## 1 Introduction
In [29], a significant change in the absorption capacity of metallic materials depending on the thickness of the oxide film was reported, as well as the need to consider thermochemical processes on metal surfaces in an oxygen-containing environment. These processes significantly change the heating dynamics since the absorption capacity significantly changes as a consequence of oxide film formation, which in turn changes the temperature time dependences, introducing a significant non-linearity in them [30]. The high spatial coherence of the laser beam makes it possible to achieve good focusing, which in turn facilitates the occurrence of chemical reactions, including the heterogeneous oxidation of metals, locally, with high spatial resolution [31]. Chemical processes in the kinetics of which the thermal effect of the laser beam plays an essential role and which occur in conditions close to thermodynamic equilibrium are the subjects of the study of laser thermochemistry [32]. Under these conditions, the problems to be solved may be related to the particularities of the formation of a spatial and temporal distribution of the temperature field in a chemically active medium under the influence of a laser beam [33]. Nonisothermicity provides the opportunity to effectively control the chemical processes, and laser irradiation can be a unique tool to achieve this goal. Successful implementation of such control is only possible if a certain spatial intensity profile is formed in the target area of the material. In this case, the desired result of laser-matter interaction can only be achieved by using appropriate beam parameter conversion systems that can produce the required temperature exposure [34]. The effect of such laser irradiation on opaque materials is largely determined by the capability of the optical system to redistribute the energy over their surface with a predetermined intensity [35]. The variety of optical systems in use and under development demonstrates the widespread interest of researchers on this important topic [36]. If an optical element is specified, then by solving the wavefront shaping problem, we can obtain the field distribution in the area of interest. This is the forward problem of focusing [37]. In order to create an optical element that focuses the beam into a desired region of space, it is necessary to solve the inverse problem of focusing. In this case, it is necessary not only to focus the beam into a predetermined area of the focal plane, but also to form in it a predefined intensity distribution [38; 39]. From a mathematical point of view, the problem of creating an optical element that produces the desired wavefield is incorrect: firstly, the solution may not exist at all; secondly, it may be unstable. However, a solution to such problems is possible [40; 41]. A typical inverse problem of focusing is the problem of an optical element that focuses the beam into a predetermined region of space, e.g., the focusing of a beam into a certain figure or a line in the focal plane. Since only the intensity distribution is specified in the focusing region and the phase is arbitrary, the specified degree of freedom makes it possible to design such an optical element [42]. As a rule, it is necessary to create such dimensions of the focusing area, which are comparable with the width of the diffraction-limited spot; therefore, the diffraction has a decisive role in the formation of the predetermined intensity profile. Some tasks require focusing the beam into a zone, the longitudinal dimensions of which are much larger than the transverse dimensions, usually coinciding with the width of the diffraction-limited spot. In such cases, the problem of beam focusing into a line (a segment, a set of segments) or a curve is solved [43]. In [44; 45; 46], new techniques of pulse-periodic laser irradiation were created and a number of nanomaterials ranging from nanoporous to layered ones based on oxide-metallic nanowires were synthesized.