Broadband achromatic metalens in Terahertz regime

We demonstrate the achromatic metalens with C-shape units working from 0.3 to 0.8 THz with a bandwidth of approximately 91% over the center wavelength. The achromatic focusing has been observed both experimentally and numerically, with the size of focal spot closed to $\lambda $. Meanwhile, the designed metalens possesses high working efficiency of more than 68% at the peak and a relatively high numerical aperture (NA) of NA=0.385 in order to obtain high spatial resolution. We further show the robustness of our C-shape structures, considering lateral shape deformations and deviations of etching depth. Our work successfully demonstrates the THz achromatic metalens, which opens an avenue for the future applications of terahertz meta-devices on spectroscopy, time-of-flight tomography and hyperspectral imaging system.