We report on experimental evidence of unidirectional transmission of terahertz waves through a pair of metallic gratings with different periods. The gratings are optimized for a broadband transmission in one direction, accompanied with a high extinction rate in the opposite direction. In contrast to previous studies, we show that the zero-order nonreciprocity cannot be achieved. Nonetheless, we confirm that the structure can be used successfully as an asymmetric filter.
In this chapter, a review of the recent advances in optical metalenses is presented, with special emphasis in their experimental implementation. First, the Huygens' principle applied to ultrathin engineered metamaterials is introduced for the purpose of giving curvature to the wavefront of free-space wave fields. Primary designs based on metallic nanoslits and holey screens occasionally with variant width are first examined. Holographic plasmonic lenses are also explored offering a promising route to realize nanophotonic components. More recent metasurfaces based on nano-antenna resonators, either plasmonic or high-index dielectric, are analyzed in detail. Furthermore, 2D material lenses in the scale of a few nanometers enabling the thinnest lenses to date are here considered. Finally, dynamically reconfigurable focusing devices are reported for creating a scenario with new functionalities.
The force on a dielectric nano particle close to graphene sheet covering a plane dielectric sustrate is analyzed. We illustrate large pulling force due to the self-action effect of the particle through the graphene interface.
To focus the incident light, both convex metalenses with hyperbolic phase profile and autofocused Airy (AFA) beams are widely used. To obtain AFA-based bifocal metalenses, we propose two methods in this article. One is to bring the two conjugate focal points of an AFA beam into the real space by applying a proper convex lens phase profile. The other is to use the inefficient central space between the two launched Airy beams with zero amplitude and constant phase profile as an independent metalens. We also show that combining the two introduced methods, leads the design to upgrade to a multifocal structure.
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