Optimizing broadband lossy waves excitation at ultrathin metal layer interface for enhanced photothermal solar water evaporation

Broadband lossy electromagnetic waves (BLEWs) can be excited in ultrathin (<20 nm) layers of lossy metals such as Ni, Cr, and W. This excitation is interpreted as a result of multiple reflections from the two metal interfaces when it is thinner than the skin depth. Surprisingly high absorption is obtained when the light is incident on the metal from a high refractive index medium, over a wide range covering the whole solar spectrum, and over a wide angular range. Based on these facts we demonstrate enhanced water evaporation using solar radiation by optimizing the excitation conditions of BLEWs. A 16 nm Ni thin film is used as an absorber in a high index prism configuration for interfacial photothermal conversion at the interface with water. The experiment was conducted under controlled real-world and laboratory conditions while monitoring the water height using the laser beam triangulation method. Results indicate a significant improvement in the evaporation rate, with the Ni layer exhibiting an increased water evaporation rate nearly three times compared to evaporation from a region without it. The evaporation rate is estimated at almost 3 L/hr.m² with a halogen lamp in the lab, reaching 6.6L/hr.m² under the sun without any radiation concentration. From an estimation of the required heat to achieve this rate we concluded that interfacial heating has an evaporation efficiency of more than twice that of bulk heating. This underscores the potential of BLEWs for developing efficient and sustainable solar-powered desalination systems. Continued research and optimization of these materials and BLEWs excitation geometries could be vital in addressing global water scarcity and advancing sustainable water management practices.