Chinese University of Science and Technology Reveals a New Physical Mechanism of Photoinduced Particle Rotation

Light has angular momentum properties. Circularly polarized or elliptically polarized beams carry spin angular momentum (SAM), while beams with helical phase wavefronts carry orbital angular momentum (OAM). During the interaction between light and particles, the transfer of angular momentum can generate optical torque, driving particles to rotate. Among them, the transfer of optical spin angular momentum will drive particles to spin around the axis of rotation, while the transfer of orbital angular momentum can drive particles to rotate around the optical axis. Photoinduced rotation provides a new dimension for micro particle manipulation and has been widely applied in fields such as optical sensing, optorheology, and microrobots.

Recently, Associate Professor Gong Lei's research group from the Department of Optics and Optical Engineering at the University of Science and Technology of China collaborated with Professor Qiu Chengwei from the National University of Singapore to reveal a new physical mechanism of photo induced particle spin. It was found that even if the incident beam does not carry spin angular momentum, it can generate controllable spin torque after strong focusing. This mechanism utilizes the optical Hall effect to achieve local transfer of spin angular momentum in the focusing field by regulating the spin orbit interaction, thereby driving the captured particles to generate continuous spin motion.

Figure 1. Schematic diagram of the physical mechanism of photo induced particle spin

The relevant research results were published online on June 21st in the internationally renowned academic journal Physical Review Letters under the title "Controllable Microparticle Spinning via Light without Spin Angular Momentum".

Due to the spin orbit interaction, the two spin components of a linearly polarized or radially polarized beam will undergo lateral separation under tight focusing conditions, which is a type of optical spin Hall effect [Figure 1. (a, b)]. However, the spacing of this spin splitting is only on the subwavelength level, and it cannot effectively transfer spin angular momentum when interacting with particles, and cannot drive particle spin [Figure 1. (d, e)]. The research team cleverly uses the optical orbit Hall effect to regulate the distribution of spin angular momentum density in the focusing field. By introducing an orbital angular momentum superposition state in the incident radially polarized light field [Figure 1. (c)], the radial spacing of the two spin components is effectively controlled, achieving the effect of spin angular momentum in the focusing field on microscopic particles. Local transmission ultimately achieved controllable rotation control of particles [Figure 1. (f)].

On this basis, the research team further developed the parallel manipulation function of holographic optical tweezers, which achieved simultaneous capture of multiple particles, independent translation and rotation manipulation by adjusting the wavefront of the incident light field. This study reveals the principle of orbital angular momentum controlling the spin of the focused light field, and provides new ideas for the study of mechanical effects caused by optical spin orbit interactions.

Dr. Wu Yijing from the Department of Optics and Optical Engineering at the University of Science and Technology of China is the first author of the paper, while Associate Professor Gong Lei and Professor Qiu Chengwei from the National University of Singapore are the corresponding authors of the paper. The above research has been supported by the National Natural Science Foundation of China and the Anhui Provincial Natural Science Foundation.

Source: Guangxing Tianxia