Contemporary techniques of material fabrication allow one to produce stoichiometrically perfect ultrathin metallic films of controlled thickness down to a few monolayers [1-3]. Such films are the major components used to create ultrathin metasurfaces for advanced applications in modern optoelectronics and quantum optics including the ultrafast information processing, microscopy, imaging, sensing, and probing the light-matter interactions at the nanoscale [4,5]. As opposed to bulk metals whose electron plasma spectrum and associated optical response are controlled by the material band structure, the plasma properties and optical response of metallic films can also be controlled by adjusting their thickness, chemical composition, and by using properly chosen substrate and superstrate materials [6]. This unique tunability makes ultrathin metallic (or plasmonic) films an attractive platform for the design of advanced multifunctional metasurfaces, metasurfaces with reduced losses that are capable of utilizing the true quantum nature of light to achieve new improved functionalities [7].