Optical Trapping Of Sub-wavelength Nanoparticles Using Fano Enhanced Metasurface

Manipulation,sensing and trapping of biomolecules are important technologies in many fields of life science,such as single-molecule biophysics,cancer detection and drug development.Traditional optical trapping can rarely trap nanoscale particles because of the diffraction limitation.Researchers have proposed lots of methods to overcome this difficulty,and among them,plasmonic optical tweezers are of special importance which are stable,efficient,non-invasive,and offer fast response time.However,because of the absorption loss caused by thermal effect in plasmonic nanostructures,it's of vital importance to achieve optical trapping under low-power illumination.In this paper,we innovatively introduce Fano resonance into plasmonic nanostructures,inducing enhanced optical forces by higher-order modes thus realizing optical trapping of subwavelength nanoparticles under low-power illumination by single potential well.The optical forces induced by the asymmetric structure are increased 23 times that by the symmetric structure.Here we design symmetric split ring cavities(SSRC)and asymmetric split ring cavities(ASRC)based on Babinet principle.Simulation is done by Finite Difference Time Domain method and structure-unsymmetry caused Fano resonance has been observed under linearly polarized light.The resonance mode P1 of SSRC splits into P2 and P3 quatrimodes of ASRC,leading to enlarged quality factors Q.These two bonding magnetic resonances lead to a local electric-magnetic field enhancement inside the cavities.When the system is placed in a water environment,enhanced optical forces exerted on polystyrene particles are obtained by integration with Maxwell Stress Tensor method.When the wavelength of light source is 2020 nm,20nm-radius polystyrene particles can be trapped with an optical intensity threshold of 28.7 m W/?m2,the potential well is above the upper split ring cavity and the optical force along z direction always points towards the structure.The optical force increases when the radius of particle becomes bigger or the particle gets closer to the structure.ASRC can be simply processed and is practical in near-infrared region.We innovatively exploit the metasurface-caused Fano resonance to enhance the near field optical forces,which can be potentially used to manipulate and trap nanoparticles for biochemistry applications with low energy consumption.Meanwhile,the trapping position can be tuned by changing the size of structure or tailoring the wavelength of light source.And since the Fano-enhanced optical forces are sensitive to wavelength and radius of the particles,our findings also have important implications for sorting and sieving of particles with different sizes.