Active Graphene Plasmonic Tweezers: Size Based Nanoparticle Trapping and Sorting

In this article, a novel graphene based Optical tweezer (OT) and size based particle sorter is proposed and numerically investigated by finite difference time domain (FDTD) method and Maxwell's stress tensor (MST) analysis. Each unit cell of the structure is composed of a graphene layer on top a metallic nano-ring which is embedded in SiO₂. Strong hotspots with field enhancements reaching values as high as 140 are obtained inside the rings with no need to sharp edges conventional in plasmonic structures. Size based trapping can be achieved by controlling the fermi energy of the graphene through a gate. By cascading several unit cells, each operating at a different gate voltage, a particle size based sorter is designed which can effectively sort Polystyrene particles with their diameter in the range of smaller than 10 nm to 100 nm. The structure is illuminated by a laser beam in the mid-IR frequency range of 4-8 μm and its wavelength and intensity are kept constant even for sorting functionality. Instead, tunable plasmonic characteristics of graphene is utilized for active particle sorting. The fermi level-particle size-gate voltage relationship of the proposed OT is fully investigated at the article. Moreover, it is shown that the proposed structure is capable of sorting particles according to their refractive indices as well.