Optical trapping and manipulation typically relies on shaping focused light to control the optical force, usually on spherical objects. However, one can also shape the object to control the light deflection arising from the light-matter interaction and, hence, achieve desired optomechanical effects. In this work we look into the object shaping aspect and its potential for controlled optical manipulation. Using a simple bent waveguide as example, our numerical simulations show that the guided deflection of light efficiently converts incident light momentum into optical force with one order-of-magnitude improvement in the efficiency factor relative to a microbead, which is comparable to the improvement expected from orthogonal deflection with a perfect mirror. This improvement is illustrated in proof-of-principle experiments demonstrating the optical manipulation of two-photon polymerized waveguides. Results show that the force on the waveguide exceeds the combined forces on spherical trapping handles. Furthermore, it shows that static illumination can exert a constant force on a moving structure, unlike the position-dependent forces from harmonic potentials in conventional trapping.

No abstract

Electromagnetic waves carry angular momenta, and, due to spin-orbit interaction, an encounter with a gradient of refractive index leads to transport of spin similar to the electronic spin Hall effect. We show here that transversal spin transport is possible even when the symmetry of optical interaction is of higher dimensionality. We demonstrate that for a wave in a pure state of polarization, the spin-orbit interaction results in a spiraling power flow that is determined by the extent of the interaction. As a result, the spin transport can be resonantly enhanced in a spherical geometry. Our results open the possibility for developing new functionalities for photonic devices.

No abstract

We demonstrate both analytically and numerically the existence of optical pulling forces acting on particles located near plasmonic interfaces. Two main factors contribute to the appearance of this negative reaction force. The interference between the incident and reflected waves induces a rotating dipole with an asymmetric scattering pattern while the directional excitation of surface plasmon polaritons (SPP) enhances the linear momentum of scattered light.The strongly asymmetric SPP excitation is determined by spin-orbit coupling of the rotating dipole and surface plasmon polariton. As a result of the total momentum conservation, the force acting on the particle points in a direction opposite to the incident wave propagation. We derive analytical expressions for the force acting on a dipolar particles placed in the proximity of plasmonic surfaces. Analytical expressions for this pulling force are derived within the dipole approximation and are in excellent agreement with results of electromagnetic numerical calculations. The forces acting on larger particles are analyzed numerically, beyond the dipole approximation.

We demonstrate the existence of a class of optical beams where the nonconservative forces can be locally oriented in a direction opposite to the propagation wave vector. Objects placed in the vicinity of these locations will move toward the source of light. The behavior of these negative forces is discussed for the particular case of nondiffracting rotating scale-invariant vector electromagnetic waves.

When circularly polarized wave scatters off a sphere, the scattered field forms a vortex with spiraling energy flow. This is due to the transformation of spin angular momentum into orbital one. Here we demonstrate that during this scattering an anomalous force can be induced that acts in a direction perpendicular to the propagation of incident wave. The appearance of this lateral force is made possible by the presence of an interface in the vicinity of scattering object. Besides radiation pressure and tractor-beam pulling forces, this lateral force is another type of non-conservative force that can be produced with unstructured light beams. I. INTRODUCTIONUpon interaction with matter, there is an exchange between the spin and the orbital parts of the momentum carried by an optical wave. This optical spin-orbit interaction is responsible for a number of wave polarization effects [1,2,3,4,5]. Moreover, the conservation of total momentum also involves momentum transfer to matter. When analyzing this conservation law, the symmetry of the field is a critical component. For instance, when the azimuthal symmetry of the field is preserved around an axis, the projection of the total angular momentum along that axis is conserved according to Noether's theorem. As a result, the mechanical action on matter is along this axis of symmetry. When the rotational symmetry is broken as a result of interaction, the direction of the mechanical action is affected in order to obey the conservation of canonical momentum.

We show that in the canonical case of two lossless spheres that are electromagnetically coupled there is interplay between conservative and nonconservative forces, which is controlled by the polarization of the bounding field. We demonstrate that this phenomenon leads to new mechanisms to induce torques on spherically symmetric, optically isotropic, and lossless objects. The electromagnetic interaction can be exploited to apply orbital torque about the mutual center of mass of the bounded spheres as well as spin around the individual axes. When the incident field is linearly polarized, the torques are mostly conservative and affect only transient behaviors while for circularly polarized fields, the torques are entirely nonconservative, resulting in steady rotations. Means to control the magnitudes of orbital and spin torques are presented and applications to nanorotator machines are discussed.