Tunable particle trapping based on a polarization-sensitive transverse bifocal metalens

In this work, we design a polarization-sensitive transverse bifocal metalens whose phase modulation is governed by a hybrid mechanism combining Pancharatnam-Berry (PB) phase and propagation phase. Under linearly or elliptically polarized illumination, the metalens generates two different focal spots along the x-axis. However, for left- or right-circularly polarized (LCP/RCP) light, only a single focal spot is formed on one side. We further investigate the optical trapping performance of the proposed metalens under different polarization states using finite-difference time-domain (FDTD) numerical simulations. The numerical results show that, under linearly and elliptically polarized illumination, simultaneous trapping of particles at the two focal positions can be achieved. By tuning the ellipticity of the incident light, the relative intensity of the two trapping sites can be flexibly controlled. In addition, by switching between LCP and RCP light, the trapped particles can be controllably transferred between the two focal points. Then we analyze the influence of particle size on the transfer performance. Simulation results indicate that for 1000 polystyrene (PS) particles with a radius of 100 nm, the transfer rate of 92.1% is achieved within 0.4 ms after switching the polarization state of the incident light. Finally, we compared the manipulation performance of this metalens on PS particles of different sizes. The results showed that the speed of particle transfer is directly proportional to particle size. This study provides more flexibility and tunability for optical trapping, while presenting new alternatives for particle manipulation and integrated photonic systems based on metalenses operating in the near-infrared regime.