ABSTRACT
The goal of progressively widening the range of wavelengths at which coherent monochromatic radiation is generated has been pursued ever since the successful demonstration of laser action in 1959. At long wavelengths the range extends through the far infra-red to microwaves and radio waves via molecular lasers, free electron lasers, masers and conventional radio transmitters, a range of ten orders of magnitude in wavelength. In contrast at short wavelengths lasing has been achieved relatively routinely into the far ultra-violet (1500Å), but only with difficulty and at limited wavelengths in the soft X-ray region (greter than 40Å)—a range of only two decades. This is despite the fact that short wavelength lasers offer significant technological reward in materials, biological and general research applications, and that as a consequence there has been significant international effort directed towards this goal. It may correctly be concluded from these general remarks that there are severe technological impediments to the development of X-ray lasers. These stem from two fundamental precepts which must always be born in mind in considering conceptual X-ray lasers.
Photon energies of the order of keV require large energy transitions, with correspondingly short lifetimes (picoseconds) in the lasant. In contrast visible photon energies of the order of eV, and time scales of microseconds, match un-ionised valence atomic transitions and electrical discharge pumping. In addition, the need to provide high excitation energy at high speed implies much higher pump powers, which must not only be delivered, but the rejected energy must be dissipated without destroying the laser.
Optical materials in the conventional sense do not exist at X-ray wavelengths. A visible laser is made up of a weak gain medium enclosed within a cavity, and at the simplest level requires mirrors, windows and lenses. These do not exist for 294X-rays, and in consequence the laser must operate in a high gain mode in which the spontaneous emission from one end is amplified along the element. This is a serious handicap as the coherence properties of a visible laser are established by the multiple transits of the light beam within the cavity, which through the amplification and long path progressively select the signal from essentially a single transition.
