Homogeneous Nanostructuring of a Molybdenum Surface by Dual-Color Correlated Femtosecond Laser Irradiation for Solar Absorber Applications

Disordered ridge-splitting on laser-induced periodic surface structures (LIPSSs) often reduces structure quality and degrades application performance but introduces a potential route for laser structuring accuracy with access to the deep-subwavelength scale. Here, nanolithography based on the homogeneous ridge-splitting mechanism is implemented on a molybdenum surface using two-color (400 and 800 nm) temporally delayed femtosecond laser pulses with identical linear polarizations. The achieved splitting LIPSS, with a periodicity down to 140 nm and a feature size down to 70 nm, was uniformly distributed in the long range without any wavy or interruption defects, presenting improvements on the structure's accuracy and quality relative to the observation of single-beam femtosecond 400 nm laser irradiation. The generation and suppression of the homogeneous ridge-splitting phenomenon can be manipulated via altering the time delay or the fluences of the two-color laser pulses, which results in transitions between the regular near-subwavelength and deep-subwavelength LIPSSs. The underlying physical origins are attributed to the excitation of various electromagnetic field enhancement modes during the transiently correlated dynamic process of two-color laser-material interactions. Our investigations facilitate the laser nanostructuring of metals with an accessible 100 nm feature size, and the nanostructured Mo surface enables specific applications in the field of concentrated solar energy devices.