Nanohole-array size dependence of soft x-ray generation enhancement from femtosecond-laser-produced plasma

Nanostructured targets are very attractive for enhancing the intensity of x-ray pulses generated from laser-produced-plasma. In order to clarify the enhancement mechanism, the nanohole-array size dependence of the characteristics of soft x-ray pulse generation from femtosecond-laser-produced plasma was investigated in detail. We found that the highest x-ray intensity can be obtained and the x-ray pulse duration kept relatively short with a nanohole-array alumina target with a 500 nm hole interval and a 450 nm hole diameter. A 40-fold soft x-ray fluence enhancement and a nine-fold soft x-ray pulse peak intensity enhancement can be obtained. The relatively short x-ray pulse duration of 19 ps can be maintained because the target structure has high local density and nanometer-sized spaces. Similar enhancement effects can be expected by using a nanostructured target with wall thickness of less than 100 nm, space size of around a few 100 nm, and nanostructure depth larger than 20 μm.