Nano-sized hollow bump array generated by single femtosecond laser pulse

The generation of a nano-sized hollow bump array of gold thin film by uniformly spaced melting and inflation of the film induced by a single shot of four interfering fs laser beams is reported. The shape changed from a bump, to a bead on bump, to a standing bead on a hole, and finally to a hole, with the increase in fs laser fluence. The bump height, diameter, and shape as functions of laser fluence are shown. Moreover, the basic shape of the bump changed upon changing the number of interfering beams.