Maser amplification of 9.5-gigahertz elastic waves in sapphire doped with divalent nickel impurity ions.
The three spin energy levels of divalent nickel impurity ions in sapphire interact strongly with ultrasonic waves whose frequency corresponds to certain allowed transitions between the levels. Under population inversion of the levels it is possible to achieve significant amplification of very high frequency ultrasonic waves by stimulated emission from the spin system.
Peterson
P D
PD
Jacobsen
E H
EH