Longitudinal orbital rotation of nanoparticles in misaligned tightly-focused dual-beam optical trap

A dual-beam optical trap can provide increased stiffness and stability, thus having various important applications. Misaligned counter-propagating beams usually induce adverse effects and should be avoided. However, in this work, we investigate the potential of the misaligned tightly-focused dual-beam lensed optical trap to build an orbital rotation system in the longitudinal plane. The phase diagrams for the trapping state or motion state of the particle have been calculated, depending on the beam transverse offset, longitudinal offset, numerical aperture of the lenses and particle size. The simulation results show that longitudinal orbital rotation of nanoparticles can be realized with a sub-wavelength orbital radius of about 0.16 µm and a high rotation frequency of about 100 Hz in water when each beam has a power of 100 mW. Our results also provide theoretical support for various applications with a dual-beam optical trap and help to calibrate/correct the beam alignment. This work provides implications for the development of micro-machines, levitated mechanics, biological applications, and benefits the advanced optical manipulation technologies.