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A tightly focused, linearly polarized laser beam, so-called optical tweezers, is proven to be a useful micromanipulation tool. It is known that there is a stiffness asymmetry in the direction perpendicular to the optical axis inherited from the polarization state of the laser. In this Letter, we report our experimental results of stiffness asymmetry for different bead sizes measured at the optimal trapping condition. We also provide the results of our generalized Lorenz-Mie based calculations, which are in good agreement with our experimental results. We also compare our results with previous reports.

Although diffusing-wave spectroscopy has already been successfully applied to study dynamic properties of foams, we still lack a clear understanding of the diffusive transport of photons in foams. In this paper, we present a thorough study of photon diffusion in the Kelvin structure as an example for a three-dimensional model foam. We consider the photons' random walk as they are reflected or transmitted by the liquid films according to the rules of ray optics. For constant reflectance and special one- and two-dimensional photon paths, we are able to calculate diffusion constants analytically. Extensive numerical simulations reveal a remarkable similarity with our previous two-dimensional investigations. To implement a more realistic model, we use thin-film reflectances. The simulated diffusion constants exhibit oscillations for varying film thickness d which vanish when disorder is introduced in d. Absolute values and the behavior at small d agree with measurements in very dry foams providing a strong argument for the importance of liquid films in the diffusive photon transport. An analytical theory with a minimum of input parameters reproduces the numerical results.

Optical tweezers are noncontact and noninvasive force transducers. Techniques for multiple optical trapping are of much interest. In this paper, we investigate, numerically, optical trapping of multiple particles using so-called petal beams by trapping every single particle in a separate petal of the focused beam. We have used the generalized Lorenz–Mie theory to compute the optical trapping forces. Results demonstrate that the intensity is equally distributed over the lobes along the intensity ring for circularly polarized petal beams, but in the case of linear polarization, the intensity is not equally distributed over the petals, which means a particle trapped by every petal of a circularly polarized beam experiences the same force. It is shown that for a particle size range, the axial trapping stiffness for the l = 4 mode of a petal beam is about threefold greater than that of the l = 1 mode, and in the lateral direction, the trapping stiffness of the l = 2 mode is about twofold greater than that of the l = 4 mode. Finally, the optimum choice of topological charge for the stronger trap stiffness based on the particle size and trapping depth is identified.

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