Efficiency of spin-injection terahertz oscillator
  Energy efficiency in terahertz range is evaluated experimentally of a spin-injection oscillator based on a ferromagnetic rod-film structure with point contact between the components. Choice of the film material influences substantially the efficiency. A magnetic flux concentrator is used to improve the efficiency. It is found from the measurements that the quantum efficiency can exceed unity. The latter indicates substantial contribution of stimulated radiative transitions. 
Yu. V. Gulyaev, S. G. Chigarev, I. V. Malikov, G. M. Mikhailov, P. E.   Zilberman, E. M. Epshtein
cond-mat.mtrl-sci