Since the advent of optical trapping technology in 1986, it has had a profound impact on various fields, including biomedicine, materials engineering, and colloidal interactions. Traditional optical tweezers are primarily designed to manipulate high refractive index (HRI) particles, which tend to move toward regions of high light intensity. Thus, they are typically used to trap "light-seeking particles" such as those near the focus of a fundamental Gaussian beam. However, challenges arise when attempting to trap and manipulate low refractive index (LRI) or light-absorbing particles, as these particles are often repelled by strong light, exhibiting "dark-seeking" characteristics. To trap LRI particles, special "dark traps" must be constructed, but conventional optical tweezers platforms struggle to simultaneously achieve stable and reliable bright and dark traps, thereby hindering the concurrent capture and manipulation of HRI and LRI particles.
To overcome these limitations, Professor Jianping Ding's research group at Nanjing University developed an innovative dual-curvilinear optical vortex beam (DC-OVB) technique aimed at the simultaneous and flexible manipulation of LRI and HRI particles. The DC-OVB technique generates two overlapping curved optical vortex beams, where the bright curve controls HRI particles, and the dark channel can manipulate LRI particles. The novelty of this method lies in its dual-curve structure and the distribution of orbital flow density, which provides customized movement paths and local movement speeds for both types of particles. Relevant research results were recently published in Photonics Research, Volume 12, Issue 7, 2024. [Zheng Yuan, Chenchen Zhang, Yuan Gao, Wenxiang Yan, Xian Long, Zhi-Cheng Ren, Xi-Lin Wang, Jianping Ding, Hui-Tian Wang, "Dual-curvilinear beam enabled tunable manipulation of high- and low-refractive-index particles," Photonics Res. 12, 1427 (2024)]
Under the theory of tight focusing, the researchers designed orbital flow densities distributed along curves to provide orbital motion and exert torque on particles along different paths. In experiments, they successfully used circular, elliptical, and quadrilateral trajectories to achieve precise movement and control of both HRI and LRI particles, including the rotational motion of LRI particles. As shown in Figure 1, by adjusting the parameters on the inner and outer quadrilateral curves, they could precisely confine HRI particles to specific trajectories while allowing LRI particles to move within the dark channel and rotate clockwise under the influence of optical driving forces. Additionally, by adjusting parameters, the researchers could control the rotational speed of LRI particles.
Figure 1 Experimental results: Simultaneous manipulation of HRI and LRI particles using quadrilateral DC-OVB (appearing as smaller and larger particles, respectively, in the figure). The phase topological charges for the inner and outer curves in the first, second, and third rows are -5 and -40, -5 and -30, and -5 and 40, respectively.
Compared to traditional optical trapping technologies, the DC-OVB technique demonstrates superior flexibility and efficiency in customizing particle movement paths and speeds. This study's significance lies in its potential applications across multiple fields, including enhancing drug delivery and cell interaction technologies in biomedicine, improving microstructure assembly and manipulation in materials science, and providing a new method for studying colloidal interactions and fluid dynamics. Precise control of particle movement in these fields is crucial for developing more efficient and multifunctional optical manipulation tools.
Corresponding author Professor Jianping Ding stated: "This study successfully achieved high freedom path selection and precise local speed regulation for two types of particles using dual-curve optical vortex technology. Notably, the synergistic effect of the positive momentum flow on the outer ring and the negative momentum flow on the inner ring allows the low refractive particles in the dark channel to be influenced by a counterclockwise torque, exhibiting counterclockwise self-rotation. This phenomenon challenges the conventional understanding that particle rotation is typically caused by the spin of light, providing a new perspective and possibilities for the field of particle manipulation."
Looking ahead, the research team will continue exploring the construction of three-dimensional optical potential traps and the corresponding design of optical momentum flow density. They aim to achieve optical manipulation of particles with different properties using multimodal structured light fields to address more complex practical needs, further advancing optical manipulation technology and its applications in various fields.
