## 2 Summary of the Articles of the Issue
Thus, it has been demonstrated that low-cost Bi\({}_{1.9}\)Sb\({}_{0.1}\)Te\({}_{2}\)Se ultrathin topological insulators, which can be easily integrated into various nanophotonic devices, enable the conversion of femtosecond infrared radiation to terahertz radiation. The research presented in [7] focuses on the development and investigation of a new type of robust photonic topological insulators for laser array applications. It was constructed from identical prism resonators connected to each other. The author experimentally demonstrated a topologically protected propagating state due to the disconnected faces of the edge resonators. A new and accurate, fully explicit finite-difference time-domain method for modeling a nonlinear electromagnetic medium has been proposed in theoretical work [8]. The developed algorithms describe the interaction of the laser pulse with metals and interfaces of nonlinear dielectric media well, where Kerr and Raman effects, as well as multiphoton ionization and metal dispersion, occur simultaneously. The author also modeled the nonlinear propagation of an ultrafast laser pulse through a dielectric medium using this method. In [9], the generation of nanosecond photocurrent pulses due to the surface photogalvanic effect (the second-order nonlinear optical phenomenon) was studied in thin semitransparent CuSe/Se films as a function of the angle of incidence and polarization of exciting femtosecond laser pulses at 795 nm. It was found that the evolution of the temporal profile of the helicity-sensitive transverse photocurrent with a change in incident angle strongly depends on polarization. In particular, at circular polarization, the generation of unipolar and bipolar pulses is possible, with the waveforms strongly depending both on the angle of incidence and the sign of circular polarization. A kinetic theory of the edge photogalvanic effect for the intraband electron transport in two-dimensional materials was developed in [10]. It should be noted that a photogalvanic effect in 2D structures can be considered as a low-dimensional analog of the surface photogalvanic effect. The authors obtained an analytical expression for the edge current valid for arbitrary dispersion law and scattering mechanism and analyzed the result for single-layer and bilayer graphene for electron scattering by short-range defects and Coulomb impurities. The authors believe that the edge photogalvanic effect will find applications in fast terahertz radiation and terahertz radiation polarization sensors. This work was supported by the Ministry of Education and Science of the Russian Federation (state registration number 1021032422167-7-1.3.2.). **Acknowledgments:** I consider it my duty to express my gratitude to all the authors of the articles who sent their insightful papers to this Special Issue. Special thanks to all the reviewers who provided substantive comments and suggestions to improve the quality of the submitted articles. I am grateful to the entire editorial team of _Applied Sciences_ for the high editorial quality. I would also like to thank [PERSON] for discussing the articles published in this Special Issue. **Conflicts of Interest:** The author declares no conflict of interest.