Visualization of Optical Vortex Forces Acting on Au Nanoparticles Transported in Nanofluidic Channels

Abstract

The optical manipulation of nanoscale objects via structured light has attracted significant attention for its various applications, as well as for its fundamental physics. In such cases, the detailed behavior of nano-objects driven by optical forces must be precisely predicted and controlled, despite the thermal fluctuation of small particles in liquids. In this study, the optical forces of an optical vortex acting on gold nanoparticles (Au NPs) are visualized using dark-field microscopic observations in a nanofluidic channel with strictly suppressed forced convection. Manipulating Au NPs with an optical vortex allows the evaluation of the three optical force components, namely, gradient, scattering, and absorption forces, from the in-plane trajectory. We develop a Langevin dynamics simulation model coupled with Rayleigh scattering theory and compare the theoretical results with the experimental ones. Experimental results using Au NPs with diameters of 80–150 nm indicate that our experimental method can determine the radial trapping stiffness and tangential force with accuracies on the order of 0.1 fN/nm and 1 fN, respectively. Our experimental method will contribute to broadening not only applications of the optical-vortex manipulation of nano-objects, but also investigations of optical properties on unknown nanoscale materials via optical force analyses.

Optical trapping in micro- and nanoconfinement systems: Role of thermo-fluid dynamics and applications

“…4. Recently, a nanofluidic device of 300 nm height is used to evaluate optical forces acting Au nanoparticles [107]. The effect of thermophoresis is more difficult to eliminate.…”

Real-time phase measurement of optical vortex via digital holography

“…Owing to the singular properties of spiral phase and polarization distributions, optical vortex is widely used in many research and application fields. For example, tightly focused vortex beams are used in optical tweezers to manipulate particles such as cells and targeted drug [4,5]. Due to the orbital angular momentum (OAM) of the vortex beam, such particles can be rotated around the axis of the beam.…”

“…Due to the orbital angular momentum (OAM) of the vortex beam, such particles can be rotated around the axis of the beam. In the stimulated emission depletion (STED) microscopy technique, the tight focusing of the vortex beam can produce a hollow ring focus, which can be used to deplete the fluorophores around a desired area and keep the fluorophores in the desired target area and that enables spatial resolution beyond normal optical diffraction limits [5,6]. In addition, in the field of optical communication, the introduction of optical vortex can increase the number of optical modes for information transmission, thereby increasing significantly the bandwidth of optical communication.…”