Generation of coherent phonons in ruby

In conditions of inversion of spin-level populations at T = 1.7° K the resonant stationary coherent phonon emission has been revealed in ruby at frequency ν = 9.12 GHz. The intensity of emission is I_S ≈ 10^?6 W·cm^?2. The frequency spectrum consists of very narrow lines generated simultaneously on several modes of hypersonic ruby cavity. Monochromatism of emission in each line is Δν/ν ? 2 × 10^?6. The spectrum nature permits to suppose the levelwidth of elementary spin excitation to be Δν_s < 3 × 10⁵ Hz, i.e. at least 2–3 orders it is narrower than the inhomogeneously broadened resonant line in ruby. The lifetime of resonant phonons in ruby was defined: τ = 7 × 10^?6 sec.