Molding light with coaxial plasmonic geometries [electronic resource] : toward subwavelength active plasmonic waveguides and direct optical manipulation of nanoscale specimens
Keywords: Electrical Engineering.

The ability to manipulate light has enabled the development of unprecedented technologies that reshaped our lives. Optical fibers, digital cameras and liquid crystal displays are just few examples. At the nanoscale, however, light manipulation with conventional optics becomes a challenge due to the diffraction limit. One of the viable routes to overcome the diffraction limit is through the utilization of surface plasmon polaritons. They are hybrid electromagnetic/electronic surface waves that can be guided along a metal/dielectric interface and are subwavelength by nature. Thus, they facilitate the development of subwavelength optics which allow for conquering new scientific and technological frontiers. In this thesis, we focus on studying coaxial plasmonic apertures for two particular applications; active subwavelength optical interconnects and plasmonic optical tweezers. The integration of photonics and electronics has been hampered by the large size of the photonic components (set by the diffraction limit). Thus, the realization of subwavelength optical interconnects is of fundamental importance as it is the first step toward on-chip optical data communications and eventually full optical data processing. Here, we focus on using active coaxial plasmonic waveguides as subwavelength optical interconnects. Using empirically-determined optical constants, we systematically study the dispersion, propagation length, threshold gain, modal gain, and confinement factor of these structures. We show that coaxial plasmonic waveguides can achieve lossless propagation with that threshold gain as low as 500 cm^{-1} with overall waveguide diameter less than 200 nm. Coaxial waveguides also showed significant enhancement in the confinement factor where modal gain can exceed threshold gain by 10 to 100x across visible and near-infrared frequencies. Our results indicate the promise of the coaxial plasmonic waveguides for low-loss optical networking, and provide a roadmap for the design of optimized nanoscale plasmonic laser cavities. The other application we focus on in this thesis is plasmonic optical tweezing of nanoscale dielectric particles. Conventional optical tweezers use sculpted laser beams accelerate, manipulate, or trap small objects with light alone. Since their introduction, optical tweezers have emerged as a powerful means of probing and controlling micrometer-scale objects. In the biosciences, for example, optical tweezers have been utilized for bacterial trapping as well as noninvasive manipulation of organelles and filaments within individual living cells. They have also advanced our understanding of biomolecular systems and the physics of molecular motors, ranging from kinesin and myosin to the polymerases involved in DNA transcription and replication. Despite these advances, direct optical trapping and manipulation of individual subwavelength particles remains a considerable challenge, due to the diffraction limit of light. In this thesis, we introduce a new optical tweezer design - coaxial plasmonic apertures - that can trap sub-10nm dielectric particles. These coaxial apertures are capable of strongly confining light in all transverse directions, creating a highly-localizing dual trapping potential in the nearfield. First, using full-field electromagnetic simulations, we show that an optimized coaxial design can trap dielectric particles as small as 2nm in diameter. The required optical power for nanoscale trapping and manipulation is less than 100 mW, and the dual potential well can be rotated with the incident polarization, allowing for specimen rotation. Then, we discuss our experiments to characterize the optical forces of individual coaxial fibers using atomic force microscopy. This technique enables direct detection of the mechanical action of light at the nanoscale with high spatial resolution and sub-pico Newton force resolution. Finally, we describe how coaxial aperture integration onto an optical fiber can enable full, three-dimensional manipulation of particles. Looking forward, coaxial apertures provide a promising a route toward non-invasive nano-specimen optical trapping at extended-working-distances.