X-ray laser

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 (1100 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. Again, different active media — excited atomic nuclei — must be used if yet higher frequency, gamma ray lasers are to be constructed.

X-ray laser active media

The most often used media include highly ionized plasmas, 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:

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.

A different approach to optically induced coherent X-ray generation is high-harmonic generation, stimulated Thomson scattering or radiation produced by oscillating electrons during laser wakefield acceleration.

Applications

Applications of coherent X-ray radiation include research into dense plasmas (not transparent to visible radiation), X-ray microscopy, phase-resolved medical imaging, material surface research, and weaponry.

A soft x-ray laser can perform ablative laser propulsion.

See also

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