Optical Trapping And In-situ Spectral Characterization Of A Single Nanoparticle Based On Metal Ring Nanostructure

Optical tweezers have had a major impact on physcs and medical research.It has emerged as a powerful tool for immobilizing and manipulating small particles in three dimensions.Traditional optical tweezer provides a three-dimensional potential well formed by laser focusing spot for the trap.The basic principle is the mechanical effect formed by the interaction of light and matter.Laser tweezer is a non-contacting and scatheless technique with high precision.However,traditional laser optical tweezer is achieved at far field,which foucses a laser beam to produce a gradient force.It’s difficult to break through the diffraction limit to trap particles below the sub-wavelength scale,and it’s hard to achieve in-situ characterization.Near-field optical tweezer based on metal micro-nano structure has been rapidly developed,which benefits from the field enhancement of surface plasmon polarition.Such plasmonic optical tweezer based on metal micro-nano structures can break through the diffraction limit and expand the optical trap to the nano-scale.It can provide strongly optical gradient force and trap micro/nano particles at a lower incident light.Moreover,the field enhancement effect of surface plasmons can increase the Raman scattering signal,which is the surface-enhanced Raman spectroscopy technology(SERS).In this thesis,a kind of structure named metal-based ring structure is used to stably capture and achieve in-situ characterization of single nanoparticles.Self-induced back-action(SIBA)optical trapping has the potential for stably capture,and raman spectroscopy can be used to achieve in-situ characterization.Specific contents are as follows:(1)In this thesis,a metal coaxial nanowaveguide used in optical band is designed,in which transverse electromagnetic mode(TEM)can be transmitted.Such mode can not exist in optical fiber.TEM mode has a radially symmetric transverse electric field distribution and annularly symmetric transverse magnetic field distribution.Single particle will break the symmetry of the mode and lead to high transmission efficiency at far-field,thereby strongly altering light momentum and inducing a large-scale back-action on the particle.By numerical simulation,the maximum photon force of 7.78 p N/m W·μm~2 can be provided for 10 nm particles.The potential well is up to 14 k _BT.The particle plays an active role in the trapping mechanism,which called SIBA optical trapping.It provides a new idea for the new optical trapping.(2)Besides,in this thesis,a plasmonic bullseye optical tweezers system based on surface plasmon polarition is constructed,it consists of a bowtie nanohole and concentric ring groove in golden film.By theoretical analysis,numerical simulation and experimental method,such structure can trap single 15 nm polystyrene particle stably.In addition,in-situ Raman spectroscopy of single particle can be achieved simultaneously,which lays the foundation for the study of specific biomolecules below 20 nm in life science and virology.