## 2 Metasurfaces  
Metasurfaces, also known as 2D metamaterials, possess all the properties of traditional metamaterials while overcoming their limitations. In research in recent years, metasurfaces have been utilized in the design and fabrication of optical elements and systems, offering performance capabilities that exceed those of conventional diffractive optical elements [1]. Metamaterials and their planar versions, metasurfaces, are artificially constructed materials [2]. Their properties mostly depend on the physical characteristics of their structural elements but also on the geometry and dimensions of the unit cell [3]. Their extraordinary properties can be engineered and artificially refined, in contrast to the properties of ordinary, naturally occurring materials, which are constrained by their chemical composition [3; 4].  
The use of metasurfaces in industrial applications is growing and can be very different depending on the frequency range in which the metasurfaces are applied. Thus, metamaterials are widely used in the medical [5; 6], automotive [7], and aerospace sectors [8], as well as in devices such as biosensors [9; 10; 11], crowd control systems [12], absorbers [13; 14; 15], antennas [16; 17; 18; 19; 20], and energy harvesters [21; 22; 23; 24], but they also have wide application in sustainable sectors of the industry [25].  
The unique properties of metamaterials and metasurfaces depend, among other things, on the geometry of their unit cells. The unit cells serve as the foundation of a metamaterial.  
In the process of creating a metamaterial, it is important to differentiate between the various methods of connecting these unit cells [26]. The substrate of a metasurface can be composed of dielectric and applied conductive parts [27; 28; 29], although there are also metamaterials composed of dielectric materials only [30]. The geometry of a unit cell of a metasurface strongly depends on its target application and also on the frequency range in which this application will take place.  
Some of the unit cell geometries of metasurfaces mean that these structures can also find their application in the field of ecology and the 17 Sustainable Development Goals of the United Nations. One of the applications and processes already mentioned in this article, which is very actively used in the field of ecology, is the process of energy harvesting. Energy harvesting encompasses the process of capturing energy from waste, environmental, or mechanical sources and converting it into a usable form. This broad field includes well-established methods and materials [31]. Metasurfaces can be applied in the process of energy harvesting in the microwave range. The microwave signals can be rectified to direct current by a simple electronic circuit with a composed metasurface structure in it to provide the higher energy efficiency of the device. A metasurface structure compatible with the incident radiation range should be fitted to the proposed electronic circuit. Such an electronic device is an example of a sensor that, by means of a receiving antenna, a converter, and a metasurface, could be a self-powered device by means of microwave waves. In order to know what metasurface structure to use, or, more precisely, the geometry of the unit cell of the metasurface to be applied to the material substrate, one can be assisted by the intelligent learning tools of neural networks.