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We investigate how the optical gain or loss (characterized by isotropic complex refractive indexes) influence the ideal Kerker scattering of exactly zero backward scattering. It was previously shown that, for non-magnetic homogeneous spheres with incident plane waves, either gain or loss prohibit ideal Kerker scattering, provided that only electric and magnetic multipoles of a specific order are present and contributions from other multipoles can all be made precisely zero. Here we reveal that, when two multipoles of a fixed order are perfectly matched in terms of both phase and magnitude, multipoles of at least the next two orders cannot possibly be tuned to be all precisely zero or even perfectly matched, and consequently cannot directly produce ideal Kerker scattering. Moreover, we further demonstrate that, when multipoles of different orders are simultaneously taken into consideration, loss or gain can serve as helpful rather than harmful contributing factors, for the elimination of backward scattering.

A general acoustic force field can be decomposed into a conservative gradient force (GF) and a non-conservative scattering force (SF), which have very different physical and mathematical properties. However, the profiles of such forces for Mie particles are unknown, let alone their underlying physics. Here, by using a fast Fourier transform approach, we calculated the GF and SF for spherical particle of various sizes and various incident waves. For the same focused incident waves, the normalized GF and SF are similar for different particle sizes, while the total force can be quite different owing to the varying relative strength between the GF and SF. GF and SF possess symmetries that are not found in the incident waves, indicating that these physically and mathematically distinct forces have symmetries that are hidden from the beam profile. For a vortex beam carrying a well-defined topological charge, acoustic forces alone cannot trap particles.

Abstract: We present an experimental demonstration of multiple optical tweezers based on interference of two co-propagating beams that intersect at a given angle and form interference fringes (asymmetric optical traps) at the focal plane of a focusing lens. Since this arrangement provides only two-dimensional trapping when the objects are pushed against the coverglass, we added the third counter-propagating beam. This beam did not interfere with the previous two but compensated their radiation pressure. Therefore, stable three-dimensional confinement into multiple fringes is achieved. We quantified experimentally the maximal optical forces exerted on 1μm polystyrene bead in both configurations and compared them with theoretical predictions. Reasonable good coincidence was found especially for two-dimensional trapping. [Copyright &y& Elsevier]

Manipulating small objects with the optical force has benefitted various fields ranging from nano/micro-patterning techniques to bio-applications. In this paper, we theoretically study the modulation of the local optical force on a Rayleigh or Mie dielectric sphere at a given location, when the superposition of the zeroth-order Bessel beam as an incident light has a phase profile which depends on the cone angle of the Bessel beam. Based on vector spherical harmonic analysis, we decompose the optical force into multipole interaction components, where each component is expressed as a function of the phase profile. Using a genetic algorithm, we optimize the phase profile to maximize the magnitude of the local optical force or to flip the direction of the force. The optimization is performed at various optical conditions (e.g., the ratio between the size of the sphere and the wavelength of the incident light and the cut-off cone angle), and we analyze the enhanced properties of optical force in terms of multipole interaction components. We find that the smaller sphere can take more advantage of optical force modulation. For example, the phase profile can amplify the strength of axial optical force on a Rayleigh sphere by several orders of magnitude (~103). [ABSTRACT FROM AUTHOR]