Excitation, Relaxation, and Continuous Maser Action in the 2.613-Micron Transition of Ca${\mathrm{F}}_2$:${\mathrm{U}}^{3+}$

G. D. Boyd, R. J. Collins, S. P. S. Porto, A. Yariv, and W. A. Hargreaves

Phys. Rev. Lett. 8, 269 – Published 1 April 1962