X-ray and y-ray lasers

Due to the many applications of laser light it is, of course, highly desirable to build lasers with very short wave-lengths. With respect to the X-ray and y-ray region so far only proposals exist. Possible laser-active quantum systems could be excited atomic nuclei which can emit y-rays. With respect to resonators the principle of the distributed feedback laser has been suggested. Because of the factor v² in the laser, the realization of such lasers seems to be very difficult, however.