Manipulation of mesoscopic particles using a structured beam in optical tweezers

Abstract

We use spin-orbit interaction (SOI) effects of light in tight focusing by optical tweezers to engineer the dynamics of birefringent microparticles at different spatial locations close to the focal region of the tweezers. Thus, we tightly focus radially and azimuthally polarized first order vortex (Laguerre-Gaussian) beams - that do not carry intrinsic orbital angular momentum (OAM) - into a refractive index stratified medium, and observe multiple birefringent particles orbiting around a single particle trapped stably at the beam center. This is due to the fact that tight focusing induces a longitudinal component of the electric field in the case of radial polarization, which completely modifies the intensity distribution, creating finite intensity at the center - which is typically dark for vortex beams. The intensity at the beam center and off-axis - in an annular ring - are both enhanced on introducing a refractive index stratified medium in the path of the optical tweezers, so that particles are trapped in both regions. In addition, the presence of the longitudinal component leads to an additional transverse spin angular momentum (TSAM) and extrinsic transverse orbital angular momentum (ETOAM). The latter causes single or multiple birefringent particles trapped in the annular ring to rotate around the beam axis, while a single particle is also trapped without displaying rotation or translation. This demonstrates the effectiveness of SOI in engineering the dynamics of mesoscopic particles in optical tweezers.