Optical binding and lateral forces on chiral particles in linearly polarized plane waves

Abstract

We demonstrate numerically and analytically the optical binding between a pair of chiral nanoparticles immersed in a plane wave of linear polarization. The numerical results based on the full wave simulation show that, when the electric field of the incident wave is polarized along the pair axis (the line connecting the particle pair), the closest dimer can be formed at particle separation around $1.25\ensuremath{\lambda}$, with $\ensuremath{\lambda}$ denoting the incident wavelength, while when the electric field is polarized normal to the pair axis, the nearest dimer is formed with a separation of one wavelength. In both cases, there exist multiple stable dimer states due to the spatially oscillatory decaying behavior of the binding force with the increase of particle distance. Interestingly, there also appears a lateral optical force exerted on each particle in the direction normal to both the pair axis and light flow, when either or both particles have chirality. Analytical analysis within the dipole approximation indicates that the binding force comes dominantly from the gradient of the optical potential, irrespective of particle chirality. The lateral force, on the other hand, has multiple origins. In addition to the gradient force, the radiation force, the curl force, and the spin force may make significant contributions dependent on the particle chirality, as the particles approach each other.