Large-area resonant and non-resonant optical nanostructures

Li, Ping-Chun, active 21st century
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Manipulation of light via subwavelength nanostructures is currently a subject of intense research interest, and is enabling the development of nanostructured photonic crystal, metamaterials and metasurfaces that provide a variety of new optical and electromagnetic functionalities, or that enable existing functionalities to be realized in new and often extremely compact form factors. This dissertation will include wide-angle wavelength-selective metasurface, omnidirectional enhancement in photovoltaic performance via subwavelength gradient anti-reflection coating, and applications of birefringent nanocylinders for single-molecule spectroscopy. In wide-angle wavelength-selective metasurface, high and broad reflectance (~95%) with low absorption (<5%) are shown to be achieved with multilayer metasurface structures. These characteristics are shown to be independent of interlayer misalignment and defects within individual layers. Interactions between different metasurface layers due to Fabry-Perot resonance are also examined with analytical models and numerical simulations. Wavelength-selective focusing at optical wavelengths which is enabled by large-area nanosphere lithography on a flexible substrate is demonstrated. In omnidirectional enhancement in photovoltaic performance via subwavelength gradient anti-reflection coating, large-area "moth-eye" structure fabricated on a flexible substrate is shown to have high transmittance (>85%) at large angle of incidences (>70°) and insensitivity to polarizations. Integration of the "moth-eye" anti-reflection coating together with nanostructured gradient A1₂O₃/TiO₂ on a GaAs solar cell shows significant improvements on external quantum efficiency (EQE) and short circuit current over all angle of incidences compared with conventional thin film anti-reflection coating. Detailed design, simulation, and fabrication of these nanostructured anti-reflection coating for reducing the discontinuity in refractive index profile will also be discussed. In application of birefringent nanocylinders for single-molecule spectroscopy, the design and fabrication method for large quantity of subwavelength birefringent nanoparticle are also discussed. These birefringent nanoparticles are shown to be stably trapped in an optical torque wrench setup, and enable observation of the dynamical response of a double-stranded DNA under torsional and extensional forces.