X-ray laser is a device that uses stimulated emission to generate or amplify the electromagnetic radiation in the near X-ray or extreme ultraviolet region, usually in the order of several nanometer or tens of nm.
Because of high gain in the medium, short upper state lifetime (1 - 100 ps) and problems associated with construction of X-ray mirrors, the X-ray laser usually operates without any resonator. The emitted radiation based on amplified spontaneous emission has relatively low spatial coherence. The line is mostly Doppler broadened (which depends on the ion temperature).
As the common laser transitions between electronic or vibrational states correspond to energies up to 10 eV, a different active medium is needed.
X-ray laser active media
Most often used media include highly ionized plasma created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance to the Sah 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 plasma usually correspond to energies in the order of 100s eV.
* Capillary plasma discharge medium: In this setup, a several centimeters long capillary made of resistant material (e. g. alumina) confines a high current, sub microsecond electrical pulse in low-pressure gas. The Lorentz force causes further compression of the plasma discharge (see pinch). A pre-ionisation electric pulse and/or optical pulse is often used. An example is the capillary neon-like Ar8+ laser (generating at 47 nm).
* Solid slab target medium: After being hit by optical pulse, the target emits highly excited plasma. Again, a longer prepulse is often used for the 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 refractive index gradient causes the amplified pulse to bend from the target surface. This can be compensated using curved target or multiple targets in series.
* Plasma excited by optical field: At optical densities high enough to cause effective electron tunelling or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize the gas without contact with any capillary or target. Usually a collinear setup is used, enabling to synchronize pump and signal pulses.
Alternative amplifying medium is the relativistic electron beam in free electron laser.
A completely different approach to X-ray generation is the high-harmonic generation.
Applications of X-ray coherent radiation include research of dense plasma (not transparent in optical region), X-ray microscopy, material surface research.
* Strategic Defense Initiative X-ray laser