## 1 Introduction  
The demand for faster and more reliable wireless communication systems is constantly increasing, and traditional wireless technologies face challenges in meeting these demands [1]. Reconfigurable Intelligent Surfaces (RISs) have emerged as a potential solution to these challenges [2]. RISs are an innovative technology that uses electromagnetic surfaces to manipulate and control electromagnetic waves, offering a new way to improve wireless communication [3]. With the ability to create multiple virtual channels and selectively block or allow specific frequencies of electromagnetic waves, RISs can significantly enhance the performance of wireless communication systems, increase energy efficiency, and even enhance imaging and sensing technologies [4]. As a result, there has been significant research interest in RISs, and they are being investigated as a promising technology for the next generation of wireless communication systems. RISs are a class of metamaterials that consist of a planar array of closely spaced, subwavelength-sized elements that can manipulate electromagnetic waves in a controllable manner [2]. These surfaces are designed to modify the propagation and radiation of electromagnetic waves, enabling them to selectively reflect, refrac, amplify, attenuate, or even redirect electromagnetic waves in a desired direction [5]. RISs are \"intelligent\" because they can be programmed to adapt to changing environmental conditions, which makes them highly versatile and applicable to a wide range of wireless communication,sensing, and imaging applications [6]. By reconfiguring the geometry, material properties, or phase of individual elements on the RIS, the surface can be customized to meet specific application requirements [6]. RISs have the potential to revolutionize wireless communication, energy harvesting, sensing and imaging, privacy, and security technologies and applications [7]. The potential applications and benefits of RISs are improved wireless communications, enhanced energy efficiency, advanced sensing and imaging, and increased privacy and security [6]. Regarding wireless communication, RISs can create virtual channels between a transmitter and receiver, effectively increasing the capacity of wireless networks [5]. By selectively reflecting or refracting electromagnetic waves, RISs can also reduce interference, increase coverage, and improve the overall quality of wireless communications [5]. Concerning energy efficiency, RISs can be employed to improve the efficiency of energy harvesting technologies by manipulating the electromagnetic waves that are used to extract energy from the environment [8,9]. By focusing and amplifying these waves, RISs can improve energy conversion efficiency and reduce the size and cost of energy harvesting systems [9]. In terms of sensing and imaging, RISs can be used to enhance the performance of such technologies by manipulating the propagation and scattering of electromagnetic waves. By focusing, redirecting, or amplifying these waves, RISs can improve the resolution, sensitivity, and range of sensing and imaging systems [10]. Regarding privacy and security, RISs can be used to create \"smart\" environments capable of selectively blocking or allowing specific frequencies of electromagnetic waves [11]. This could enhance privacy and security by preventing unauthorized access to wireless networks, or creating secure areas protected from electromagnetic interference [12]. RISs are a relatively new area of research that has gained significant attention in the last few years. Using electromagnetic surfaces to manipulate and control electromagnetic waves has been around for decades, but reconfigurable surfaces are relatively new [6]. Here is a brief history of the development of RISs:  
* In the late 1990s and early 2000s, researchers began investigating the concept of metamaterials, artificially engineered materials that can manipulate electromagnetic waves in unique ways [13].