Optical trapping force and torque on a Rayleigh spheroidal particle by a non-diffracting optical needle

As a kind of non-diffracting beam, optical needle is gaining increased attention for its interesting properties and various potential applications. In this paper, we theoretically investigate the optical trapping force and torque on a Rayleigh spheroidal particle by an optical needle. A strategy to realize a non-diffracting optical needle by focusing a radially polarized vortex (RPV) beam with a high numerical aperture (NA) lens system is introduced and a diffractive optical element (DOE) is adopted to control the field distribution of the optical needle. Based on the field expressions of the optical needle derived through the Richards and Wolf theory, the factors affecting the properties of the optical needle are examined. Within the framework of dipole approximation, the optical trapping force and torque of an optical needle on a prolate spheroid with any spatial orientation are calculated. Numerical results show that the trapping force and torque are significantly affected by the field distribution of the optical needle, as well as the ellipticity and orientation of the spheroid. The present study may provide useful insights into the dynamic behavior during an actual optical trapping.