  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.
