Researchers at the University of Science and Technology of China have made significant strides in optical manipulation, showcasing how controlled light can precisely apply three-dimensional (3D) optical torque via vectorial spin angular momentum (SAM) transfer. This groundbreaking study, published on January 16, 2025, highlights the ability to manipulate the rotation of microparticles trapped by laser beams, opening doors for advanced applications in fields such as biophysics and micro-engineering.
The ability to rotate micro-objects under light is not just a neat trick; it holds enormous potential for technologies ranging from optical tweezers used to study biological samples to creating highly sensitive sensors. Typically, controlling 3D rotations around arbitrary axes has proved challenging, often requiring complex setups. This research simplifies the challenge by using a single beam of light, demonstrating full control over the optical torque acting on birefringent microparticles.
To achieve this, the researchers developed theoretical frameworks relating the 3D SAM vector of tightly focused laser beams to the local polarization helicity of the incident light. At the core of their method is the use of light modified with what they termed 'asymptotic spiral phase,' which allows the generation of specific light fields capable of achieving desired SAM manipulations. The practical application of this involves utilizing optical traps—commonly executed with high numerical aperture lenses—to hold and mechanically rotate particles with precision.
Through simulations and empirical demonstrations, the researchers showcased the manipulation of cubic calcite microparticles, illustrating how these particles can be dynamically rotated about various axes simply by altering the light's phase patterns. "Our work provides new perspectives for light-induced torque and 3D rotation of microscopic objects with great potential in optomechanical and microfluidic applications," the authors stated, underlining the transformative capability of their findings.
The significance of these advancements extends to possible applications involving living biological cells, as the controlled 3D rotations demonstrated with non-living microparticles suggest similar techniques could be employed with biological samples. This is particularly promising for scientific inquiries aimed at exploring cellular biomechanics or probing the mechanical properties of biomolecules through techniques like optical torque wrenches.
The controlling factor of this method—the vectorial transfer of SAM—offers unprecedented precision. The researchers are optimistic about its prospects: "The ability to control 3D SAM vector with arbitrary orientation lays the foundation for vectorial SAM transfer to microparticles," they explained. This innovation can potentially usher new techniques for controlling particle dynamics within optical traps, allowing researchers to effectively study material and biological systems on very small scales.
Overall, the simplicity and effectiveness of manipulating 3D optical torque not only enhances light manipulation capabilities but also sets the stage for developing new functionalities and applications of optical tweezers, making this research pivotal for future advancements. By using just one focused beam, the research presents opportunities to apply time-varying optical spin torque broadly across multiple disciplines – from enhancing imaging techniques to improving the usability of microfluidic devices.
These findings herald exciting new possibilities for research and applications centered around optical communication, advanced microscopy, and even the development of micro-machines actuated by light. Scientists expect the new method to be instrumental for applications aimed at detailed investigations of materials, potentially revolutionizing how microscale movements and interactions are observed and manipulated under light.
Future work may focus on exploring the limits of this technique, including the manipulation of more complex biological systems, promising even greater innovation and discovery fueled by the power of light.