Multiscale Microparticle Trapping Through Tapered-Fiber Structures and Refractive-Index Modulation

Tapered optical fibers reshape optical fields in confined spaces, enabling versatile microparticle trapping. This study establishes a continuous framework spanning near-field confinement, far-field extension, and environmental modulation by comparing flame-tapered and machine-pulled fibers. Flame tapering produces sharp tips with micrometer-scale foci; taper angles of 24°, 40°, and 60° yield focal positions of 0.2, 5.1, and 7.0 µm, respectively. A 980 nm laser is used, and stable trapping and probe-guided transport of 20 µm polystyrene microspheres were achieved at a threshold of 84 mW, with dual-particle trapping demonstrated via microlens-induced secondary potential wells. In contrast, machine-assisted tapering generates smooth end faces and long transition regions, extending working distances to 67, 122, and 163 µm. A long-transition probe further produced dual axial potential wells that stably confined 5 µm microspheres at 143 and 286 µm at powers above 500 mW. Environmental modulation revealed that increasing the external refractive index from 1.33 to 1.36 shifted the simulated focal positions from 40 to 55 µm, while ethanol-water evaporation experimentally reduced the trapping distances from 353 to 236 µm. These results show that fiber geometry defines the trapping range, whereas refractive index tunes the precise trapping position, offering a compact platform for multiscale manipulation with potential applications in cell positioning and sorting in liquid environments.