Rotational Jamming of Optical Matter Driven by Chiral Light (2511.10170v1)

Abstract: 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.

• The paper demonstrates that maintaining triangular or hexagonal symmetry in plasmonic nanoparticle assemblies is critical for sustaining rotational motion under chiral light.
• Experimental evidence and EDLD simulations reveal that adding a particle beyond a symmetry threshold triggers a jamming transition that halts rotation.
• The study outlines a framework for engineering reconfigurable optical micromachines by modulating particle number, structural order, and laser beam parameters.

Rotational Jamming in Optically Bound Plasmonic Assemblies Driven by Chiral Light

Introduction and Motivation

The paper examines the collective rotational dynamics of optically bound plasmonic nanoparticle assemblies, referred to as "optical matter" (OM), when irradiated with focused, chiral (circularly polarized) laser beams. While previous research has extensively characterized both the optical binding and the ability of light’s spin angular momentum (SAM) to induce rotation in small particle systems, this work addresses a critical knowledge gap: the interplay between structural symmetry, particle number, and the onset of jamming-like transitions in the torques that drive collective rotation. The relevance of these observations extends to the design and control of micromachines, active matter, and mesoscale kinetic arrest phenomena.

Experimental Evidence for Rotational Jamming

Rotational Arrest in Small Assemblies

The experimental focus is initially on assemblies of 400 nm Au nanoparticles confined in two dimensions by a 1064 nm, circularly polarized laser beam tightly focused by a high-NA objective. Assemblies of three particles spontaneously organize into a stable, triangular configuration and exhibit continuous, unidirectional rotation that tracks photon SAM transfer. When a fourth particle is introduced, the assembly’s symmetry is lost—transitioning into a fluctuating square—and the rotation is abruptly arrested.

Figure 1: Addition of a fourth particle to a rotating three-particle assembly disrupts triangular symmetry and immediately halts rotational motion; the effect is reversible upon removal of the defect particle.

Particle tracking and analysis reveal that the angular displacement of particles ceases precisely when the fourth particle joins. This correlation is quantified using a threefold structural order parameter, which sharply decreases during the jammed state.

Figure 2: Order parameter and angular trajectories confirm that rotational jamming coincides with disruption of triangular symmetry by the fourth particle.

Larger Assemblies and Effect of Beam Size

The paper is extended to larger assemblies using a wider beam (lower NA), allowing up to eight particles to cluster. A dynamic phase transition is observed: while the seven-particle (7P) assembly maintains a stable hexagonal order and rotates efficiently, the addition of an eighth particle (8P) induces structural disorder and rotational arrest. The local hexagonal order parameter drops from values near unity (hexagonal) to ~0.4 (disordered), consistent with a "fluid-like" structural regime.

Figure 3: Transition from a rotating, hexagonal 7P assembly to a stalled, disordered 8P assembly, tracked in angular position and quantified via the local hexagonal order parameter.

This demonstrates that the critical number for rotational jamming depends on beam size and the corresponding packing density, with the system reliably transitioning between order and disorder as particle number increases past a threshold.

Structure-Rotation Interdependence and Phase Behavior

Direct imaging of the evolving assemblies reveals a clear sequence: up to seven particles, the assembly forms a well-ordered, compact hexagon; with increasing size, structural defects proliferate, and non-hexagonal (often square) packings emerge. The rotational degree of freedom is suppressed in these disordered/fluid-like assemblies.

Figure 4: Progressive breakdown of rotational order, with square symmetries supplanting hexagonal symmetry as assembly size increases beyond seven particles.

This symmetry-dependent behavior reflects fundamental light-matter coupling rules: only configurations with discrete angular order allow efficient, constructive transfer of angular momentum from the optical field to the assembly.

Computational Analysis: Electrodynamic Simulations

To elucidate the mechanism of rotational jamming, static generalized Mie theory (GMMT) simulations quantify both the scattering cross section and the net electromagnetic torque experienced by assemblies of different symmetry classes (triangular/hexagonal vs. square).

