Tailoring the geometry and arrangement of metasurface structures yields a complete control of the reflection/transmission amplitude, phase, and polarization states. The planar nature of the structures can be readily fabricated using existing technologies, which makes them more accessible particularly in the optical regime and potentially revolutionizes the design of integrated photonics. Although the ultrathin thickness in the wave propagation direction can greatly suppress the absorption, the impedance mismatching in many metasurfaces results in undesirable high insertion losses, and the performance of single-layer metasurface devices is yet unsatisfying in real world applications. In contrast, few-layer meta-surfaces can circumvent this impedance mismatching issue. Recently, we have developed a three-layer metasurface structure that is capable of rotating the incident linear polarization by 90° with a very high efficiency over a bandwidth of nearly two octaves. More importantly, the phase of the output light can be tuned over the entire 2π range with sub-wavelength resolution through simply tailoring the structure geometry of the basic building blocks. This creates an important opportunity in designing highly efficient optical devices for wavefront engineering, such as at lenses working in the microwave, terahertz, and infrared frequency ranges. Here we will present the design, fabrication, and characterization of high-performance terahertz metasurface lenses based on this concept.
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