Perfect metal crystals are needed to bring plasmonics toward its limits. Surface plasmons enable appealing optical effects, one of them being a strong scattering of light with controlled amplitude and phase. This property is pivotal in the design of metasurfaces, artificial structures manipulating light beyond the Snell law. For example, beam steering can be achieved with gap-plasmon metasurfaces, which consist of an array of metal nano-antennas of different shapes deposited on a transparent thin spacer backed by a metal mirror. However, the optical power reflected in the visible by current metasurfaces of this kind remains lower than the theoretical optimum. One reason is the imperfect crystalline quality of the metal mirror. Indeed, it usually presents a polycrystalline structure and a rough surface inherent to the process used for its fabrication: evaporation. Therefore, to achieve gap-plasmon metasurfaces with mitigated losses in the visible, Sergejs Boroviks and coworkers used monocrystalline and smooth chemically prepared gold nanoflakes as a mirror. They quantified that a maximum efficiency gain near 5% can be achieved by such an approach, if limitations related with the nano-antenna fabrication are overcome.
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