An X-ray laser (or Xaser) is a device that uses stimulated emission to generate or amplify electromagnetic radiation in the near X-ray or extreme ultraviolet region of the spectrum, that is, usually on the order of several of tens of nanometers (nm) wavelength.
Because of high gain in the lasing medium, short upper-state lifetimes (1–100 ps), and problems associated with construction of X-ray mirrors, X-ray lasers usually operate 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 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.
Read more about X-ray Laser: X-ray Laser Active Media, Applications