Abstract
We study the effect of pulling optical force acting on a nonmagnetic anisotropic bead in electromagnetic fields without intensity gradient. Extreme anisotropy can be realized by a hyperbolic metamaterial made of metal-dielectric multilayers. We find that a passive anisotropic Rayleigh particle cannot be pulled by the electromagnetic beam without intensity gradient and the nonparaxial incident beams can exert backward negative force acting on anisotropic dipole spheres. We investigate the validity of the dipole approximation and establish the conditions for pulling hyperbolic-metamaterial particles. It is important to note that the loss in hyperbolic metamaterial does not suppress the effect of pulling force. We notice that the nonradial components of the permittivity tensor strongly affect the optical force and propose the way of material engineering to ensure the optical pulling.
Highlights
Optical force can be divided in two parts: gradient and nonconservative force
We have revealed that mainly the transverse permittivity component influences the pulling optical force without intensity gradient
Since the radial component can vary in wide ranges resulting in Fz < 0, the permittivity tensor can be engineered in a way to reduce the loss parameter εt and obtain the proper value of εr
Summary
Optical force can be divided in two parts: gradient and nonconservative force. The force of the first type as F = ∇V (r) does not perform work in a closed loop and, is called conservative. Light field without intensity gradient usually creates the pushing force (light pressure) This well-known effect was discovered experimentally a hundred years ago by Lebedev [6]. The interaction of multipole momenta induced by the incident light in the particle can originate the negative backward optical force [7,8,14]. It was shown preciously for isotropic particles that nonparaxial light beams should be used to obtain pulling force [7,8,9] and the force in dipole approximation can be negative for a wide range of material and size parameters of the particle [15].
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