ИСТИНА

In the past years multicycle terahertz sources became very demanded in different fields of photonics, some of the main of which are: creating compact and convenient table-top free-electron lasers, where THz frequency range paves a way to higher field strength for electron acceleration and deceleration with significant excess of damage threshold depending on central frequency and pulse duration; developing narrowband sources for the efficiency and frequency resolution increase in THz nonlinear generation physics and resonant nonlinear spectroscopy. The most constructively convenient way to generate THz radiation is a nonlinear source usage. Certain molecular crystals have resonant phonon modes falling in the low-THz frequency range. Despite a widespread point of view that low-energy phonons negatively affect the generation of THz radiation, since first order and second order susceptibilities are affected by the high Q-factor phonon resonances, we can tune the conditions in such a way that the nonlinear conversion efficiency will be extremely high and noncritical phase matching conditions will be achieved only in a narrow spectral region around the phonon peaks. Semiorganic molecular crystal guanylurea hydrogen phosphite (NH2)2CNHCO(NH2)H2PO3 (GUHP) has several phonon resonances in the THz frequency range, which are defined by the molecular basis structure glued with hydrogen bonds and may be selectively tuned by the orientation and temperature of the sample [1-3]. We report on the nonlinear excitation the molecular phonon subsystem and multicycle THz pulse gen-eration in GUHP crystal. The generated THz pulses are multicycle and thus have narrow-band emission spectra, which width can be reduced up to 6.2 GHz, and the peak position may be tuned from 1.03 to 1.67 THz. Based on the THz-TDS and Raman experimental re-sults we assume that the spectral properties of generated THz radiation in GUHP molecular crystal are determined by the stimulated Raman scattering on the both IR- and Raman- ac-tive vibrational resonances. We have also investigated the possibility of narrowband THz generation in molecular crystals of phthalic acid: potassium, rubidium, cesium and ammonium acid phthalate. It turned out that, despite the same syngony and the presence of absorption peaks in the THz frequency range, the narrowband THz generation is possible only in crystals with all both IR- and Raman- active vibrational modes.