We report a quasi-unitary broadband absorption over the ultraviolet-visible-near-infrared range in spaced high aspect ratio, nanoporous titanium oxynitride nanotubes, an ideal platform for several photothermal applications. We explain such an efficient light-heat conversion in terms of localized field distribution and heat dissipation within the nanopores, whose sparsity can be controlled during fabrication. The extremely large heat dissipation could not be explained in terms of effective medium theories, which are typically used to describe small geometrical features associated with relatively large optical structures. A fabrication-process-inspired numerical model was developed to describe a realistic space-dependent electric permittivity distribution within the nanotubes. The resulting abrupt optical discontinuities favor electromagnetic dissipation in the deep sub-wavelength domains generated and can explain the large broadband absorption measured in samples with different porosities. The potential application of porous titanium oxynitride nanotubes as solar absorbers was explored by photothermal experiments under moderately concentrated white light (1-12 Suns). These findings suggest potential interest in realizing solar-thermal devices based on such simple and scalable metamaterials.

CEET Nanotechnology Centre VŠB Technical University of Ostrava 17 Listopadu 2172 15 Ostrava Poruba 708 00 Czech Republic

Czech Advanced Technology and Research Institute Regional Centre of Advanced Technologies and Materials Department Palacký University Olomouc Šlechtitelů 27 Olomouc 78371 Czech Republic

Department of Chemical and Pharmaceutical Sciences INSTM and ICCOM CNR University of Trieste via L Giorgieri 1 Trieste 34127 Italy

Department of Chemistry and NIS Centre University of Turin Turin 10125 Italy

Department of Electrical and Computer Engineering Rice University 6100 Main Street Houston Texas 77005 United States

Department of Physical Chemistry Faculty of Science Palacký University 17 listopadu 1192 12 779 00 Olomouc Czech Republic

Department of Physics Politecnico di Milano Piazza Leonardo da Vinci 32 20133 Milano Italy

Istituto di Fotonica e Nanotecnologie Consiglio Nazionale delle Ricerche Piazza Leonardo da Vinci 32 1 20133 Milano Italy

Istituto Italiano di Tecnologia via Morego 30 16163 Genoa Italy

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