Structured beam generation and optical trapping of low-index particles using a semi-nonlinear Michelson interferometer

In this work, we propose a semi-nonlinear Michelson interferometer (SNMI) for controllable laser beam shaping and optical manipulation. By placing a Kerr-type nonlinear medium into one arm of the interferometer, the phase difference between the two arms becomes intensity-dependent, enabling dynamic control of the spatial profile of the output beam through the input laser power. Numerical simulations show that a conventional Gaussian beam can be transformed into a variety of structured beams, including flat-top, ring, bottle, and hollow Gaussian beams, under appropriate operating conditions. The generated hollow beam is proposed to trap “low-index” nanoparticles, whose refractive index is lower than that of the surrounding medium. The transverse optical gradient force acting on dielectric nanoparticles is analyzed under the Rayleigh scattering regime. The results indicate that the generated hollow-type beams can produce stable trapping regions for low-index nanoparticles, which are typically difficult to trap with standard Gaussian beams. The proposed SNMI provides a simple, tunable, and flexible platform for structured beam generation, with potential applications in trapping nanoparticles, as numerically observed and discussed.