational Jamming of Plasmonic Optical Matter Driven by Chiral Light

TL;DR

The study uncovers a rotational jamming transition in plasmonic optical matter driven by spin angular momentum of a focused chiral beam. By combining experiments on 3–8 particle Au nanoparticle assemblies with symmetry-aware static torque calculations via generalized multiparticle Mie theory and dynamic EDLD simulations, it demonstrates that stable rotation requires hexagonal or triangular symmetry, while adding defects disrupts symmetry and arrests rotation in a reversible manner. The work links structural order to torque transfer and reveals symmetry as a controllable parameter for switching OM between rotating and non-rotating states, offering a route toward reconfigurable light-driven micro-machines and soft-robotic applications. It also highlights limitations of 2D modeling and points to future 3D and material-diversity explorations to refine the mechanistic picture of jamming in optical matter.

Abstract

Plasmonic Optical matter (OM), composed of optically bound metallic particles, can be rotated by transferring the spin angular momentum (SAM) of chiral light to the assembly. Rotating OM is a promising platform for optical micromachines, with potential applications in plasmofluidics and soft robotics. Understanding the dynamic states of such Brownian, micro-mechanical systems is a relevant issue. One key problem is understanding kinetic jamming and clogging. Studies of driven multiparticle systems have revealed that under suboptimal driving, the systems can stop moving, showing jamming transitions. It is important to identify dynamic regimes where crowding competes with driving and is susceptible to jamming in the context of optical micromachines. Through experiments supported by numerical simulations, we reveal assemblies with well-defined hexagonal or triangular symmetry that efficiently harness the SAM of incident chiral light, resulting in stable rotation. However, as the plasmonic-particle assembly grows and its dimensions approach the beam waist, new particles can disrupt this order. This causes a transition to a fluid-like state with less-defined symmetry, correlated with a significant reduction in transferred torque, causing rotation to stagnate or cease. We suggest this behaviour is analogous to a rotational jamming transition, where the rotational motion is arrested. Our findings establish a clear relationship between the structural symmetry of the OM assembly and its ability to harness SAM, providing new insights into controlling chiral light-matter interactions and offering a novel platform for studying jamming transitions.

Figures (6)

• Figure 1: Arresting the rotation of the optically bound assembly. a) A schematic figure shows that when circularly polarised light is used to form and rotate Optical matter composed of 3 particles, it rotates stably. Additionally, the assembly has a triangular structure. However, with the addition of one more particle, the assembly loses its triangular symmetry and becomes a fluctuating square. The four-particle assembly also stops rotating. b) The time series of experimental data shows the rotation of the three particle assembly until the addition of a fourth particle. One of the particles is marked by a red circle to improve visualisation. After the fourth particle leaves the assembly due to thermal noise, the assembly starts rotating again.
• Figure 2: Evolution of optical matter from 3 to 4 particles. a) Snapshots of three-particle (3P) and four-particle (4P) assemblies corresponding to the supplementary video 1. This assembly is created with a 1.2 NA objective, creating a tighter spot. The assembly fluctuates from three to four particles intermittently. b) Angular trajectories of particles are shown, showing rotation by an increase in the angles. The 4P assembly times are highlighted in grey. The rotation stagnates whenever the 4th particle joins the assembly, distorting the triangular symmetry. c) An order parameter that quantifies the triangular nature of the assembly is plotted in blue for the whole time. It can be seen that the order parameter drops sharply as the fourth particle joins the assembly. The red line with the y-axis on the right side shows the instantaneous number of particles in the assembly.
• Figure 3: Evolution of optical matter from 7 to 8 particles. a) Snapshots of seven-particle (7P) and eight-particle (8P) assemblies corresponding to the supplementary video 2. b) Mean angular rotation of the assembly plotted as a function of time. The transition boundary is marked with a change in colour from grey to white. The rotation of the assembly ceases dramatically upon the addition of the 8th particle. Insets show the position of one peripheral particle in the assembly in Cartesian coordinates for both cases. This again illustrates the rotation before the transition and the stochastic motion that follows. c) The instantaneous local hexagonal order parameter (HOP) of the assembly is plotted for the whole duration of the video. The assembly transitions from 7P to 8P at 5.7 seconds. The 7P assembly is comparatively more hexagonally ordered than the 8P assembly. The distribution of the HOP for both halves is plotted in the adjacent plot.
• Figure 4: Evolution and rotation of optical matter from three to eight particles in focused beams. 400 nm AuNPs are assembled with a 1064 nm laser. Initially, the particles assemble in a hexagonal symmetry up to 7 particles. After seven particles, the hexagonal assembly becomes unstable, and some states begin to emerge with square packing symmetry.
• Figure 5: Symmetry-dependent electromagnetic interaction and force. a) The scattering cross-section of optical matter assemblies is studied as a function of the assembly symmetry, specifically comparing a hexagonally-packed and a square-packed array. b) The force on particles for 3P and 4P assemblies with different symmetries from experimental data is calculated using GMMT-based simulations. c) The force on particles for 7P and 8P assemblies. The particle arrangement corresponds to representative positions with different symmetries as shown in Figure \ref{['evolution']}b. d) and e) show the total torque on the assemblies as a heatmap for different numbers of particles with hexagonal and square symmetry, respectively. The torque is calculated for different particle separations. The torque for hexagonal arrays with 800 nm separation increases with the number of particles. Meanwhile, the torques are mostly small and negative for a square array.