Design of Metallic Nanostructures for Wavelength and Angle Selective Light Management
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Sub-wavelength metal nanoparticles demonstrate a resonant coupling to incident optical fields known as the localized surface plasmon resonance, enabling enhanced absorption, scattering, and nano-focusing of light. In this work, plasmonic properties of metal nanoparticles and nanorods are studied and engineered for management of light in photovoltaic (PV) and selectively transmissive / absorptive systems.
Placed behind a thin film PV absorbing layer, an array of silver nanospheres acts as a wavelength dependent backreflector, allowing engineering of selective transparency vs. absorption and modulation of photocurrent. Further tuning the array by considering anisotropic particle shape (increasing the aspect ratio), backscattering can also be controlled as a function of incident angle – we propose such a structure to enable building integrated angle selective PV window coatings which maintain normal direction transmission for visibility while harvesting direct sunlight from elevated angles. Angle selective metal nanorods can also be integrated with actuating micro-origami structures which rotate their orientation with respect to incident light. We detail a potential method to realize balanced 0 – 95% transmission modulation of the full visible spectrum for application to next generation smart glass.
Large area patterning of deeply sub-wavelength (10's of nm) metal nanorods remains a challenge for traditional nanofabrication. We investigate ways to realize the structures of interest on transparent substrates via electrochemical synthesis of self-assembled nanoporous anodized aluminum oxide (AAO) films, including both bottom-up (electroplating) and top-down (reactive ion etching) approaches. Finally, the anisotropic and angle selective optical scattering properties of high aspect ratio AAO itself (without metal) are considered for similar light management applications.