Researchers have investigated how the shape, aspect ratio, and orientation of non-spherical micro- and nanoparticles influence the optical forces and torques generated in paraxial trapping systems.
Study: Impacts of particle morphology and rotation on optical manipulation. Image Credit: l i g h t p o e t/Shutterstock.com
Using a line-shaped, circularly polarized 532 nm laser beam, they demonstrated that geometric asymmetry breaks the optical field's symmetry, producing counterintuitive negative optical torques that compete with positive torques and ultimately freeze particle rotation. This rotational equilibrium enables stable lateral drift and reversible optical forces, showing that particle geometry governs the transfer of energy and momentum in light fields.
Dynamics of Optical Tweezers in Light-Matter Interaction
Optical tweezers manipulate micro- and nanoscale objects by transferring momentum from light, generating gradient and scattering forces that enable stable trapping. In conventional single-beam optical tweezers, tightly focused laser beams draw dielectric particles toward the beam center. When the light is circularly polarized, it carries spin angular momentum that exerts a positive optical torque, causing non-spherical particles to rotate in the direction of the light's helicity.
However, under paraxial or slightly defocused conditions, the balance between axial radiation pressure and lateral gradient forces shifts particles away from the optical axis. This off-axis displacement creates asymmetries in the scattered electromagnetic field, allowing the particle geometry to alter the magnitude and direction of the optical torque, challenging the assumption that the optical torque is always positive.
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Experimental Setup and Computational Validation
To investigate how particle geometry and orientation influence optical forces and torques, researchers developed a custom optical manipulation system using a 532-nm, 500-mW laser. A pair of cylindrical lenses with focal lengths of 400 mm and 30 mm focused the beam into a line-shaped profile, simplifying the three-dimensional (3D) trapping problem into a two-dimensional (2D) planar system. Polystyrene microparticles with various geometries, including cylinders with different length-to-diameter ratios, triangular prisms, and irregular trapezoids, were suspended in deionized water inside a microfluidic chamber.
A quarter-wave plate switched the beam between right- and left-handed circular polarization, while particle motion was recorded using a 10× objective lens and a high-resolution camera. The experimental observations were validated using finite element method (FEM) simulations in COMSOL and finite-difference time-domain (FDTD) simulations in Ansys Lumerical. By positioning non-spherical particles away from the beam center, the study introduced spatial asymmetry to analyze off-axis optical forces and torques. Optical forces were calculated from the time-averaged Maxwell stress tensor, while scattered fields were expanded into vector spherical harmonic functions using a transition-matrix formulation.
Negative Optical Torques and Rotational Dynamics
The experiments showed that placing non-spherical particles slightly off the optical axis generated counterintuitive negative optical torques that opposed the incident light's spin angular momentum. At a paraxial offset of 200 nm, geometric asymmetry redistributed scattered-wave momentum, producing positive or negative optical torques depending on the particle's orientation. Unlike previously reported negative-torque mechanisms, this effect arose solely from particle morphology and off-axis positioning within a paraxial trapping field.
At specific orientation angles, positive and negative torques balanced to produce zero net torque, causing rotational freezing in which particles remained locked at stable equilibrium angles. Under left-handed circular polarization, stable orientations occurred above 90°, while right-handed circular polarization shifted the equilibrium below 90°. For example, a micro-cylinder measuring 6.2 μm in diameter and 11 μm in length stabilized at approximately 155° under left-handed polarization and about 50° under right-handed polarization.
Rotational freezing also converted the spin angular momentum of light into controlled lateral motion. The direction of particle drift depended on both aspect ratio and polarization handedness. Cylinders with small aspect ratios moved upward under right-handed circular polarization and downward under left-handed polarization, while high-aspect-ratio cylinders demonstrated the opposite behavior.
Applications in Optical Sorting and Manipulation
The coupling between particle morphology, rotation angle, and directional translation creates new opportunities for advanced optical manipulation and sorting. Because particles with different shapes and aspect ratios freeze at distinct equilibrium angles and drift in opposite directions, optical systems can separate heterogeneous microparticles based on morphology without relying solely on refractive index or chemical labeling.
These findings have implications in optofluidics, microfluidics, and light-driven microrobotics. Tailored optical fields could drive micro-rotors, manipulate flexible bioparticles, coordinate the assembly of micro-components, and enable controlled microfluidic stirring. This extends to soft-matter physics and biophysics, where geometry-dependent optical torques could improve the manipulation of non-spherical biological structures, including bacteria and red blood cells, while supporting cell stretching and high-precision optical torque sensing.
Future Directions in Optical Force Control
Particle morphology, aspect ratio, and spatial orientation are key factors governing optical forces and torques in paraxial light fields.
By showing that off-axis trapping generates negative optical torques and zero-torque rotational freezing, the researchers uncovered the mechanism responsible for predictable lateral drift. They addressed a long-standing question in light-matter momentum transfer.
Future work should extend these principles to structured-light systems, nanophotonics, and optomechanics while enabling advanced optical tweezers and automated optofluidic sorting platforms.
The ability to control morphology-dependent optical forces may also improve the manipulation of complex biological structures, including cells and proteins, across biomedical and physical sciences.
Journal References
Yi, W., et al. (2026). Impacts of particle morphology and rotation on optical manipulation. Light Sci Appl 15, 313. DOI: 10.1038/s41377-026-02403-5, https://www.nature.com/articles/s41377-026-02403-5
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