## 2 Summary of the Articles of the Issue  
This Special Issue presents the results of studies conducted in various fields of nonlinear optics. These studies range from experimentally investigating the generation of higher harmonics of laser radiation in laser-induced plasma to theoretically examining the generation of edge currents in graphene caused by second-order nonlinearity. In [1], the peculiarities of the generation of the 17 th and 18 th harmonics of the femtosecond laser radiation (at a wavelength of 806 nm) in the laser plasma were studied. The plasma was excited in bulk Sn and Sn nanoparticles glued onto a glass substrate by powerful, focused radiation of picosecond and nanosecond lasers. Additionally, plasma probing was performed using bi-harmonic radiation at wavelengths of 806 and 403 nm (the second harmonic of a femtosecond laser) as well as the radiation of a femtosecond parametric light generator (1280-1440 nm) and its second harmonic in the single-frequency and double-frequency modes. It is shown that the use of a two-frequency mode allows achieving a 12-fold increase in the intensity of even harmonics. It was found that for the effective generation of higher harmonics pulses, it is necessary to provide an optimal delay (130-180 ns) between the plasma excitation laser pulses and pulses of probing radiation. The results of a study on the generation of higher harmonics in the wavelength range of 30-100 nm by a femtosecond laser at 806 nm in a laser-induced plasma from six carbon-containing materials (graphite, fullerenes, carbon nanotubes, carbon nanofibers, diamond nanoparticles, and graphene) are presented in [2]. It was demonstrated that the presence of nanoparticles (C\({}_{60}\), carbon nanofibers, and multi-walled carbon nanotubes) and their fragments in the laser plasma leads to an increase in the efficiency of 9-17 th harmonic generation at wavelengths of 50-95 nm. The generation of laser radiation harmonics can be utilized to develop unique tools for studying the surfaces of materials. In [3], magnetization-induced second and third harmonic generation of a femtosecond laser radiation of an optical parametric oscillator at 740-1800 nm has been applied for the visualization of the interface magnetic domains of 10 \(\upmu\)m thick (LuBi)\({}_{3}\)Fe\({}_{5}\)O\({}_{12}\) garnet film. It has been shown that the technique of nonlinear optical microscopy based on third harmonic generation provides better spatial resolution compared to microscopy based on second harmonic generation. In the paper [4], comparative studies of the nonlinear optical properties of aqueous suspensions of high pressure-high temperature (HP-HT) nanodiamonds and detonation nanodiamonds under femtosecond laser excitation are performed. It was shown that at the same parameters of femtosecond laser pumping and experimental conditions, saturable absorption occurred in an aqueous suspension of detonation nanodiamonds accompanied by short-term nonlinear bleaching, while in an aqueous suspension of HP-HT nanodiamonds, two-photon absorption appeared, leading to optical limiting. In study [5], the nonlinear up-conversion luminescence in the aqueous suspensions of carbon dots with polyfunctional and carboxylated surfaces synthesized by a hydrothermal method was observed. It was shown that the discovered nonlinear up-conversion luminescence in these carbon dots was caused by two-photon absorption. The optimal laser excitation wavelength at which nonlinear up-conversion luminescence manifests itself most strongly was found. The results obtained can be used to determine the pH of liquid media in a wide range of pH values. A 40 nm thick film of topological insulator (Bi\({}_{1.9}\)Sb\({}_{0.1}\)Te\({}_{2}\)Se) was used for efficient generation of terahertz radiation under femtosecond laser radiation at 1560 nm excitation in [6]. It was shown that the generation of terahertz radiation in this film occurred due to photo-excited bulk carriers and their relaxation through the surface states in the presence of the constant electric field.