Active samples were cut from monocrystaline Ge(Ga doped) with an impurity con- centration of NA=7 x1013 cm-3 in the form of a rectangular parallelepiped, cooled by liquid helium and placed in a superconducting magnet. The magnetic field was directed along the samples' long axis. A pulsed electric field was applied to the lateral sides through evaporated ohmic contacts, thus E was orthogonal to H. The laser generation in the far-infrared was due to the gain on intersubband hole optical transitions.
V. N. Shastin,
"Linewidth of the p-ge laser", Proc. SPIE 1929, 17th International Conference on Infrared and Millimeter Waves, 19292K (14 December 1992); doi: 10.1117/12.2298209; https://doi.org/10.1117/12.2298209