Gallium nanostructure fabrication by irradiation with femtosecond laser.

Gallium surfaces were irradiated with a 350 fs, 1040 nm femtosecond laser while submerged in liquid nitrogen, producing laser-induced periodic surface structures (LIPSS) spanning multiple length scales. Scanning electron microscopy reveals a hierarchical morphology with characteristic periodicities of approximately 10 μm, 1 μm, 250 nm, and down to ~ 30 nm. The largest structures (10 μm and 1 μm) are oriented perpendicular to the laser polarization, consistent with conventional wavelength-scale LIPSS, whereas the subwavelength (~ 250 nm) and deep-subwavelength (~ 30 nm) features are oriented at large angles relative to the larger patterns. The coexistence of multiple periodicities and their systematic angular relationships indicate a multistep self-organization process that cannot be explained solely by linear optical interference. We propose that plasmonic interactions play a central role in this hierarchical pattern formation. Specifically, wavelength-scale ripple ridges support propagating surface plasmon polaritons (SPPs), while their elongated ridge-like geometry enables localized longitudinal charge oscillations that strongly reshape the near field. Coupling between propagating SPPs and these localized plasmon modes enhances electromagnetic confinement, facilitating the emergence of both subwavelength and deep-subwavelength periodic structures. These results indicate a plasmon-mediated feedback mechanism underlying multiscale surface patterning on gallium under femtosecond laser irradiation.