Broad band optical absorption and thermoplasmonic response from bio-inspired hierarchical copper nanostructures fabricated by pulsed laser deposition

We report fabrication and characterization of bio-inspired hierarchical metallic nanostructures as an efficient absorber of broadband solar light radiation for photo-thermal conversion applications. We used pulsed laser deposition(PLD) to deposit different thicknesses of Cu nanostructures on bottom (hydrophobic) and top (hydrophilic) surface of Palash leaf (scientific name "Butea Monsperma").Optical image and reflectivity data of two surfaces coated with Cu showed clear advantage of coating metal on hydrophobic side of leaf for efficient absorption and temperature increment upon solar irradiation. The naturally occurring hierarchical bio-structures on the hydrophobic bottom surface of these leaves are responsible for similar hierarchical micro/nano structures of Copper. It enables excitation of propagating as well as localized surface plasmons over a broad range of optical spectrum. For enhanced photo-thermal conversion studies, the deposition time of Cu on hydrophobic side was optimized to generate highly absorbing bio-metamaterial. Thermal images of these surfaces under solar light illumination showed six-fold rise of surface temperature as compared to the temperature rise in planar Cu film of similar thickness deposited on a glass plate. Maximum temperature rise has effectively been correlated with absorption strength and local field enhancement invoking a simple model based on plasmonic resonances (propagating Surface Plasmons SPR and Localised Surface Plasmon Resonance LSPR). The analytical study of themoplasmonic response of complex nano/microstructure and planar metal film gives a clear insight for synthesizing efficient optothermal materials. Fabrication of such plasmonic structures on bio-inspired surfaces can be utilized as strong absorber of solar light in many applications including photovoltaic, thermo-plasmonic, solar heating applications etc.