The present paper studies the generation mechanism of terahertz (THz) radiation from tightly focused femtosecond laser pulses in a gas medium. We measured the angular radiation pattern under different focusing conditions and observed that, with the deepening of focus, the angular radiation pattern changes and optical-to-THz conversion efficiency increases. The analysis of the observed phenomena led to the assumption that the dipole radiation prevails in most cases despite the existing conception regarding the dominating role of the quadrupole mechanism of radiation. Based on these assumptions, the transient photocurrent theory of the phenomenon presented in this paper was developed by us and used for the numerical fit of the experimental data.

We show that an individual droplet of liquid metal can be a source of coherent terahertz radiation when it is excited by two femtosecond laser pulses of the same frequency. Under certain delays between these pulses, the intensity of terahertz radiation increases by more than 3 orders of magnitude. We describe the experimental results with the model of dynamic gain control, which considers the interaction of both laser pulses with the droplet and explains the terahertz-generation process by taking into account the dynamics of electrons and ions after photoionization of the metal droplet. The spatial distribution of terahertz radiation has a forward-directed contribution, whose polarization properties are well described by a nonlinear susceptibility of the second order. Our theoretical estimations based on the experimental data show that under the dynamic gain control the observed terahertz output can be considerably increased. Joint generation of x-ray, ultraviolet, and, as shown in the present work, terahertz radiation allows one to forecast that a free-falling photoexcited droplet of liquid metal is a promising source of multifrequency electromagnetic radiation for extreme nonlinear laser science.

In this paper we describe the properties of the crystal of guanylurea hydrogen phosphite (NH $$_2$$2 ) $$_2$$2 CNHCO(NH $$_2$$2 )H $$_2$$2 PO $$_3$$3 (GUHP) and propose its application in terahertz photonics and optoelectronics. GUHP crystal has a wide window of transparency and a high optical threshold in the visible and NIR spectral regions and narrow absorption bands in the terahertz frequency range. The spectral characteristics of absorption and refraction in the THz range were found to be strongly dependent on crystal temperature and orientation. Computer simulations made it possible to link the nature of the resonant response of the medium at THz frequencies with the molecular structure of the crystal, in particular, with intermolecular hydrogen bonds and the layered structure of the lattice. The possibility of application of the crystal under study for the conversion of femtosecond laser radiation from visible an NIR to terahertz range was demonstrated. It was shown that dispersion properties of the crystal allow the generation of narrow band terahertz radiation, whose spectral properties are determined by conditions close to phase matching. The properties of the generated terahertz radiation under various temperatures suggest the possibility of phonon mechanism of enhancement for nonlinear susceptibility of the second order.

In this review, we present a survey on the use of molecular nonlinear crystals in the context of terahertz (THz) photonics. The fundamentals of nonlinear optics for converting optical and infrared radiation into THz radiation with the basic theory of femtosecond optical rectification and difference frequency generation are described. Various types of phase-matching conditions that can be achieved in molecular crystals are discussed. It is shown that one of the unique features of molecular crystals is the ability to generate tunable narrowband terahertz radiation using femtosecond lasers. We also provide a detailed description of the most commonly used and promising molecular crystals such as DAST, DSTMS, OH1, HMQ-TMS, DCMBI, and GUHP. This review also presents a description of recent publications which show the prospects of using molecular nonlinear optical crystals in THz photonics.

The present article studies narrow-band terahertz (THz) emission stimulated by femtosecond laser pulse in molecular crystal guanylurea hydrogen phosphite (NH $_{2})_{2}$ CNHCO(NH $_{2}$ )H $_{2}$ PO $_{3}$ (GUHP). We demonstrate that this emission is closely connected with the excitement of high-quality phonon oscillations in the crystal, which is proved by the temperature dynamics of the spectra and DFT calculations. For the purposes of studying the origin of this stimulated THz emission and creation of the adequate model of the phenomenon, we analyzed the polarization sensitive spectra of spontaneous Raman scattering and THz transmission spectra while considering their polarization features in relation to crystallographic axes of GUHP crystal. In this paper we show that molecular crystals provide an effective means to convert vis-NIR laser light regardless of wavelength into the THz frequency range. This approach can lead to the creation of “laser-like” source with the desired THz frequency for a range of medical, scientific, and technological applications.