Optomechanics of Optical Vortex Trapping Induced by Plasmonic Nanostructure Interacting with Incident Light

The optomechanics of optical vortex induced by the interaction of plasmonic clustered nanostructure with incident light will be studied. The optical force and torque exerted on a dielectric nanoparticle in the vicinity of an optical vortex will be investigated. Because the local confinement of optical vortex on a nanoparticle is beyond the diffraction limit, it can trap these nanoparticles or molecules in its vicinity for the physical & chemical manipulation. The potential applications of optical vortex (noncontact-mode trapping) are using plasmonic nanostructure array on microfluidic chip to perform the plasmon-enhanced biochemical sensing or reaction Lab-on-chip. Additionally, this technology can be applied on the trapping of PM 2.5 and aerosols in air for the monitoring of air pollution. The academic contributions are the study of optical vortex and photon’s spin-orbital interaction. The proposal includes two parts: numerical analysis and experimental measurement. Numerical Analysis The multiple multipole (MMP) method and finite element method (FEM, COMSOL) will be used to simulate the electromagnetic field of arrayed cluster plasmonic nanostructure (dimer, trimer, tetramer) and a nearby nanoparticle (NP) irradiated by a Gaussian beam. The optical force and torque on the nanoparticle can be obtained by the surface integrals of Maxwell stress tensor to study the optomechanical effect. In particular, the streamline of optical force will be used to investigate the optical vortex (non-contact mode trapping). Through the analysis of angular and orbital momenta, we study the effect of spin-orbital interaction on the optical force and torque. For example, we will characterize the optical vortex induced by a gold dimer irradiated by linearly or circularly polarized Gaussian beam in detail. In particular, the gap effect of cluster on the longitudinal/transverse spin/orbital angular and linear momenta in the vicinity of optical vortex will be analyzed. In addition, we can identify an optical vortex by searching the stagnation point with zero-force. We also condiser Brownian motion, and use NP’s dynamic equation to calculate the drift range of trapped NP to evaluate the optical trapping performances of various plasmonic nanostructures. Experimental MeasurementWe will use optical tweezers technique to focus a linearly polarized/circularly polarized laser beam (532 nm, 830 nm or 1064 nm) on a sample with arrayed nanostructure (dimer, trimer, or tetramer), and use microscopy with dark-field condenser, CCD and a preciously controlled xyz stage to measure the trajectory of a fluorescent nanoparticle for studying the optical trapping of optical vortex. These samples will be prepared using focus ion beam/electron beam lithography. The results of experiment will be compared with those of numerical analysis. Additionally, this technology will be combined with microfluidic chip for detection of nanoparticles in fluid.

Project ID:PB10907-3541
External Project ID:MOST109-2221-E182-061