Surface plasmon resonance effect on laser trapping and swarming of gold nanoparticles at an interface

Gaining control on optical force by the stimulated-emission resonance effect

The resonance between an electronic transition of a micro/nanoscale object and an incident photon flux can modify the radiation force exerted on that object, especially at an interface. It has been theoretically proposed that a non-linear stimulated emission process can also induce an optical force, however its direction will be opposite to conventional photon scattering/absorption processes. In this work, we experimentally and theoretically demonstrate that a stimulated emission process can induce a repulsive pulling optical force on a single trapped dye-doped particle. Moreover, we successfully integrate both attractive pushing (excited state absorption) and repulsive pulling (stimulated emission) resonance forces to control the overall exerted optical force on an object, validating the proposed non-linear optical resonance theory. Indeed, the results presented here will enable the optical manipulation of the exerted optical force with exquisite control and ultimately enable single particle manipulation.

Unravelling 3D Dynamics and Hydrodynamics during Incorporation of Dielectric Particles to an Optical Trapping Site

Mapping of the spatial and temporal motion of particles inside an optical field is critical for understanding and further improvement of the 3D spatio-temporal control over their optical trapping dynamics. However, it is not trivial to capture the 3D motion, and most imaging systems only capture a 2D projection of the 3D motion, in which the information about the axial movement is not directly available. In this work, we resolve the 3D incorporation trajectories of 200 nm fluorescent polystyrene particles in an optical trapping site under different optical experimental conditions using a recently developed widefield multiplane microscope (imaging volume of 50 × 50 × 4 μm3). The particles are gathered at the focus following some preferential 3D channels that show a shallow cone distribution. We demonstrate that the radial and the axial flow speed components depend on the axial distance from the focus, which is directly related to the scattering/gradient optical forces. While particle velocities and trajectories are mainly determined by the trapping laser profile, they cannot be completely explained without considering collective effects resulting from hydrodynamic forces.

The primeval optical evolving matter by optical binding inside and outside the photon beam

Optical binding has recently gained considerable attention because it enables the light-induced assembly of many-body systems; however, this phenomenon has only been described between directly irradiated particles. Here, we demonstrate that optical binding can occur outside the focal spot of a single tightly focused laser beam. By trapping at an interface, we assemble up to three gold nanoparticles with a linear arrangement which fully-occupies the laser focus. The trapping laser is efficiently scattered by this linear alignment and interacts with particles outside the focus area, generating several discrete arc-shape potential wells with a half-wavelength periodicity. Those external nanoparticles inside the arcs show a correlated motion not only with the linear aligned particles, but also between themselves even both are not directly illuminated. We propose that the particles are optically bound outside the focal spot by the back-scattered light and multi-channel light scattering, forming a dynamic optical binding network.

Optical Force-Induced Chemistry at Solution Surfaces

When an intense 1,064-nm continuous-wave laser is tightly focused at solution surfaces, it exerts an optical force on molecules, polymers, and nanoparticles (NPs). Initially, molecules and NPs are gathered into a single assembly inside the focus, and the laser is scattered and propagated through the assembly. The expanded laser further traps them at the edge of the assembly, producing a single assembly much larger than the focus along the surface. Amino acids and inorganic ionic compounds undergo crystallization and crystal growth, polystyrene NPs form periodic arrays and disklike structures with concentric circles or hexagonal packing, and Au NPs demonstrate assembling and swarming, in which the NPs fluctuate like a group of bees. These phenomena that depend on laser polarization are called optically evolved assembling at solution surfaces, and their dynamics and mechanisms are elucidated in this review. As a promising application in materials science, the optical trapping assembly of lead halide perovskites, supramolecules, and aggregation-induced emission enhancement-active molecules is demonstrated and future directions for fundamental study are discussed.