Angular and position stability 
of a nanorod trapped in an optical tweezers

Abstract

We analyze the trap stiffness and trapping force potential for a nano-cylinder trapped in the optical tweezers against its axial and lateral shift and tilt associated to the natural Brownian motion. We explain the physical properties of the optical trapping by computing and integrating the radiation stress distribution on the nano-cylinder surfaces using the T-matrix approach. Our computation shows that the force stiffness to the lateral shift is several times higher than that to the axial shift of the nano-cylinder, and lateral torque due to the stress on the side-face is 1-2 orders of magnitude higher than that on the end-faces of a nano-cylinder with the aspect ratio of 2 - 20. The torque due to the stress on the nano-cylinder surface is 2-3 orders of magnitude higher than the spin torque. We explain why a nano-cylinder of low aspect ratio is trapped and aligned normal to the trapping beam axis.

FDTD simulation of trapping nanowires with linearly polarized and radially polarized optical tweezers

“…However, trapping is achievable for radially polarized beams and the axial trapping position is located 1.5µm behind the focus. The axial trapping potential corresponding to the area under the curve Q z (z) is defined as $U\left(\text{z}\right)=-{\displaystyle \int F_{\text{z}}}\text{dz}=-(\text{np/c }){\displaystyle \int Q_{\text{z}}}\text{dz}$ [7], which represents the energy required for nanowires to escape from the trap. Although the axial trapping potential of radially polarized beam in Fig.…”

“…The vector spherical wave functions (VSWFs) are a complete and orthogonal set of solutions to the vector Helmholtz equation [7,9]. For tightly focused beams, the fifth-order Gaussian beam description provides a significantly improved solution to Maxwell’s equations in comparison with commonly used paraxial Gaussian beam descriptions [10].…”

“…Direct solutions of Maxwell’s equations are suitable to get the electromagnetic field, and then the force on a nanowire can be calculated from the Maxwell stress tensor or Poynting’s vector method. In this regard the T-matrix method has been used to simulate laser trapping of nanorods with a circularly polarized beam [7] and linear nanostructures composed of identical nano-spheres with a linearly polarized beam [8]. The three dimensional finite-difference time-domain (FDTD) method is also a robust approach to solve Maxwell’s equations.…”

Pulling cylindrical particles using a soft-nonparaxial tractor beam

“…But position of a free-standing cylinder is not stable: it should be exerted by the optical torque 35 . Nevertheless, we expect that the results obtained in this article could be exploited in a periodic metamaterial with dielectric cylinders as meta-atoms.…”

Real-time monitoring and visualization of the multi-dimensional motion of an anisotropic nanoparticle

“…We used a single-mode, continuous-wave Nd:YAG laser (SLOC; IR1064H-800, λ = 1064 nm) with 2.5 W maximum power as the trapping source. The laser intensity entering the objective lens is controlled by a neutral density (ND) filter to make a bigger gradient force than the scattering force exerted on the SNW, to optimizing the stable trap30. To increase trapping efficiency, the size of the laser beam was expanded threefold by collimated lens sets, L1 and L2, to overfill the objective lens aperture31.…”