Abstract
Typically, metal particles are difficult to manipulate with conventional optical vortex (OV) tweezers, because of their strong absorption and scattering. However, it has been shown that the vortex field of surface plasmonic polaritons, called plasmonic vortex (PV), is capable of stable trapping and dynamic rotation of metal particles, especially those of mesoscopic and Mie size. To uncover the different physical mechanisms of OV and PV tweezers, we investigated the force distribution and trapping potential of metal particles. In OV tweezers the stronger scattering force causes a positive potential barrier that repels particles, whereas in PV tweezers the dominant gradient force contributes to a negative potential well, resulting in stably trapped particles. Compared with OV, the orbital angular momentum of PV produces an azimuthal scattering force that rotates the trapped particles with more precise radius and position. Our results demonstrate that PV tweezers are superior in manipulation of metal particles.
Highlights
Metal particles are difficult to manipulate with conventional optical vortex (OV) tweezers, because of their strong absorption and scattering
To probe the different interactions between particles and the two vortex fields, we numerically investigated the force distributions exerted on metal particles in both OV and plasmonic vortex (PV) fields, using finite difference time domain (FDTD) and Maxwell stress tensor (MST) methodology
We aimed to reveal the different mechanisms of metal particle trapping and rotation in OV and PV fields, and to prove the superiority of our novel PV tweezers in trapping and manipulating metal particles
Summary
Metal particles are difficult to manipulate with conventional optical vortex (OV) tweezers, because of their strong absorption and scattering. The electric charge polarization induces a larger attractive gradient force in metal particles than in dielectric particles, and the repulsive scattering force increases more quickly with increasing particle size than the gradient force does, due to its strong absorptive and scattering properties[9,10,11] This means that metal particles in a traditional OV field are more pushed away than trapped, especially large metal mesoscopic particles (with radius a ~ λ) and Mie particles (a λ). PV was shown be capable of attracting and stably trapping micrometer-sized gold particles, and dynamically rotating them with OAM delivered from an incident OV beam This implies that PV could be the better candidate for an “optical spanner” of metal objects. The radial gradient force produced by the non-uniform field intensity contributes to a negative potential well that stably traps particles This is more effective than the OV tweezers, which possess a stronger scattering force and a positive potential barrier that repels particles. We suggest that PV tweezers are a more stable, convenient, precise and general method of trapping and manipulating metal particles
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