Manipulating nanoparticles by coupling wedge and plasmonic modes on non-uniformly biased graphene strips

Abstract

Realizing tunable high intensified and localized multiple hot spots are crucial for developing integrated optical tweezers in lab-on-a-chip devices to study interparticle interactions, routing and delivering nanoparticles. In this work, we utilize from a wedge-shaped Si structure to create and guide an extremely localized optical wedge mode and also as a non-uniform gate to non-uniformly control the chemical potential of topped graphene strips. Using three-dimensional finite-difference time-domain numerical method, we show that the wedge mode can excite localized surface plasmon (LSP) modes on the surface of each graphene strips with two distinct hot spots (trapping sites). The position of the trapping sites can be electrically manipulated continuously along the length of each graphene strip and discretely along the direction of propagation of the wedge mode by tuning the gate voltage. The calculated plasmonic forces by Maxwell stress tensor (MST) method reveals that 7 V change in the gate voltage leads to displacement of trapped particles by ∼ 49 nm. Moreover, we show that in the proposed structure by smoothly varying the gate voltage of graphene strips, active plasmonic focusing and defocusing lenses can be realized. We believe the proposed system can be implemented in lab-on-a-chip devices for electrically manipulating multiple nanoparticles without the need to use large scale expensive optical instruments.