An unusual nonlinear resonance was experimentally observed in a ruby phonon laser (phaser) operating at 9 GHz with an electromagnetic pumping at 23 GHz. The resonance is manifested by very slow cooperative self-detunings in the microwave spectra of stimulated phonon emission when pumping is modulated at a superlow frequency (less than 10 Hz). During the self-detuning cycle new and new narrow phonon modes are sequentially "fired" on one side of the spectrum and approximately the same number of modes are "extinguished" on the other side, up to a complete generation breakdown in a certain final portion of the frequency axis. This is usually followed by a short-time refractority, after which the generation is fired again in the opposite (starting) portion of the frequency axis. The entire process of such cooperative spectral motions is repeated with high degree of regularity. The self-detuning period strongly depends on difference between the modulation frequency and the resonance frequency. This period is incommensurable with period of modulation. It increases to very large values (more than 100 s) when pointed difference is less than 0.05 Hz. The revealed phenomenon is a kind of global spin-phonon self-organization. All microwave modes of phonon laser oscillate with the same period, but with different, strongly determined phase shifts -as in optical lasers with antiphase motions.PACS numbers: 05.65.+b, 42.65.Sf, 43.35.+d Phonon amplification by stimulated emission of radiation was predicted theoretically in [1] and experimentally observed in microwave range by several groups [2] in 1960-1970-th. This phenomenon is very similar to the usual paramagnetic maser gain of electromagnetic field [3,4]. But if the phonon gain is large enough to exceed the phonon losses in the solid-state resonator, the self-excitation of laser-like phonon emission is possible [5,6,7]. The wavelength of generated microwavefrequency phonons in such phonon laser lies usually in optical or near-infrared range (due to very small velocity of sound in crystals, which is about 5 orders less than the light velocity). In this sence microwave phonon laser (phaser) is more close relative of the optical laser, than terahertz phonon laser (saser), having much shorter wavelength (see [8] and references therein).Almost all early experiments with microwave phonon lasers [5,6,7] were carried out in autonomous regime, when the control parameters of the active system remain unchanged during the whole time of measurement. Various regular and chaotic processes of generation of microwave phonons in a multimode nonautonomous phaser was experimentally observed and studied [9] for ω m ≈ ω R ≈ 20-300 Hz, where ω m is the pumping modulation frequency, ω R is the relaxation frequency of a nonequilibrium autonomous acoustic system (ω R depends on pumping level). The existence of a single dominating component exp(iω R t) for the transient processes in the autonomous phonon laser reflects the collective character of a multimode stimulated emission [10] in the quantum ...
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