FDTD approach to optical forces of tightly focused vector beams on metal particles

Optical trapping of gold microparticles using linearly polarized, centrally obstructed Gaussian beams

Metallic microparticles larger than the illumination wavelength are commonly considered poor optical trapping candidates due to their high extinction coefficient. This paper presents a numerical and experimental study on the three-dimensional (3D) trapping of gold microparticles using a centrally obstructed Gaussian beam based on the T-matrix method. The range of particle size for stable optical trapping is determined. For the trapping numerical aperture of 1.32 and illumination wavelength of 1.064 µm, numerical analysis proves that 3D trapping of gold microparticles with a radius bigger than 1.0 µm can be readily achieved. By imprinting a digital lens to the spatial light modulator, we slightly defocus the centrally obstructed Gaussian beam to shift the trapping location to the focal plane for clear observation. Experimental results demonstrate stable trapping of gold microparticles with a radius greater than 1.4 µm at high-power illumination, agreeing well with the theoretical predictions. The presented work should be of interest to the community applying metallic microparticles to relevant research.

Dual focused coherent beams for three-dimensional optical trapping and continuous rotation of metallic nanostructures

Metallic nanoparticles and nanowires are extremely important for nanoscience and nanotechnology. Techniques to optically trap and rotate metallic nanostructures can enable their potential applications. However, because of the destabilizing effects of optical radiation pressure, the optical trapping of large metallic particles in three dimensions is challenging. Additionally, the photothermal issues associated with optical rotation of metallic nanowires have far prevented their practical applications. Here, we utilize dual focused coherent beams to realize three-dimensional (3D) optical trapping of large silver particles. Continuous rotation of silver nanowires with frequencies measured in several hertz is also demonstrated based on interference-induced optical vortices with very low local light intensity. The experiments are interpreted by numerical simulations and calculations.

Radiation pressure of active dispersive chiral slabs

We report a mechanism to obtain optical pulling or pushing forces exerted on the active dispersive chiral media. Electromagnetic wave equations for the pure chiral media using constitutive relations containing dispersive Drude models are numerically solved by means of Auxiliary Differential Equation Finite Difference Time Domain (ADE-FDTD) method. This method allows us to access the time averaged Lorentz force densities exerted on the magnetoelectric coupling chiral slabs via the derivation of bound electric and magnetic charge densities, as well as bound electric and magnetic current densities. Due to the continuously coupled cross-polarized electromagnetic waves, we find that the pressure gradient force is engendered on the active chiral slabs under a plane wave incidence. By changing the material parameters of the slabs, the total radiation pressure exerted on a single slab can be directed either along the propagation direction or in the opposite direction. This finding provides a promising avenue for detecting the chirality of materials by optical forces.

Optically driven oscillations of ellipsoidal particles. Part II: ray-optics calculations

We report numerical calculations on the mechanical effects of light on micrometer-sized dielectric ellipsoids immersed in water. We used a simple two-dimensional ray-optics model to compute the radiation pressure forces and torques exerted on the object as a function of position and orientation within the laser beam. Integration of the equations of motion, written in the Stokes limit, yields the particle dynamics that we investigated for different aspect ratios k. Whether the beam is collimated or focused, the results show that above a critical aspect ratio k(C), the ellipsoids cannot be stably trapped on the beam axis; the particle never comes to rest and rather oscillates permanently in a back-and-forth motion involving both translation and rotation in the vicinity of the beam. Such oscillations are a direct evidence of the non-conservative character of optical forces. Conversely, stable trapping can be achieved for k < k(C) with the particle standing idle in a vertical position. These predictions are in very good qualitative agreement with experimental observations. The physical origin of the instability may be understood from the force and torque fields whose structures greatly depend on the ellipsoid aspect ratio and beam diameter. The oscillations arise from a non-linear coupling of the forces and torques and the torque amplitude was identified as the bifurcation control parameter. Interestingly, simulations predict that sustained oscillations can be suppressed through the use of two coaxial counterpropagating beams, which may be of interest whenever a static equilibrium is required as in basic force and torque measurements or technological applications.

