The nonlinear response of liquids in the terahertz regime has recently attracted significant interest, even though very few measurements have been reported. Here, we report on our measurements based on a $z$-scan technique of the nonlinear refractive-index coefficient $n_2$ at terahertz frequencies for several liquids with noncentrosymmetric molecules, specifically, water, ethanol, and $\alpha$-pinene. We describe how the value of $n_2$ depends on the physical parameters of these molecules. The measured values of $n_2$ of the liquids in the terahertz region are as much as 6 orders of magnitude larger than their corresponding values in the visible or near-IR. Through a simple theoretical model, we confirm that the predominant source of this large third-order nonlinearity is the second-order perturbative component of the vibrational response of these molecules, which have resonances in the mid-IR.

A method is proposed for accelerating measurements for ghost imaging reconstruction of objects using spectral multiplexing of speckle patterns formed by broadband terahertz (THz) radiation. The presented method aims at the challenge of a sufficiently large number of measurements, and increases the correlation between reference and reconstructed images. It is experimentally shown that unique speckle patterns for individual frequencies can be formed by broadband THz radiation while propagating through a random phase plate. It is shown by numerical simulation that these speckle patterns can be used to decrease the number of diffusers required by more than one order compared to the integrated method. The possibility of using electro-optical detection in the proposed technique reduces the number of measurements. For example, for the spectral range from 0.2 to 1.2 THz, there is a more than 20-fold decrease in the number of diffusers used.

Metal halide perovskite nanocrystals (NCs) attract much attention for light-emitting applications due to their exceptional optical properties. More recently, perovskite NCs have begun to be considered a promising material for nonlinear optical applications. Numerous strategies have recently been developed to improve the properties of metal halide perovskite NCs. Among them, B-site doping is one of the most promising ways to enhance their brightness and stability. However, there is a lack of study of the influence of B-site doping on the nonlinear optical properties of inorganic perovskite NCs. Here, we demonstrate that Cd²⁺ doping simultaneously improves both the linear (higher photoluminescence quantum yield, larger exciton binding energy, reduced trap states density, and faster radiative recombination) and nonlinear (higher two- and three-photon absorption cross-sections) optical properties of CsPbBr₃ NCs. Cd²⁺ doping results in a two-photon absorption cross-section, reaching 2.6 × 10⁶ Goeppert-Mayer (GM), which is among the highest reported for CsPbBr₃ NCs.

Laser-driven nonlinear phenomena can both reveal the structural features of materials and become the basis for the development of various translated technologies, including highly intense terahertz sources. Here we realize a modified single-color double-pulse excitation scheme for enhancing the terahertz wave generation in flat liquid jets, and we show that the pre-ionization effect is crucial for finding the optimal input conditions. The experimental results, being supported by numerical simulations, reveal the preference for longer pre-pulses to induce the effective ionization process and shorter signals for the strong laser-plasma interaction. In addition to the identified features of the terahertz wave energy enhancement with respect to the duration change for both pulses and their ratio variation, we state the possibility of achieving the optical-to-THz conversion efficiency value up to 0.1% in the case of double-pulse excitation of an α-pinene jet.

In this paper, we demonstrate experimentally for the first time (to our knowledge) the generation of spectral supercontinuum (SC) of more than two octaves with high intensity in a water jet. The spectrum of the generated SC extends from 350 to 1400 nm, with intensities up to ${10}^{11}\mathrm{W}/{\mathrm{cm}}^2$, and its generation efficiency is more than 50%. For the pump intensity $3.0\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ in the spectral range from 400 to 800 nm, the spectrum is nearly flat (less than 40% deviation), which is useful for many applications.

Our recent theoretical studies of generation of ultrashort pulses based on collective spontaneous emission from a thin resonant medium layer excited by a pair of high-frequency (compared to the transition frequency of the medium) pulses are reviewed. The source of generated radiation is a resonant polarization pulse in the medium (“stopped polarization pulse”), which is excited and switched off by a pair of light pump pulses. This method allows obtaining attosecond pulses in the optical and ultraviolet ranges, as well as femtosecond terahertz pulses. The effect of the superradiance in generation is discussed. The use of coherent and collective processes makes it possible to achieve a high efficiency of the generation of terahertz radiation. The proposed method is an alternative method of obtaining ultrashort pulses compared to the currently well-developed methods such as the high-harmonic generation method.