Producing anomalous uniform periodic nanostructures on Cr thin films by femtosecond laser irradiation in vacuum

We report on producing unprecedentedly uniform periodic structures on chromium thin films in vacuum conditions with irradiation of femtosecond laser pulses. In sharp contrast to the observations in air, the achieved surface structures of the ablated groove arrays are surprisingly found to have not only an extraordinarily uniform distribution but also a deep-subwavelength period of 360 nm. The measured both width and depth of the ablated periodic grooves are 150 and 120 nm, respectively, showing a large depth-to-width ratio and sharp-edge profiles. Remarkably, such well-organized nanostructures can be enabled to robustly extend into an infinitely long range via the sample scanning and even have a large-area production with a cylindrical lens. Raman spectral analyses reveal that the regular formation of such nanostructures benefits from avoiding the material oxidation and thermal disturbance of the air plasma on the sample surface.