Table-top femtosecond soft X-ray laser by collisional ionization gating
Abstract
The advent of X-ray free-electron lasers has granted researchers an unprecedented access to the
ultrafast dynamics of matter on the nanometre scale(1-3). Aside from being compact, seeded
plasma-based soft X-ray lasers (SXRLs) turn out to be enticing as photon-rich(4) sources (up to 10(15)
per pulse) that display high-quality optical properties(5,6). Hitherto, the duration of these sources was
limited to the picosecond range(7), which consequently restricts the field of applications. This bottleneck
was overcome by gating the gain through ultrafast collisional ionization in a high-density plasma
generated by an ultraintense infrared pulse (a few 10(18) W cm(-2)) guided in an optically pre-formed
plasma waveguide. For electron densities that ranged from 3 x 10(18) cm(-3) to 1.2 x 10(20) cm(-3), the
gain duration was measured to drop from 7 ps to an unprecedented value of about 450 fs, which paves
the way to compact and ultrafast SXRL beams with performances previously only accessible in
large-scale facilities.