Broadband Achromatic Terahertz Metalens Constituted by Si-SiO2-Si Hybrid Meta-Atoms

Metasurfaces have proved to possess extraordinary properties and propelled the development of versatile functional devices in the terahertz domain, but the intrinsic dispersion severely impedes their broadband applications. Sporadic studies on achromatic terahertz metasurfaces are either subject to specific polarization operation or challenged in the fabrication process for extended bandwidth and numerical aperture. Herein a novel meta-atom scheme made up of a silicon-silica-silicon sandwich-shaped structure is presented that enlarges the propagation phase as well as improves the dispersion engineering capability at low losses. As a classic case, an achromatic metalens is constituted by these meta-atoms and experimentally demonstrated which shows a remarkable achromatic focusing performance from 0.5 to 1.1 THz with a numerical aperture of 0.47 and an average efficiency as high as 43.1%. Continuous wavelet transformation analysis further proves the metalens has negligible group delay dispersion. This study not only demonstrates an outstanding terahertz achromatic metalens but also provides a novel meta-atom paradigm for constructing achromatic metasurfaces such as holographic plate, beam deflector, and Bessel axicon, further not limited to THz bands.