Enhanced X-ray generation from femtosecond-laser-produced plasma by using a nanohole-alumina target

X-ray generation from femtosecond laser-produced plasma is an attractive way to obtain short pulse X-rays. However, due to the formation of solid density plasma at a target surface, most of the incident femtosecond laser pulse is reflected and thus the conversion efficiency is limited. There is a need for higher efficiency of conversion to the X-ray wavelength region from the viewpoint of practical application. One way to increase the conversion efficiency is to form a pre-plasma before the incidence of an intense main pulse. X-ray generation enhancement by using a femtosecond pre-pulse has been demonstrated by several authors. However, the emitted X-ray pulse duration becomes considerably long with this method. Another attractive way to increase the efficiency is to adopt a structured surface target. Experiments with metal (gold and aluminum) cluster targets made by evaporating metal in a background of several Torr of gas and porous Si targets made by anodizing have been demonstrated. They achieved X-ray conversion efficiency enhancement of one or two orders of magnitude in hard X-ray energy regions (>1 kev). However, no large enhancement was obtained in soft X ray energy regions (<1 kev). The report shows that by using a nanohole-alumina target made by anodizing an Al plate (99.99%), X-ray emission can be enhanced even in soft X-ray energy regions (<1 kev).