Omnidirectional Ultra‐Broadband Cu/TiO ₂ Metasurface Absorbers for High‐Efficiency Solar Thermal Conversion and Photothermal Plasmon‐Driven Catalysis

ABSTRACT Efficient solar absorption is vital for renewable energy applications such as solar thermal systems. Wavelength‐selective devices are necessary to minimize thermal re‐radiation. Structured absorbers, which leverage their geometry to influence electromagnetic properties, allow greater flexibility in design and performance. However, achieving near‐ideal energy conversion remains challenging, requiring strong UV–NIR absorption while suppressing mid‐ to far‐infrared losses. This optimization is crucial for improving energy utilization in technologies such as desalination, wastewater treatment, and steam generation. In this study, we experimentally validate an omnidirectional, ultra‐broadband selective solar absorber comprising a copper‐titania‐based metasurface that exhibits an average spectral absorption efficiency of >90% from 250 to 1220 nm and >85% for the 250–1350 nm range. Importantly, this near‐perfect solar absorption arises from the excitation of magnetic and electric fields governed by Fabry–Perot and localized surface plasmon resonances. Moreover, the metasurface exhibits a low emissivity of <5% in the mid‐infrared and far‐infrared regions. Together, near‐ideal broadband absorption and effective heat retention enable near‐ideal solar thermal conversion efficiencies approaching 90% at 380 K. Omnidirectionally, the proposed absorber consistently achieves an average absorption efficiency exceeding 80% across the entire operational range (250–1350 nm) under both TM and TE polarizations for incident angles of up to 50°, underscoring its potential for a wide range of practical applications. We utilize this Metasurface as an extremely efficient SERS platform for the study of LSPR‐SLR co‐induced dimerization of 4NBT and pATP. Compared to bare Cu and TiO 2 films, highly accelerated reaction kinetics are achieved on the metasurface, determined quantitatively via the Raman intensity of the (N═N) band in the generated DMAB product. The photothermal effect of the Metasurface effectively lowers the energy barrier and generates more hot electrons. This opens new avenues for enhancing the reaction rate and broadening the research area of metasurface‐driven catalysis.