Applied Nanophotonics, Liquid Crystals and Biomedical Optics

Photonic metamaterial (MTM) perfect absorbers have gained significant attention due to their exceptional performance and desirable properties for sustainable energy and more efficient energy devices. One such application is the development of an interfacial photothermal conversion MTM for solar water desalination processes. This enhanced light-to-heat conversion improves efficiency in seawater and wastewater treatment, ultimately supporting freshwater production. Developing such efficient photothermal converters represents a significant step towards addressing global water scarcity issues. Here, a metal–insulator-metal (MIM) stacked MTM device for interfacial solar photothermal conversion is presented. The structure comprises a top MTM metal layer for wide-angle light trapping, a middle dielectric spacer for resonant mode enhancement, and a bottom reflective tungsten layer to nullify transmission. It exhibits broadband, wide-angle absorption (±40°) and polarization-independent characteristics, with a remarkable photothermal conversion efficiency in the 400–2400 nm spectral range. An average absorption of ≈77% is achieved both theoretically and experimentally. The device has shown a promising photothermal conversion efficiency of 73%, an evaporation rate (ER) above 10 L/m²·h under focused sunlight conditions (≈8.5 kW/m²), and with the potential to double under optimized conditions, showing that it is a strong candidate for efficient water desalination and wastewater treatment. Several experiments were conducted to assess the MTM environment's effect on the ER and found that, due to its small area, surrounding it with water helps utilize lost thermal energy, thereby increasing the ER by more than a factor of x2.