Optical and Magnetic Manipulation of Hybrid Micro and Nanoparticle Sensors.

Abstract

Microparticles and nanoparticles have been used in a wide variety of applications ranging from biomedical to optical and electronic technologies. The microscopic and mesoscopic size scale of single particles makes them ideal tools for probing the local environments of biological cells, sensing the viscous properties of fluids and surfaces on the microscale, and interacting with photonic and magnetic fields. But the effectiveness of these particle systems is limited by the ability to manipulate and control them in predictable ways.

In this work, two methods of microparticle and nanoparticle manipulation are investigated, namely optical tweezers (OT) and magnetic rotation. OT provide a mechanically non-invasive means of grasping microparticles and nanoparticles, utilizing focused laser light. Moreover, particles driven by magnetic rotation in viscous media exhibit nonlinear dynamical motion and are a subclass of systems known as nonuniform oscillators. Both the individual and combined synergistic use of these control schemes is studied, in particular, on hybrid particles systems comprised of several materials, including both dielectric microspheres and metallic or magnetic colloids.

Classical electromagnetic theory was developed to describe the wavelength dependence of OT forces acting on a trapped, resonantly absorptive particle. Enhancements in the trapping strength could be obtained via near-resonance tuning of the laser wavelength. Experimental observation of this phenomenon on our hybrid particles was inhibited by increased destabilizing forces at the micron scale and the emergence of heating effects at high laser intensities often used in OT.

Using reduced laser intensities in conjunction with magnetic rotation, hybrid particles could be two-dimensionally trapped and rolled at a substrate surface. Changes in the nonlinear dynamical motion of the particles were measured to distinguish particle roughness and surface friction.

The response of rigid dimers of hybrid particles to optical and magnetic manipulation was studied. Observed changes in the dynamical motion with increased optical perturbation strength, using both numerical modeling and experiment, were investigated in terms of scattering forces, magnetization and heat generation from absorptive interactions.

Finally, the escape into the third-dimension of a magnetic dimer of hybrid particles undergoing nonuniform rotation was studied experimentally and compared to both theory and numerical simulation.