Abstract
Our previous paper reported on pulsed CO2 laser induced periodic microstructures on the silicon substrates, which are coated with a thin material layer at the bottom surface to promote optical absorption. Our further study shows that two sets of parallel fringes generated at right angles can overlap and produce crossed-grid patterns when the laser pulse energy just crosses the thershold required for fringe formation. The fringe period is of sub-wavelength scale and the crossed-grid pattern is of grid size 2 μm by 2 μm. The fringes are considered to be due to the laser induced periodic surface structure (LIPSS) effect. The crossed-grid patterns are only possible when thermal bumps due to the thermal capillary wave effect are minimized, and they become re-writable at higher pulse energies.