Classical Electromagnetic Properties And Optical Macromanipulation Of Metamaterial Dimers

Metamaterial dimer is one of the most common metamaterial atom, and supportssurface plasmonic resonance （SPR） since it’s usually composed of adjacent noble metalnanoparticles. Controlling the distance between the particles can adjust the surfaceplamonic resonance frequency and the local field intensity at the gap region. This isoften utilized to design optical nano-antenna, ultrasensitive biosensors, and controllablenear field optical tweezers. Most of the previous research concern individual dimer,barely considering the influence of a substrate on the properties of the dimer and opticalmicromanipulation of the dimer on substrate. On the other hand, heterodimer becomes ahot research topic because it can sustain Fano resonance. But there is no systematicresearch about the effects on optical forces by Fano resonance.We develop a method and tool for calculating optical forces of3D arbitrarystructure based on Finite Integration Technique （FIT） and Maxwell Stress Tensor（MST）, named FIT-MST. After comparing data obtained by other different methodssuch as discrete dipole approximation plus Lorentz force （DDA-LF）, we show thatFIT-MST is reliable and precise, especially the differ is less than5%between opticalforces calculated by FIT-MST and DDA-LF for several typical models.Next, in a system of a gold homodimer coupled to a metallic slab, we study thenear field distribution and optical forces by varying with the geometry parameters whenSPR happens. It is also found that the plasmonic resonance wavelength λ_res of thesystem shifts obviously as the nanorod gap g and h are changed. More specifically,numerical results show that when both g and h are both far less than the resonantwavelength λ_res, the resonant optical binding force （OBF） between the two nanorodsdecreases, while the force acting on each nonorod towards the slab increasesmonotonically as h is reduced. When continue to increase the dimmer-slab spacing h,there is a local maximum force at h≈λ_res/4for various g, which is as4times large asbare dimer.Finally, we present calculations of the optical forces on heterodimer of goldnanorods aligned head-to-tail, under plane wave illumination. It is found that adipole-quadrupole Fano resonance can happen and near which, the optical binding forcebetween the nanorods reverses, indicating an attractive to repulsive transition. Ourcalculations show that the unusual force reversal is associated with the strong phasevariation between the hybridized dipole and quadrupole modes near the Fano dip.