Because of high gain in the lasing medium, short upper-state lifetimes (1–100 ps), and problems associated with construction of mirrors that could reflect X-rays, X-ray lasers usually operate without mirrors; the beam of X-rays is generated by a single pass through the gain medium. The emitted radiation, based on amplified spontaneous emission, has relatively low spatial coherence. The line is mostly Doppler broadened, which depends on the ions' temperature.
As the common visible-light laser transitions between electronic or vibrational states correspond to energies up to only about 10 eV, different active media are needed for X-ray lasers. Again, different active media — excited atomic nuclei — must be used if yet higher frequency, gamma ray lasers are to be constructed.
The most often used media include highly ionizedplasmas, created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance with the Saha ionization equation, the most stable electron configurations are neon-like with 10 electrons remaining and nickel-like with 28 electrons remaining. The electron transitions in highly ionized plasmas usually correspond to energies on the order of hundreds of electron volts (eV).
The vacuum chambers at the PALS laboratory in Prague, where a 1 kJ pulse creates plasma for X-ray generation
Common methods for creating X-ray lasers include:
Capillary plasma-discharge media: In this setup, a several centimeters long capillary made of resistant material (e.g., alumina) confines a high-current, submicrosecond electrical pulse in a low-pressure gas. The Lorentz force causes further compression of the plasma discharge (see pinch). In addition, a pre-ionization electric or optical pulse is often used. An example is the capillary neon-like Ar8+ laser (generating radiation at 47 nm).
Solid-slab target media: After being hit by an optical pulse, the target emits highly excited plasma. Again, a longer "pre-pulse" is often used for plasma creation and a second, shorter and more energetic pulse is used for further excitation in the plasma volume. For short lifetimes, a sheared excitation pulse may be needed (GRIP - grazing incidence pump). The gradient in the refractive index of the plasma causes the amplified pulse to bend from the target surface, because at the frequencies above resonance the refractive index decreases with matter density. This can be compensated for by using curved targets or multiple targets in series.
Plasma excited by optical field: At optical densities high enough to cause effective electron tunnelling, or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize gas without contact with any capillary or target. A collinear setup[clarification needed] is usually used, enabling the synchronization of pump and signal pulses.
An alternative amplifying medium is the relativistic electron beam in a free-electron laser, which, strictly speaking, uses stimulated Compton scattering instead of stimulated emission.
Other approaches to optically induced coherent X-ray generation are:
^Chang, Zenghu; Rundquist, Andy; Wang, Haiwen; Murnane, Margaret M.; Kapteyn, Henry C. (20 October 1997). "Generation of Coherent Soft X Rays at 2.7 nm Using High Harmonics". Physical Review Letters. 79 (16): 2967. Bibcode:1997PhRvL..79.2967C. doi:10.1103/PhysRevLett.79.2967.
^Popmintchev1, Tenio; Chen, Ming-Chang; Popmintchev, Dimitar; Arpin, Paul; Brown, Susannah; Ališauskas, Skirmantas; Andriukaitis, Giedrius; Balčiunas, Tadas; Mücke, Oliver D.; Pugzlys, Audrius; Baltuška, Andrius; Shim, Bonggu; Schrauth, Samuel E.; Gaeta, Alexander; Hernández-García, Carlos; Plaja, Luis; Becker, Andreas; Jaron-Becker, Agnieszka; Murnane, Margaret M.; Kapteyn, Henry C. (8 June 2012). "Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers". Science. 336 (6086): 1287–1291. Bibcode:2012Sci...336.1287P. doi:10.1126/science.1218497. hdl:10366/147089. PMID22679093. S2CID 24628513.
^Whittum, David H.; Sessler, Andrew M.; Dawson, John M. (1990). "Ion-channel laser". Physical Review Letters. 64 (21): 2511–2514. Bibcode:1990PhRvL..64.2511W. doi:10.1103/PhysRevLett.64.2511. PMID10041731.