Since its invention, metasurface has been widely utilized to achieve nearly arbitrary wavefront control based on phase-only modulation at a single wavelength. To achieve better performance or exotic functions, it is desirable to demonstrate metasurfaces capable of realizing both phase and amplitude modulations. Meanwhile, the wavelength-dependent behavior of the metasurface is one of the critical limitations in existing metasurface structures. Specifically, single-layer metasurfaces with the capability to tailor both phase and amplitude at multiple wavelengths have not been reported so far. Here, we propose a class of ultrathin metasurfaces with complete phase and amplitude modulations at two THz wavelengths. These metasurfaces enable dual-wavelength metalenses and nondiffractive Airy beam generators operating at two THz wavelengths.

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The Airy beam has attracted considerable interests due to its remarkable non-diffracting, self-bending, and self-healing properties. An auto-focusing Airy (AFA) beam can be achieved by constructing two counter-propagation Airy beams, which is a novel method to generate focusing effect. Traditionally, because of the complexity of the generation mechanism, it is hard to realize the Airy beam with complex amplitude modulation. Here, a general scheme is proposed to launch dual-band Airy and AFA beams based on the single-layer geometric metasurface, consisting of a newly designed meta-atom that can realize independent complex amplitude modulations at two arbitrary terahertz wavelengths. Based on the proposed meta-atom, the aforementioned attractive properties of the Airy beam are demonstrated at two terahertz wavelengths. Besides, the dual-band AFA beams are illustrated to have the practically important adjustability of the focal length. 

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