• Hexagonal and triangular assemblies experience substantial positive torque under right-circularly polarized light, with magnitude increasing monotonically with particle number (e.g., 0.768 pN·μm for 3P, 4.7 pN·μm for 7P, 6.0 pN·μm for 8P hexagonal).
• Square and disordered arrangements generate near-zero or even negative torque, with force vectors predominantly pointing to the assembly center or producing opposing (clockwise) torques.

  Figure 5: Calculated net forces and torques reveal that only hexagonal/triangular symmetry yields significant, positively directed torque, while square symmetric assemblies exhibit near-zero or negative torque.

Torque heatmaps across a range of lattice spacings and particle numbers reinforce this point: positive torque is sustained only for hexagonal symmetry at the theoretical optical binding distance, while square arrangements either show diminished or negative torques.

Dynamic Simulations: Electrodynamic-Langevin Modeling

Stochastic, time-resolved Electrodynamic-Langevin Dynamics (EDLD) simulations are used to model the assembly evolution and dynamically probe stability vs. disorder. These simulations incorporate both deterministic optical forces (from GMMT) and stochastic Brownian noise.

• 3P triangular assemblies consistently show persistent, rapid rotation; when a 4P square initial configuration is imposed, it rapidly degenerates into a non-rotating, lower-symmetry state.
• 7P hexagonal assemblies rotate robustly; 8P assemblies with non-hexagonal (square or circular) initial symmetry stall, confirming the experimental phase boundary.

  Figure 6: EDLD simulation tracks corroborate experiment—only highly symmetric assemblies rotate persistently; square/circular 8P assemblies are unstable and do not support collective rotation.

Changing beam waist or laser power within the simulations scales the angular velocity and stable orbital radius, further recapitulating the observed dependence of jamming on confinement and intensity.

Quantitative Highlights and Contradictory Claims

Key numerical results:

• Torque in hexagonal 7P/8P assemblies: $4.7$–$6.0$ pN·μm; Torque in 8P square/circular assemblies: $-0.17$–$-1.37$ pN·μm.
• Hexagonal order parameter $\psi_6$ transitions from $\approx 1$ (ordered) to $\approx 0.4$ (jammed/fluid-like) at the jamming threshold. Crucially, the paper demonstrates that increased particle number—commonly presumed to enhance mechanical response—can actually suppress rotation if symmetry is compromised, overturning naive expectations about the scaling of collective optical torque.

Implications and Outlook

Theoretical:

The results provide a rare mesoscopic realization of kinetic arrest (jamming) in an active, light-driven system, with close parallels to classical jamming in granular and colloidal matter but uniquely controlled via photonic boundary and symmetry. The direct correspondence between angular symmetry class and rotational motion validity affirms that light-matter angular momentum transfer is strictly symmetry-protected.

Practical:

This work establishes symmetry as a practical control knob for the operation of optical micromachines and rotors. It suggests new paradigms for reconfigurable, light-programmable microactuators, fluidic pumps, or systems for cargo sorting, where function can be dynamically and reversibly switched by modulating particle number, symmetry, or field profile.

Future Directions:

• Exploration of jamming and kinetic arrest in larger, polydisperse, or more complex chiral assemblies.
• Systematic paper of out-of-plane fluctuations and 3D traps, potentially extending the discovered phenomena to volume-confined architectures.
• Integration with optothermal and electrokinetic control modalities for advanced soft robotics, active matter, and non-equilibrium statistical mechanics.

Conclusion

This paper conclusively demonstrates a rotational jamming transition in optically bound plasmonic assemblies driven by focused chiral light. The onset of this kinetic arrest is finely and reversibly controlled by the symmetry and number of particles, mediated by the nontrivial dependence of collective optical torque on angular order. The results unify the physics of driven colloidal assemblies, soft matter jamming, and angular momentum transfer at the mesoscale, with implications for the rational engineering of photo-actuated microdevices and the fundamental non-equilibrium dynamics of active matter.