Femtosecond laser based “Janus-material/function” processing paradigm for photothermal and radiative cooling modulation

The “Janus” concept, integrating asymmetric functions like radiative cooling (RC) and photothermal (PT) effects is gaining prominence for multi-scenario applications and function enhancement. In this work, we present a novel “Janus-material/function” processing paradigm by femtosecond laser nanomaterial synthesis and 600 °C thermal post annealing. This technique enables universal preparation of PT-active surfaces and in situ colorful RC functional transitions on transition metals (Nb, Mo, Ta, and W), while Ti-based PT interfaces exhibit uniquely thermal resistantance. Using custom-built systems, the PT/RC effects of as-prepared “Janus” black/white PT/RC-WO_3−x interfaces are verified, with interfacial temperatures 2 °C lower and 9 °C higher, respectively, than that of the benchmark tungsten plate under 1-sun irradiation. These interfaces demonstrate good thermal/mechanical stabilities, withstanding 4 h of 100 °C/200 °C annealing, vigorous hand shaking, and tape peeling. Femtosecond laser in situ PT-WO_3−x patterning on high-thermal-conductivity, high-infrared-reflective W-substrate demonstrates high-contrast infrared display in the emission and reflection modes by 80 °C sample heating and hot water excitation, respectively; whereas annealed white/dark-blue RC-PT-WO_3−x matrix, characterized by low thermal conductivities, exhibits convergent low-contrast infrared display. The flexibility of this paradigm, enabling convenient integration of double-sided “Janus” PT-RC functionalization and single-sided PT-RC “LASER JANUS” patterning on a single refractory metal, highlighting its potential for advanced Janus-conceptional customization in different thermal regulation scenarios.