Zhifang Lin

Theory of Optical Trapping by an Optical Vortex Beam

We propose a theory to explain optical trapping by optical vortices (OVs), which are emerging as important tools to trap mesoscopic particles. The common perception is that the trapping is solely due to the gradient force and that it may be characterized by three real force constants. However, we show that the OV trap can exhibit complex force constants, implying that the trapping must be stabilized by ambient damping. At different damping levels, particles exhibit remarkably different dynamics, such as stable trapping and periodic and aperiodic orbital motions.

Photonic clusters formed by dielectric microspheres : Numerical simulations

We show through rigorous calculations that small dielectric microspheres can be organized by an incident electromagnetic plane wave into stable geometric configurations, which we call photonic clusters. The long-ranged optical binding force arises from the multiple scattering between the spheres. A photonic cluster can exhibit a multiplicity of distinct geometries, including quasicrystal-like configurations, with exotic dynamics. Linear stability analysis and dynamical simulations show that the equilibrium configurations can correspond with either stable or a type of quasistable states exhibiting periodic particle motion in the presence of frictional dissipation.

Strong Light-Induced Negative Optical Pressure Arising from Kinetic Energy of Conduction Electrons in Plasmon-Type Cavities

We found that very strong negative optical pressure can be induced in plasmonic cavities by LC resonance. This interesting effect could be described qualitatively by a Lagrangian model which shows that the negative optical pressure is driven by the internal inductance and the kinetic energy of the conduction electrons. If the metal is replaced by perfect conductors, the optical pressure becomes much smaller and positive.

Negative Optical Torque

Light carries angular momentum, and as such it can exert torques on material objects. Applications of these opto-mechanical effects were limited initially due to their smallness in magnitude, but later becomes powerful and versatile after the invention of laser. Novel and practical approaches for harvesting light for particle rotation have since been demonstrated, where the structure is always subjected to a positive optical torque along a certain axis if the incident angular momentum has a positive projection on the same axis. We report here an interesting phenomenon of “negative optical torque”, meaning that incoming photons carrying angular momentum rotate an object in the opposite sense. Surprisingly this can be realized quite straightforwardly in simple planar structures. Field retardation is a necessary condition and discrete rotational symmetry of material object plays an important role. The optimal conditions are explored and explained.

Multiply coated microspheres. A platform for realizing fields-induced structural transition and photonic bandgap*

Under crossed electric and magnetic fields, multiply coated microspheres form columnar crystallites with an internal structure that transforms from body-centered-tetragonal to face-centered-cubic as the ratio between the magnetic and the electric fields exceeds a minimum value. The observed transition scenario is in excellent agreement with calculations. These multiply coated microspheres also serve as building blocks for photonic crystals. Robust photonic gaps exist in any periodic structure built from such spheres when the filling ratio of the spheres exceeds a threshold.

Optical Twist Induced by Plasmonic Resonance.

Harvesting light for optical torque is of significant importance, owing to its ability to rotate nano- or micro-objects. Nevertheless, applying a strong optical torque remains a challenging task: angular momentum must conserve but light is limited. A simple argument shows the tendency for two objects with strong mutual scattering or light exchange to exhibit a conspicuously enhanced optical torque without large extinction or absorption cross section. The torque on each object is almost equal but opposite, which we called optical twist. The effect is quite significant for plasmonic particle cluster, but can also be observed in structures with other morphologies. Such approach exhibits an unprecedentedly large torque to light extinction or absorption ratio, enabling limited light to exert a relatively large torque without severe heating. Our work contributes to the understanding of optical torque and introduces a novel way to manipulate the internal degrees of freedom of a structured particle cluster.

Negative optical torque

While it is now clear that optical scattering force can be negative (opposite to the propagating direction of the beam), less attention is being paid to the optical torque. Here, we show that the optical torque exerted on a structure by an electromagnetic wave can be in an opposite direction to the angular momentum of the incident beam. This negative torque is closely related to the symmetry of the structure, and by playing around with the rotational symmetry and density of the structure, one can control the magnitude of the negative as well as the positive torque.

Field-induced structural transition in mesocrystallites

Abstract We have fabricated multiply-coated microspheres that exhibit appreciable electro-magneto-rheological responses. Under crossed electric and magnetic fields, the microspheres form columnar crystallites with an internal structure that transforms from body-centered tetragonal to face-centered cubic as the ratio between the magnetic and the electric fields exceeded a minimum value. The observed transition scenarios are in excellent agreement with calculations.

Pulling particles backward using a forward propagating beam

Can the scattering force of a forward propagating beam pull a particle backward? A photon carries a momentum of ℏ*k, so one may expect light will push against any object standing in its path. However, light can indeed “attract” in some cases. For example, a focused light beam can attract particles, due to the gradient force. But it is probably more appropriate to say that the gradient force “grabs” rather than “pulls”, as the particle will remain stable in the trap after being drawn to the focus. Here, we discuss another possibility — a backward scattering force which is always opposite to the propagation direction of the beam so that the beam keeps on pulling an object towards the source without an equilibrium point. In the absence of intensity gradient, using a light beam to pull a particle backwards is counter intuitive. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a pulling for...