Optical tweezers, recognised by the 2018 Nobel Prize in Physics, employ tightly focused light to confine and manipulate microscopic objects through optical forces and torques. When the incident light carries spin angular momentum, it can induce rotation in non-spherical particles, a capability extensively exploited in microfluidic stirring, biological stretching, and quantum measurements. The conventional understanding holds that a shaped particle undergoes continuous rotation in the direction dictated by the light spin, while the influence of the particle's intrinsic morphology and orientation has remained largely underappreciated.
In a new paper published in Light: Science & Applications, a research team led by Professor Xinbin Cheng, Yuzhi Shi and Hongfei Jiao from Tongji University, China, in collaboration with Professor Cheng-Wei Qiu from the National University of Singapore and co-workers, establish that particle morphology and rotation angle constitute decisive determinants of optical torques and forces, particularly within paraxial or slightly defocused optical systems that are ubiquitous in practice yet frequently neglected.
The investigators demonstrate that a shaped particle displaced slightly from the beam axis experiences both positive and negative optical torques, the magnitude and sign of which depend on its rotation angle and dimensions. When these opposing torques attain equilibrium, particle rotation ceases and the particle instead drifts steadily in the lateral direction, a phenomenon the authors designate as rotational freezing accompanied by stable lateral drift. Significantly, this behaviour originates exclusively from the geometric asymmetry of an individual particle under paraxial trapping, a mechanism fundamentally distinct from previously reported origins of negative torque, including particle clusters, chiral objects, vortex beams, and phase-gradient fields.
To resolve these effects with clarity, the team devised a line-shaped spin light beam that reduces the three-dimensional trapping problem to a two-dimensional configuration, thereby converting orbital rotation into readily observable lateral motion. The researchers summarise the underlying principle of their methodology:
"We expose particles of diverse geometries, including long and short cylinders, triangles, trapezoids, and irregular morphologies, to a focused line-shaped beam. Rather than rotating continuously as conventional theory predicts, each particle rotates toward a specific stable angle, becomes rotationally frozen, and subsequently undergoes lateral translation. The direction of this drift reverses upon inverting the light polarization or modifying the particle's length-to-diameter ratio."
The team substantiated their theoretical predictions through comprehensive numerical simulations and experimental observations. In cylinder particles, stable angles below ninety degrees were obtained under right-handed circular polarisation, whereas those above ninety degrees emerged under left-handed polarisation, a trend that remained robust across variations in particle size, geometry, position, and refractive index.
"Irrespective of variations in particle size, rotation angle, position, refractive index, or the polarisation state of the light, rotational freezing and subsequent lateral motion are consistently observed. This therefore constitutes a generic phenomenon rather than a coincidental occurrence." the researchers note.
"Our work completes a hitherto missing building block in optical manipulation. By harnessing the pivotal roles of particle morphology and rotation, these reversible forces and torques may be exploited for shape-based optical sorting and binding, with substantial implications for biophysics, optofluidics, quantum sciences, and metaoptics." the researchers conclude.
