Broadband achromatic metalens in terahertz regime

Achromatic focusing is essential for broadband operation, which has recently been realised from visible to infrared wavelengths using a metasurface. Similarly, multi-terahertz functional devices can be encoded in a desired metasurface phase profile. However, metalenses suffer from larger chromatic aberrations because of the intrinsic dispersion of each unit element. Here, we propose an achromatic metalens with C-shaped unit elements working from 0.3 to 0.8 THz with a bandwidth of approximately 91% over the centre frequency. The designed metalens possesses a high working efficiency of more than 68% at the peak and a relatively high numerical aperture of 0.385. We further demonstrate the robustness of our Cshaped metalens, considering lateral shape deformations and deviations in the etching depth. Our metalens design opens an avenue for future applications of terahertz meta-devices in spectroscopy, time-offlight tomography and hyperspectral imaging systems.