An X-ray laser is a laser device which emits in the spectral region of X-rays, i.e., with wavelengths of only a few nanometers. In this extreme spectral region, traditional laser gain media can not be used. There are different technical approaches:
- The gain medium of a free electron laser is an undulator, through which very high-energy electrons are sent. For a combination of high-electron energy and short undulator period, very short wavelengths are reached .
- Optical gain in the X-ray region can also be generated in plasmas, which themselves are typically created with laser beams or with electrical discharges. (The first such lasers were pumped with X-rays from nuclear explosions.) The laser transitions are between excited states of highly charged ions (e.g. Ta45+ for 4.5 nm and Se24+ for 20 nm).
As it is difficult to construct a low-loss laser resonator for such short wavelengths, X-ray lasers are in most cases (particularly for shorter wavelengths) built without a resonator and actually operate as ASE sources (superluminescent sources). This, however, decreases the spatial and temporal coherence. Much higher spatial and temporal coherence as well as an increased output power can be achieved with injection seeding, if a suitable narrow-band seed source is available.
Lasers can also be used in other ways for generating X-rays, in particular with high harmonic generation, which is a kind of nonlinear frequency conversion. The optical power achievable in such high harmonics is fairly low, but it is possible to amplify certain harmonics in a plasma [8, 13].
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