Phase profile in superposition of Bessel beam modulates local axial optical force on Rayleigh and Mie dielectric spheres

Abstract

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).