Intensity Gradient Forces Research Articles

Spoon-like Beams Generated with Exponential Phases

In this paper, we report a new kind of beam, named “spoon-like” beams, generated with the exponential phase. The intensity distributions and transverse energy flow of the spoon-like beam at the focal plane are analyzed theoretically and experimentally. The results demonstrate that the size of the spoon-like beam becomes enlarged with the increasing power exponent n, and the length of the spoon-like intensity trajectory becomes shorter with the increasing parameter p. Furthermore, there is an intensity gradient along the spoon-like trajectory of the beam, which introduces the intensity-gradient force exerted onto microparticles. The experiment on optical tweezers demonstrates that the focused beams can create spoon-like traps for the two-dimensional manipulation of particles.

Intensity‐Gradient Force: Kerker‐Type Intensity‐Gradient Force of Light (Laser Photonics Rev. 14(4)/2020)

In article number 1900265 by Yao Zhang and co-workers, a nontrivial optical intensity-gradient force is uncovered in conventional optical tweezers set-ups. This force is excited by the Kerker effects inside high-index dielectric nanoparticles. Its non-conservative and anisotropic properties are in sharp contrast to the classical widespread picture of optical intensity-gradient forces. Its repulsive effects, moreover, could be exploited for high-resolution all-optical sorting.

Is it possible to enlarge the trapping range of optical tweezers via a single beam?

For optical tweezers, a tiny focal spot of the trapping beam is necessary for providing sufficient intensity-gradient force. This condition results in a limited small trapping range to guarantee stable trapping of the particle. Exploiting structured light, i.e., an optical vortex beam, the trapping range can be enlarged by adjusting its doughnut ring diameter. However, the trapped particle scarcely remains static due to the optical spanner action of the orbital angular momentum of the vortex beam. To enlarge the trapping range and simultaneously ensure stable trapping, we propose a beam, referred to as a mirror-symmetric optical vortex beam (MOV). Essentially, MOV is constructed by using two opposite optical spanners and a pair of static optical tweezers. The optical spanners attract the particle to the site of the static optical tweezers, which realizes long-range optical trapping. Through detailed force-field analysis, it is found that MOV could perform these setting functions. In experiments, yeast cells are manipulated in a long range of ∼25 μm, which is 3 times longer than that of the Gaussian beam. Further, the trapping range is easily adjusted by changing a parameter as desired. This technique provides versatile optical tweezers, which will facilitate potential applications for particle manipulation.

Optical sorting using an array of optical vortices with fractional topological charge

An array of optical vortices with fractional topological charge is generated using a phase-only Talbot array illuminator and used to sort microparticles. Our theoretical analysis and experimental results reveal that when a particle passes through a fractional vortex array, it will be driven by two forces, intensity-gradient force and phase-gradient force, and the cooperation of these two forces can improve its ability in optical sorting because of the special intensity and phase distribution of the fractional optical vortex array. Larger angle separation could be obtained with moderate laser power.

Electron acceleration in high-frequency longitudinal waves, Doppler-shifted ponderomotive forces, and Landau damping.

An electron in the field of a high-frequency longitudinal wave experiences both Doppler-shifted intensity-gradient and phase-gradient ponderomotive forces. It is shown that Landau damping can be considered as a phase-gradient force effect. By taking the wave amplitude space dependence into account, we demonstrate another collisionless effect due to the intensity-gradient force. Either damping or amplification of the wave can occur depending on whether the electron density increases or decreases.