Kinetic Laser

A traditional laser involves the stimulated downward electron transition in an atom in a background “sea of photons,” with the emission of a characteristic photon matching in fequency and momentum the stimulating photons. This stimulated emission process was predicted by Einstein. In 1951, J. Weber at the University of Maryland was the first to calculate the operating principles of the ammonium maser and laser. However, as the story goes, upon asking for research monies to build the maser, a few hundred thousand dollars from the university, he lost out to the athletic department’s request for money to build up the Maryland football program.

The first operating ammonium maser was subsequently built by C. Townes in 1954, and the first operating device lasing in the optical part of the spectrum was built in 1960 by T. H. Maiman. Laser action first in the microwave region is no coincidence, for spontaneous emission is proportional to the cube of the transition frequency, and being extremely small in this part of the spectrum, can be neglected compared to stimulated emission and absorption.

Among the more exotic lasers is the kinetic laser, which is an “exploding” material that emits light and X-rays. In its simplest form, the material would be a foil of a single element such as copper that is exploded by focusing powerful laser pulses on it. How does this type of laser produce coherent laser light?

Answer

The explosion of the lasing material creates many free electrons, some of which have been blown out of lowlying atomic states, creating the needed population inversion for possible lasing action. Practically any material can be used. During an extremely short time interval after the explosion on the order of nanoseconds stimulated emission may occur as photons from the exploding material exit the expanding blast volume. These photons pass through regions of the expanding cloud of ionized debris and can stimulate the emission of many more photons into the same quantum state at the same wavelength. The resulting coherent radiation at many frequencies, including the soft X-ray region, will show intensity spikes in particular directions.

Some of the first kinetic laser explosions were done at Livermore National Laboratory in the 1970s and 1980s with exploding foils and the Nova laser system. Since the first demonstration of soft X-ray lasing emissions at about 10 nanometers or more using the collisional excitation mechanism in neon like selenium, many other neon like ions, ranging from copper (Z = 29) to silver (Z = 47), have been made to lase. However, attempts to produce lower-Z neon like X-ray lasers have been unsuccessful.

In the effort to develop a tabletop X-ray laser that would require smaller high-energy laser drivers than Nova and that could be used for applications such as biological imaging, nonlinear optics, holography, and so on, a prepulse technique has been developed. This technique has been used successfully to produce lasing in many lower- Z neon like ions such as titanium (Z = 22), chromium (Z = 24), iron (Z = 26), and nickel (Z = 28). The use of this prepulse technique has opened up a new class of neon like X-ray lasers for investigation.