Polarization insensitive achromatic terahertz metalens based on all-dielectric metasurfaces

Terahertz wave has great potential in wireless communication, biomedical monitoring and spectroscopy. However, the lack of terahertz functional devices hinders the development of terahertz technology. Metasurface is a kind of novel artificial two-dimensional electromagnetic metamaterials, which can efficiently manipulate the electromagnetic wave at sub-wavelength scale and have the advantages of small size and light weight. Therefore, metasurfaces provide an additional modality to realize terahertz functional devices. Terahertz metalens based on metasurfaces is an indispensable functional component, but there is chromatic aberration that will degrade its performance when broadband terahertz wave is incident, so it is very important to eliminate the chromatic aberration of terahertz metalens, especially for terahertz imaging, and so on. In this paper, we elaborate the principle of elimination of chromatic aberration and design a polarization insensitive achromatic metalens working in the frequency range from 0.8 THz to 1.2 THz with numerical aperture of 0.46, three types of meta-atoms with different cross-section shapes are adopted to constitute achromatic metalens. The numerical simulation has also been carried out for the proposed achromatic metalens. The maximum deviation of focal length and average focusing efficiency of the proposed achromatic metalens across the working frequency range is 4.74% and 38.47%, respectively. Meanwhile, the achromatic metalens is polarization insensitive due to the symmetric cross-section shapes of meta-atoms adopted. The polarization insensitive achromatic metalens will pave the way for the practical application of terahertz devices based on metasurfaces.