We propose a kind of novel cascade optical tweezers for the capture and isolation of particles with high/low refractive indices. The device is made of a tapered fiber-optic probe nested, inserted into capillary microtubules, and achieves the stable capture of particles with different refractive indices by combining optical force and capillary force. Based on finite element simulation, the optical field distribution and particle force are analyzed. Meanwhile, capture and suspension experiments on yeast cells, polystyrene (PS) microspheres, silica particles, and magnetic particles are carried out by adopting a 980 nm wavelength laser. The experimental results show that the cascaded optical tweezers exhibit sound performance in capturing particles with different refractive indices. The proposed cascaded optical tweezers provide a new technological means for both particle capture in the biomedical field and isolation operations in complex environments.

We develop cascaded optical tweezers based on the integration of fiber optic tweezers and capillary microtubules, which can be adopted for the capture of biological cells, non-biological particles, particles with different refractive indices, and non-uniform magnetic particles. Additionally, a light source with a 980 nm wavelength is passed into the tapered fiber optic probe which is nested with a capillary microtubule to form a particle “isolation zone”. At the same time, by employing the capillary phenomenon at the nested insertion and the joint action of optical force, the particles move from the tip of the microcavity optical waveguide to the tip of the tapered optical fiber probe, thus realizing the capture of biological and non-biological particles, and non-uniform magnetic particles. Furthermore, the “isolation zone” formed by the nesting of the microcavity optical waveguide and the tapered fiber probe can effectively isolate and protect the captured particles from the interference of the external environment, thereby enhancing the stability and controllability of the capture process.

Yeast cells, PS particles, silica particles, and magnetic particles are selected for the experiments, as shown in Figs. 6 and 7 respectively. When the yeast cells are in the microcavity optical waveguide port of the cascaded optical tweezers, due to the combined effect of the optical force generated by the fiber optic probe and the capillary force generated by the cascaded optical tweezers, the cells move toward the inside of the microcavity cascaded optical tweezers to the tapered fiber optic tip, which is isolated from the outside particles in solution. The movement process of yeast cells is shown in Fig. 6(a). Due to the different refractive indices of various particles, the optical power of the laser beam is adjusted to 0.54, 1.52, and 0.4 mW for the capture and collection experiments of PS particles, silica particles, and magnetic particles with the size of 5 $\mathrm{\mu }\mathrm{m}$. Meanwhile, the three kinds of particles are captured and collected in the “isolation zone”, with the corresponding time-displacement curves plotted as shown in Figs. 8 and 9 respectively. Moreover, we also verify the capture of low refractive index particles, and the experimental results show that the method can realize the capture of low refractive index particles. Finally, to expand the application potential of cascaded optical tweezers, we investigate the role of curved microcavity optical waveguides in regulating the behavior of particles. As shown in Fig. 13, the particles can realize the bending capture along the tube wall. The cascaded optical tweezers composed of bent microcavity optical waveguides not only maintain the ability to capture particles but also optimize the light field distribution via its unique geometric bending structure, which affects the trajectory and alignment of the particles.

We propose a novel cascaded optical tweezer system that integrates single-fiber optical tweezers and capillary microtubules to achieve efficient capture and isolation of particles with different refractive indices and non-uniform characteristics in liquid media by introducing a 980 nm wavelength laser beam via a tapered fiber probe. The experimental results show that the cascaded optical tweezers exhibit excellent capture performance in background media with various refractive indices, especially in the capture of particles with different refractive indices, such as yeast cells, PS particles, silica particles, and magnetic particles, demonstrating highly stable manipulation ability. The design of this device not only significantly improves the flexibility of capturing multiple particles, but also broadens the application of optical tweezers in complex environments. Finally, new technical means and theoretical support are provided for fine particle manipulation and efficient capture in biomedical science, materials science, and micro- and nanotechnology in the future.