Optical force acting on a particle in a reverse energy flow near the focus of a gradient lens

We show that if a dielectric nanoparticle (with a diameter of ∼70 nm) is placed on the optical axis near the surface (at a distance of less than 100 nm) of a high-aperture silicon gradient microlens with a refractive index in the form of a hyperbolic secant, and the lens is illuminated by laser radiation with a wavelength of 1.55 μm, then the particle is attracted to the lens surface with a force of a fraction of a piconewton. If there is a nanohole in the lens output surface, then the nanoparticle is pulled into it. This represents a kind of an 'optical magnet.' If a reverse energy flow is formed on the optical axis near the output surface of a gradient lens, then its presence leads to the fact that a dielectric nanoparticle with absorption will be 'attracted' to the surface with a greater force than a similar particle without absorption. In the absence of a reverse flow, both particles (with and without absorption) are attracted equally. We show also that in the nanohole, where the reverse energy flux is maximum, the light is right-hand circularly polarized, although the lens is illuminated by a left-hand circularly polarized light. The fields are calculated using the finite difference method in the time domain and the forces are calculated using the Maxwell stress tensor.