Extreme Ultra-violet and X-ray Lasers

Extreme ultra-violet (EUV) lasers have been produced by focussing infra-red laser beams in a line focus onto suitable solid targets. Lasing occurs along the line focus length due to the amplification of spontaneous emission without the need for mirrors. Usually, a plasma is formed using a pre-pulse incident in a line focus. Plasma expands normally to the solid target surface and another pumping laser pulse is incident into the expanding plasma. Ionisation of the plasma material to Ne-like or Ni-like ionisation stages occurs with electron collisional excitation in these ionisation stages producing population inversions. A review of the physics of EUV lasers has been published. Pumping with laser pulses of energy less than 1 J has been recently achieved using grazing-incidence pumping where the pumping laser is incidence at a grazing angle (14 - 20 degrees). The laser energy here is deposited at optimised electron densities below the laser critical density.

A schematic of the creation of an EUV or x-ray laser. plasma (orange) produced by focusing an infra-red laser (red) in a line.
Figrue 1. shows a schematic of the creation of an EUV or x-ray laser. plasma (orange) produced by focusing an infra-red laser (red) in a line. The EUV output is used for application experiments such as the probing of another target in order to measure opacity or laser ablation [see web reference to Laser ablation and plasma opacity measurements above].

EUV lasers have been demonstrated as probes of laser ablation rates. Ablated material becomes transparent to EUV radiation and so the EUV transmission enables the thickness of solid material remaining to be measured and hence the rate of laser ablation (see our recent published paper) Alternatively, if a solid target of mainly plastic with a buried layer of material such as 50 nm thick iron is probed by an EUV laser, it is possible to measure the plasma opacity. Figure 1. shows a schematic of an experiment where an EUV laser probes a sample target that has another incident infra-red laser acting to ablate material. The opacity of plasma material as found towards the centre of the Sun has been recently measured.