Optical trapping of gold nanoparticles by cylindrical vector beam

Optical trapping of gold nanoparticles is experimentally demonstrated using radially and azimuthally polarized beams. The transverse optical trapping stiffness of gold nanoparticles is measured. The radially polarized beam exhibits a higher trapping efficiency than the azimuthally polarized beam and the Gaussian beam. The transverse stiffness of particles with different diameters is measured experimentally and calculated via the discrete-dipole approximation method, and good agreement between theory and experiment is found.

Trapping metallic Rayleigh particles with radial polarization: reply to comment

This is a reply to the comment by Iglesias and Sáenz directed to a previous paper "Trapping metallic Rayleigh particles with radial polarization," by Q. Zhan, Opt. Express 12, 3377 (2004).

Finite-difference time-domain (FDTD) analysis on the interaction between a metal block and a radially polarized focused beam

Radially polarized focused beams have attracted a great deal of attention because of their unique properties characterized by the longitudinal field. Although this longitudinal field is strongly confined to the beam axis, the energy flow, i.e., the Poynting vector, has null intensity on the axis. Hence, the interaction of the focused beam and matter has thus far been unclear. We analyzed the interactions between the focused beam and a subwavelength metal block placed at the center of the focus using three-dimensional finite-difference time-domain (FDTD) calculation. We found that most of the Poynting energy propagates through to the far-field, and that a strong enhancement of the electric field appeared on the metal surface. This enhancement is attributed to the constructive interference of the symmetric electric field and the coupling to the surface plasmon mode.

Optical trapping of microrods: variation with size and refractive index

Optical traps can be characterized in terms of two simple parameters: the stiffness, given by the gradient of the force at mechanical equilibrium, and the strength, as expressed by the maximum restoring force available for displacement in a given direction. We present numerical calculations of these quantities for dielectric microrods of varying radius and refractive index held horizontally in pairs of holographically generated Gaussian beams. The resulting variations are seen to be influenced by optical resonances, as well as by the relative sizes of the beam waist and rod diameter. In addition, it is shown that trapping in these systems is sensitive to the polarization state of the incident field; i.e., for certain rods, trapping will occur for beams polarized perpendicular to the long axis of the rod, but not for beams polarized parallel to the long axis. Finally, friction coefficients are evaluated and used to estimate the maximum rates at which the rods may be dragged through the ambient medium.

Light forces the pace: optical manipulation for biophotonics

The biomedical sciences have benefited immensely from photonics technologies in the last 50 years. This includes the application of minute forces that enable the trapping and manipulation of cells and single molecules. In terms of the area of biophotonics, optical manipulation has made a seminal contribution to our understanding of the dynamics of single molecules and the microrheology of cells. Here we present a review of optical manipulation, emphasizing its impact on the areas of single-molecule studies and single-cell biology, and indicating some of the key experiments in the fields.

Holographic optical trapping of microrods and nanowires

Holographic optical tweezing permits the trapping of objects with less than spherical symmetry in appropriately distributed sets of beams thereby permitting control to be exerted over both the orientation and position. In contrast to the familiar case of the singly trapped sphere, the stiffness and strength of such compound traps will have rotational components. We investigate this for a simple model system consisting of multiply trapped dielectric cylinder. Optically induced forces and torques are evaluated using the discrete dipole approximation and the resulting trap stiffnesses are presented. A variety of configurations of trapping beams are considered. Hydrodynamic resistances for the cylinder are also calculated and used to estimate translation and rotation rates. A number of conclusions are reached concerning the optimal trapping and dragging conditions for the rod. In particular, it is clear that it is advantageous to drag a rod in a direction perpendicular rather than parallel to its length. In addition, it is observed that the polarization of the incident light plays a significant role. Finally, it is noted that the non-conservative nature of the optical force field manifests itself directly in the stiffness of the trapped cylinder. The consequences of this last point are discussed.

A new type of vector fields with hybrid states of polarization

We present an idea based on Poincaré sphere and demonstrate the creation of a new type of vector fields, which have hybrid states of polarization. Such a type of hybridly polarized vector fields have completely different property from the reported scalar and vector fields. The novel vector fields are anticipated to result in new effects, phenomena, and applications.