ИСТИНА

In recent years, narrowband terahertz (THz) sources have been indispensable in various fields of photonics, one of the main ones being: creating compact and convenient table-top free-electron lasers; THz resonant nonlinear spectroscopy and the construction of THz LIDARs suitable for monitoring small gaseous components of the atmosphere at kilometer tracks in real time for the purpose of environmental control. One of the ways to generate THz radiation is a nonlinear source usage. Certain molecular crystals have resonant phonon modes falling in the low-THz frequency range. Despite the widespread point of view that low-energy phonons negatively affect the generation of THz radiation, at the same time, in a nonlinear medium with high-Q lattice vibrations at THz frequencies, one can expect both the presence of noncritical phase matching and an increase in nonlinearity in the resonance frequency region [1]. Molecular-crystalline guanylurea hydrogen phosphite (NH2)2CNHCO(NH2)H2PO3 (GUHP) exhibits high-Q vibrational resonances at THz frequencies [2-4]. We report on the excitation the molecular phonon subsystem and a temperature and axis orientation dependent multicycle quasi-pulse THz generation with a strongly phonon-coupled emission pattern in GUHP crystal. A THz generation model based on the resonant both Raman and IR vibrational response of a high-Q molecular-crystalline medium is constructed, where Stimulated THz emission in a molecular crystal can be polarization selectively generated, on the one hand, by simultaneous excitation of nonlinear processes of the second and third order, and, on the other hand, by ballistic excitation of a free damped dipole oscillator. We have also investigated the possibility of quasi-pulse THz generation in molecular crystals of phthalic acid: potassium, rubidium, cesium, ammonium and sodium acid phthalate; ortho-sulfobenzoic acid crystal and sucrose single crystal.