Optical Pulling Using Chiral Metalens as a Photonic Probe

Morphology-dependent resonance induced optical forces in a multiple-sphere system

Optical fields and forces can be greatly enhanced for a microparticle when the whispering gallery modes (WGMs) are excited. In this paper, by solving the scattering problem using the generalized Mie theory, the morphology-dependent resonances (MDRs) and resonant optical forces derived from the coherent coupling of WGMs are investigated in multiple-sphere systems. When the spheres approach each other, the bonding and antibonding modes of MDRs emerge and correspond to the attractive and repulsive forces, respectively. More importantly, the antibonding mode is good at propagating light forward, while the optical fields decay rapidly for the bonding mode. Moreover, the bonding and antibonding modes of MDRs in the PT-symmetric system can persist only when the imaginary part of the refractive index is small enough. Interestingly, it is also shown that for a PT-symmetric structure, only a minor imaginary part of the refractive index is required to generate a significant pulling force at MDRs, making the whole structure move against the light propagation direction. Our work deepens the understanding of the collective resonance behavior of multiple spheres and paves the way for potential applications in particle transportation, non-Hermitian systems, integrated optical devices, etc.

Enhanced optical trapping of ZrO2@TiO2 photonic force probe with broadened solvent compatibility

Optical trapping and manipulating nanoparticles are essential tools for interrogating biomedicine at the limits of space and time. Typically, silica or polystyrene microspheres are used as photonic force probes. However, adapting those probes to organic solvents is an ongoing challenge due to the limited solvent compatibility and low refractive index mismatch. Here we report on the optical force enhancement and solvent compatibility that utilizes ZrO₂@TiO₂ core-shell nanoparticles. We experimentally demonstrate that the 450-nm-diameter ZrO₂@TiO₂ core-shell nanoparticles achieve the lateral and axial trap stiffness up to 0.45 pN µm^-1 mW^-1 and 0.43 pN µm^-1 mW^-1 in water, showing more than fivefold and ninefold improvement on the ordinary SiO₂ particle of the same size. In addition, ZrO₂@TiO₂ core-shell nanoparticles can realize stable three-dimensional trapping in both polyethylene glycol and glucose solutions. This optical trapping enhancement property, coupled with solvent compatibility, expands the range of feasible optical trapping experiments and will pave the way toward more advanced biological applications.

Manipulation force analysis of nanoparticles with ultra-high numerical aperture metalens

Metalens optical tweezers technology has several advantages for manipulating micro-nano particles and high integration. Here, we used particle swarm optimization (PSO) to design a novel metalens tweezer, which can get 3-dimensional trapping of particles. The numerical aperture (NA) of the metalens can reach 0.97 and the average focusing efficiency is 44%. Subsequently, we analyzed the optical force characteristics of SiO₂ particles with a radius of 350 nm at the focal point of the achromatic metalens. We found the average maximum force of SiO₂ particles in the x-direction and z-direction to be 0.88 pN and 0.72 pN, respectively. Compared with the dispersive metalens, it is beneficial in maintaining the constant of optical force, the motion state of trapped particles, and the stability of the trapping position.