Chih-Hsing Wang received the MS (2020) from National Taipei University of Technology. He is currently pursuing the PhD degree in condensed matter physics at Hokkaido University under the NIMS Joint Graduate School Program. His research interests include EB evaporation, sputtering deposition, refractory plasmonic materials, infrared photodetectors, and solar absorbers.

Sustainable solar energy harvesting, particularly through resonant photothermal conversion, is becoming a cornerstone for achieving global carbon neutrality and maximizing thermal energy harvesting [1]. Under the standard AM 1.5 solar spectrum (ASTM G-173), the majority of solar energy is concentrated within the 0.3–2.5 μm range, yet traditional solar collectors are often limited by inefficient photothermal conversion as well as radiation loss, necessitating sophisticated material nanostructured designs to maximize absorption limits [2]. In this work, we demonstrate the development of a perfect solar absorber based on a low-loss W/Al₂O₃/W metal–insulator–metal (MIM) structure from theoretical design to experimental realization. Based on our optimized design using particle swarm optimization (PSO) method [3], we successfully fabricated a perfect solar absorber composed of multiplex cross bar-like nanostructures. The devices were realized using radio-frequency (RF) magnetron sputtering for thin-film deposition, followed by electron-beam lithography (EBL) and capacitively coupled plasma reactive ion etching (CCP-RIE) to achieve precise surface nanopatterning. Scanning electron microscopy (SEM) confirmed uniform layer thicknesses, sharp interfaces, and high-fidelity pattern transfer, validating the reliability of our multi-step manufacturing workflow. Optical measurements demonstrated that the patterned W-MIM absorber achieves a significantly enhanced integrated absorptance of 0.9313 compared to the unpatterned trilayer absorber, while effectively suppressing mid-infrared emittance to 0.0532 at 100℃. This results in a remarkably high “total solar conversion efficiency” of 0.8827 even under heat loss via thermal radiation. This study successfully establishes a robust fabrication route for solar-plasmonic metasurfaces, by confirming the high-performance of the proposed structures for solar–photothermal harvesters. We are now focusing on wider material selection and quality improvements as well as scaling up the manufacturing area for space applications. In the future, we will extend our material choice to plasmonic superalloys as real-use candidates to further enhance the absorber's thermal stability and performance [4,5].

1. M. R. Al-Mamun et al., Solar Energy 264, 111998 (2023), DOI: 10.1016/j.solener.2023.111998
2. L.-Y. Chen, Optical Properties of Solar Absorber Materials and Structures, Springer (2021), DOI: 10.1007/978-981-16-3492-5_1
3. C.-C. Wang, J.-H. Li, and T. Nagao, Physical Chemistry Chemical Physics 27, 25868-25875 (2025), DOI: 10.1039/d5cp03164h
4. T. D. Ngo, H. D. Ngo, T. P. Tran, H. Harada, and T. Nagao, Advanced Photonics Research 6(4), 2400093 (2025), DOI: 10.1002/adpr.202400093
5. T. P. Tran, H. D. Ngo, T. D. Ngo, and T. Nagao, Applied Physics Express 14(8), 087001 (2021), DOI: 10.35848/1882-0786/ac0ecd