Optical forces from dielectric slot waveguides on dielectric nanoparticles
Abstract: Trapping and modulating the physical whereabouts of particles are in great interest for numerous areas of applications. Traditionally, this type of control is achieved with free space optical tweezers, but they do present several problems. Most prominently, they are size-limited by diffraction in free space and only allow for trapping at fixed points in space. A solution to these problems is utilizing the evanescent field from waveg- uide structures, which not only are not limited by diffraction, but offers higher trapping strengths generally. While they do offer higher trapping forces, most of the pumping power is not used for that purpose, as the modes mainly prop- agate in the solid waveguide core. This problem is solved by using slot waveguides, as in some geometrical configurations, the field propagates in the slot. In this thesis, we present and derive the fundamental relations between polarizable nanoparticles and the force exerted on them from an optical field. We then apply this theory and investigate how the geometrical parameters of the slot waveguide affects the exerted forces on a test particle with a given polarization, for 500 nm thick lithium niobate slot waveguides on a silica cladding surrounded by water, at telecom wavelength λ = 1550 nm. Our main results highlight a resonance-like behaviour for the field at cer- tain geometrical configurations, which greatly increases the trapping strength. Furthermore, we see a correlation between waveguide width in relation to slit width and depth resulting in an almost discontinuous transition behaviour discriminating the two regimes of when light is highly confined in the slot and when the field is confined in the solid core, giving rise to significant differences in trapping capability, as a result of different geometrical configurations.
AT THIS PAGE YOU CAN DOWNLOAD THE WHOLE ESSAY. (follow the link to the next page)