Sub-Wavelength Terahertz Spin-Flip Laser Based on a Magnetic   Point-Contact Array
  We present a novel design for a single-mode, truly sub-wavelength THz disk laser based on a nano-composite gain medium comprising an array of metal/ferromagnetic point contacts embedded in a thin dielectric layer. Stimulated emission of light occurs in the point contacts as a result of spin-flip relaxation of spin-polarized electrons that are injected from the ferromagnetic side of the contacts. Ultra-high electrical current densities in the contacts and a dielectric material with a large refractive index, neither condition being achievable in conventional semiconductor media, allows the thresholds of lasing to be overcome for the lowest-order modes of the disk, hence making single-mode operation possible. 
R. I. Shekhter, A. M. Kadigrobov, M. Jonson, E. I. Smotrova, A. I.   Nosich, and V. Korenivski
cond-mat.mes-hall