@@ A novel spintronic -based method of generating THz radiation is proposed. The method is based on pumping of non-equilibrium electrons into the upper (spindown) sub-band of spin-polarized half-metallic ferromagnets or ferromagnetic semiconductors, which makes it possible to build tunable, narrow-band, high-power THz sources. Non-equilibrium electrons pumped into the spin-down subband rapidly emit magnons with THz frequency, pass into highly excited states of the spin-up subband, and fall into the ground state due to interaction with the equilibrium spin-up electrons or by emitting optical phonons. The mechanism of magnon generation is similar to a three-level conventional laser, and at a critical pumping intensity, which depends on the magnon damping, magnon lasing begins. In this regime the number of excited magnons increases exponentially with time. Merging of two magnons with frequency/and wave vectors q and -q generates a THz photon with frequency 2f. Thus, a magnon laser becomes a THz photon laser.
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