Single-shot four-beam femtosecond laser interference processing of Pt thin films: From nanobumps to wall-like rims and substrate holes

Pt thin films with thicknesses of 5, 10, 20, and 50 nm on fused silica substrates, and 50 nm films on sapphire substrates, were processed by a single shot of a four-beam femtosecond laser interference pattern with a square-lattice period of 2.25 µm. The formation route branched drastically with the film thickness under the same interference pattern: nanobumps in the 50 nm films, openings with lifted edges in the 20 nm films, closed annular walls — wall-like rims — in the 10 nm films, and film removal with central substrate holes in the 5 nm films. The wall-like rim, approximately 80 nm high and much taller than the 10 nm film thickness, is the delaminated film itself standing on the substrate. Nanowhiskers with tip radii of approximately 10 nm or less were formed on the delaminated film edges. The structure diameters increased with fluence following a threshold-type behavior, and forming a nanobump of the same size on sapphire required an approximately 1.6 times higher fluence than on fused silica, consistent with the substrate thermal transport properties. The thickness dependence is explained by the absorptance-to-thickness ratio calculated by the transfer-matrix method.