Gallium Nanostructure Fabrication by Irradiation With Femtosecond Laser

A gallium substrate was irradiated by a 350-fs, 1040-nm femtosecond laser while submerged in liquid nitrogen. Using a scanning electron microscope (SEM), we observed periodic ripple structures induced by the laser radiation. The observed periodicities include approximately 10 μm, 1 μm, 250 nm and even 30 nm. The alignment of the 10 μm and 1 μm ripple structures depended on both the polarization of the laser, while the alignment of the 250 nm and 30 nm structures were perpendicular to the larger structures. These structures may result from surface waves which have wavelengths both shorter and longer than that of the laser. One possible candidate is the surface plasmon polariton (SPP) wave, a coupled mode of an electromagnetic and plasmon wave traveling along a metal-dielectric interface. The plasmonic dispersion relation shows two branches which allows for both shorter and longer wavelengths than a photon of the same frequency. The longer wavelength branch explains the larger structures, while the shorter wavelength branch explains the smaller structures. The 30 nm ripple structures may be formed by an SPP propagating along the larger ripple structures, which can be approximated by modeling the surface plasmon behavior of nanorods.