“Seeing” Periodic Structures Emerge: Ultrafast Bright‐ and Dark‐Field Microscopy During Femtosecond Laser Interference Patterning of Silicon

Micro‐ and nanopatterning of materials using ultrafast lasers has evolved into a powerful technology for device fabrication, largely owed to its versatility and single‐step performance compared to conventional lithography techniques. Yet, for the ultimate optimization of the fabricated patterns, it is necessary to comprehend the complex transformation pathways of the material at the extreme excitation intensities used. Here, a method is reported to investigate these pathways for laser interference patterning with femtosecond temporal and sub‐micrometer spatial resolution over the entire transformation window up to the nanosecond regime. Specifically, an ultrafast bright‐field pump‐probe microscopy setup combined with a line patterning system is developed to imprint amorphous and ablative line‐like structures with a 2.2 µm period in crystalline silicon using single 800 nm, 120 fs laser pulses. This system provides direct access to the dynamics of free electron generation, melting, amorphization, and ablation as a function of pulse fluence. By adding a dark‐field modality, the complex dynamics of structural elevation can be resolved and quantified. The technique can be applied to any material with polished surfaces, thus offering direct access to in situ studies of their individual transformation dynamics during laser patterning.