High-efficiency generation of narrow-band soft x-rays from carbon nanotube foams irradiated by relativistic femtosecond lasers

A number of applications such as photoelectron spectroscopy require x-rays of both a high flux and a narrow bandwidth. In this work, we experimentally demonstrate the high-efficiency generation of narrow-band soft x-rays from carbon nanotube foams irradiated by a femtosecond laser pulse at the intensity of 10^19 W/cm². The building blocks of the foam, single-walled carbon nanotube bundles with diameters smaller than the laser skin length, can be volumetrically heated and fully ionized on a femtosecond timescale. The three-dimensional network structure of the foam permits a deep penetration and a drastic absorption of the laser pulse, and results in bright line emissions without prominent Stark broadening. A single-shot yield of 3×10^14 photons in the carbon Lyα line at 3.37 nm was measured with a bandwidth of 0.013 nm.