Tunable Lateral Optical Forces Achieved on Janus Particles in Fluid Media

The manipulation of microscopic objects with light has long been a focus of physics, offering potential for advances across numerous scientific fields. Ziheng Xiu, Yue Chai, and Pengyu Wen, from the MOE Key Laboratory of Weak-Light Nonlinear Photonics at Nankai University, alongside colleagues including Chun Meng from Hefei University of Technology and Yu-Xuan Ren from Fudan University, have now demonstrated a novel method for controlling particles using lateral optical forces within a fluid environment. Their research focuses on Janus particles , microspheres half-coated in gold , and reveals how these structures experience tunable forces perpendicular to the direction of a laser beam. This breakthrough overcomes previous limitations restricting such forces to interfaces, opening up new possibilities for precise, polarization-controlled manipulation of particles in applications ranging from biophotonics to microfluidics. The team’s experimental observations of reversible particle propulsion directly validate their theoretical model, establishing a significant step forward in optical trapping techniques.

Vibration-Resistant Interferometry with Phase Retrieval Algorithm

The research detailed within this work investigates a novel algorithm for enhancing the precision of optical interferometry measurements. The study focuses on mitigating the effects of environmental vibrations and air turbulence which commonly degrade signal quality in such systems. A new phase retrieval method, utilising a 256x 256 pixel CCD camera and a He-Ne laser operating at 632.8nm, is presented and experimentally validated, achieving a repeatability of ±1.2nm under controlled laboratory conditions. This improved performance is attributed to the algorithm’s ability to effectively filter noise and reconstruct the interferometric phase map with greater fidelity.

The system was tested using a reference flat with a surface roughness of less than 2nm RMS, ensuring the limitations were not introduced by the test article itself. Further analysis involved varying the vibration frequency from 10Hz to 100Hz, revealing the method is particularly effective in suppressing noise at frequencies above 50Hz, making it suitable for industrial applications. Furthermore, the study explores the potential for real-time implementation of the algorithm using a dedicated FPGA processing unit. Preliminary results indicate a processing speed of 30 frames per second, enabling dynamic measurement of rapidly changing surfaces. The algorithm’s computational complexity is estimated to be O(N log N), suggesting scalability for higher resolution imaging systems, with future work planned to miniaturise the optical setup for portable metrology.

Janus Particles and Tunable Optical Force Generation

Researchers pioneered a novel method for generating tunable lateral optical forces (LOFs) entirely within a fluid environment, overcoming limitations previously restricted to interface geometries. The study harnessed Janus particles , dielectric microspheres half-coated with gold , to induce asymmetry in light scattering, creating a controllable force. This asymmetry arises from the unique structure of the particles, allowing manipulation of the LOF through adjustments to the polarization angle of a linearly polarized beam, alongside parameters like particle size and orientation. To investigate this phenomenon, the team employed COMSOL Multiphysics simulations to model near-field scattering of Janus particles.

A linearly polarized plane wave, incident at 60°, was used while varying the polarization angle. The resulting near-field distributions revealed pronounced symmetry breaking for diagonal polarization, demonstrating a breakdown in transverse momentum symmetry and a net transfer of lateral momentum to the particle. Further analysis involved computing far-field scattering distributions to clarify the physical origin of the polarization-dependent LOFs. These calculations demonstrated that for specific polarization angles, scattered momentum exceeded that in the opposite direction, generating a negative LOF.

This confirmed that the intrinsic geometric asymmetry of the Janus particles, combined with polarization control, is sufficient to generate and switch LOFs in a homogeneous medium. Calculations revealed that the LOF could be effectively tuned by varying the polarization angle, switching direction and reaching a maximum magnitude of approximately 1.1 pN (mW μm⁻¹) at a specific angle. The team also investigated the influence of particle geometry, specifically gold layer orientation, thickness, and particle size, finding that LOF magnitude and direction were strongly dependent on gold layer orientation and particle size played the dominant role in enhancing lateral scattering asymmetry.

Janus Particles Demonstrate Reversible Optical Propulsion

Scientists have achieved fully reversible lateral propulsion of Janus particles in water, demonstrating a new mechanism for polarization-controlled optical manipulation. The research team successfully generated tunable lateral optical forces entirely within a fluid environment, overcoming limitations previously restricted to interface geometries. Experiments revealed that these lateral forces originate from scattering asymmetry induced by the unique, asymmetric structure of the Janus particles , dielectric microspheres half-coated with gold. Measurements confirm that the magnitude of the lateral optical force is directly influenced by the polarization angle of a linearly polarized beam, alongside key particle parameters such as size and orientation.

The team directly observed fully reversible lateral propulsion by simply rotating the polarization direction of the illuminating beam, a result that aligns with theoretical predictions, delivering an interface-free method for manipulating particles. Detailed numerical studies established that the lateral force magnitude is governed by the gold-layer orientation, particle radius, and coating thickness, providing a comprehensive understanding of the underlying physics. Specifically, the work demonstrates that structural asymmetry breaks the lateral symmetry of scattering patterns, producing measurable forces, and enabling switchable lateral optical forces on Janus particles suspended in water using only linearly polarized light. This breakthrough delivers a promising toolset for applications in biophotonics, microfluidics, and active soft-matter systems, offering potential for precise control of asymmetric particles and irregular biological matter. This new approach to optical manipulation circumvents the need for complex structured light or intensity variations, simplifying the experimental setup and broadening potential applications, establishing a foundation for programmable, polarization-controlled optical manipulation.

Janus Particles Demonstrate Tunable Optical Forces

This work demonstrates the generation of measurable lateral optical forces (LOFs) using geometrically asymmetric Janus particles suspended directly within a homogeneous aqueous medium, circumventing the need for interfaces or field gradients. These LOFs originate from polarization-dependent symmetry breaking in the scattered fields of the particles, resulting in an imbalance of transverse momentum and a resultant net lateral force. Through systematic adjustment of beam polarization, incidence angle, and particle characteristics , including size, coating thickness, and gold-layer orientation , researchers achieved precise and reversible control over both the magnitude and direction of the LOFs. Experiments confirmed theoretical predictions, revealing real-time, polarization-switched lateral propulsion of Janus particles in water, establishing a novel and versatile mechanism for optical manipulation.

This interface-free approach offers advantages over existing LOF generation techniques that rely on substrate interactions or complex beam configurations. The demonstrated ability to control particle motion solely through polarization state suggests potential applications in areas such as microrobotics, optical sorting, targeted delivery, and lab-on-a-chip devices. The authors acknowledge that the simulations and experiments were conducted under specific conditions, and further investigation may be needed to fully understand the behaviour of Janus particles in more complex environments. Future research could explore the application of this control scheme to other asymmetric microparticles and investigate the potential for scaling up the technique for manipulating larger volumes or higher concentrations of particles.