All-optical motion control of metal nanoparticles powered by propulsion forces tailored in 3D trajectories

Increasing interest has been drawn to optical manipulation of metal(plasmonic)nanoparticles due to their unique response on electromagnetic radiation,prompting numerous applications in nanofabrication,photonics,sensing,etc.The familiar point-like laser tweezers rely on the exclusive use of optical confinement forces that allow stable trapping of a single metal nanoparticle in 3 D.Simultaneous all-optical(contactless)confinement and motion control of single and multiple metal nanoparticles is one of the major challenges to be overcome.This article reports and provides guidance on mastering a sophisticated manipulation technique harnessing confinement and propulsion forces,enabling simultaneous all-optical confinement and motion control of nanoparticles along 3 D trajectories.As an example,for the first time to our knowledge,programmable transport of gold and silver nanospheres with a radius of 50 and 30 nm,respectively,along 3 D trajectories tailored on demand,is experimentally demonstrated.It has been achieved by an independent design of both types of optical forces in a single-beam laser trap in the form of a reconfigurable 3 D curve.The controlled motion of multiple nanoparticles,far away from chamber walls,allows studying induced electrodynamic interactions between them,such as plasmonic coupling,observed in the presented experiments.The independent control of optical confinement and propulsion forces provides enhanced flexibility to manipulate matter with light,paving the way to new applications involving the formation,sorting,delivery,and assembling of nanostructures.

Tailored optical propulsion forces for controlled transport of resonant gold nanoparticles and associated thermal convective fluid flows

Noble metal nanoparticles illuminated at their plasmonic resonance wavelength turn into heat nanosources.This phenomenon has prompted the development of numerous applications in science and technology.Simultaneous optical manipulation of such resonant nanoparticles could certainly extend the functionality and potential applications of optothermal tools.In this article,we experimentally demonstrate optical transport of single and multiple resonant nanoparticles(colloidal gold spheres of radius 200 nm)directed by tailored transverse phase-gradient forces propelling them around a 2D optical trap.We show how the phase-gradient force can be designed to efficiently change the speed of the nanoparticles.We have found that multiple hot nanoparticles assemble in the form of a quasi-stable group whose motion around the laser trap is also controlled by such optical propulsion forces.This assembly experiences a significant increase in the local temperature,which creates an optothermal convective fluid flow dragging tracer particles into the assembly.Thus,the created assembly is a moving heat source controlled by the propulsion force,enabling indirect control of fluid flows as a micro-optofluidic tool.The existence of these flows,probably caused by the temperature-induced Marangoni effect at the liquid water/superheated water interface,is confirmed by tracking free tracer particles migrating towards the assembly.We propose a straightforward method to control the assembly size,and therefore its temperature,by using a nonuniform optical propelling force that induces the splitting or merging of the group of nanoparticles.We envision further development of microscale optofluidic tools based on these achievements.

Helix-shaped tractor and repulsor beams enabling bidirectional optical transport of particles en masse

Three-dimensional programmable transport of micro/nano-particles can be straightforwardly achieved by using optical forces arising from intensity and phase gradients of a structured laser beam. Repulsor and tractor beams based on such forces and shaped in the form of a curved trajectory allow for downstream and upstream(against light propagation) transportation of particles along the beams, respectively. By using both types of beams, bidirectional transport has been demonstrated on the example of a circular helix beam just by tuning its phase gradient. Specifically, the transport of a single particle along a loop of the helix has been reported. However, the design and generation of helix-shaped beams is a complex problem that has not been completely addressed, which makes their practical application challenging. Moreover, there is no evidence of simultaneous transport of multiple particles along the helix trajectory, which is a crucial requisite in practice. Here, we address these challenges by introducing a theoretical background for designing helix beams of any axial extension, shape, and phase gradient that takes into account the experimental limitations of the optical system required for their generation. We have found that only certain phase gradients prescribed along the helix beam are possible. Based on these findings, we have experimentally demonstrated, for the first time, helix-shaped repulsor and tractor beams enabling programmable bidirectional optical transport of particles en masse. This is direct evidence of the essential functional robustness of helix beams arising from their self-reconstructing character. These achievements provide new insight into the behavior of helix-shaped beams, and the proven technique makes their implementation easier for optical transport of particles as well as for other light–matter interaction applications. ? 2022 Chinese Laser Press.