Collision Lasers

Abstract

Reliance on spontaneous emission to relax the lower level limits the efficiency and the power output from gas discharge lasers. The level spacing required for an untrapped radiative cascade places the laser levels well up in the energy level structure. The consequence is low quantum efficiency, a small fraction of electrons with the necessary energy for excitation, and parasitic loss through excitation of lower levels. A judicious combination of inelastic electron-atom and atom-atom collisions for excitation and relaxation can maintain a population inversion in appropriate atoms. This is possible because transfer rates between levels due to atom-atom collisions are small if the energy defect substantially exceeds the average kinetic energy. Thus collisions with other atoms can selectively relax the lower level to other nearby levels without reducing the upper level population. In favorable cases we expect laser power of order 1 W/cm³ with an efficiency greater than 10%. Temperatures above 1000° are needed to produce a monatomic gas of most elements with suitably spaced low-lying levels. Two such elements are manganese and lanthanum. Mn can be contained in alumina tubing at 1350°; La in tantalum tubing at 1800°. Diffusion to cold windows is prevented by a noble buffer gas. Apparatus for testing each of these systems has been constructed. Xenon test oscillations have been observed in the apparatus at the working temperature. Measurements of fluorescence in the prospective laser transitions are being carried out.