Terahertz (THz) lens constitutes a vital component in the THz system. Metasurfaces-based THz metalenses and classical bulky lenses are severely constrained by chromatic/ spherical aberration and the diffraction limit. Consequently, achromatic super-resolution THz lenses are urgently needed. In this study, through translating the required phase distribution into a refractive index (RI) profile with a specific thickness, an innovative approach to designing THz metalenses is proposed and achieved by dielectric gradient metamaterials. The samples fabricated by 3D printing can realize achromatic super focusing with a numerical aperture (NA) of 0.555 from 0.2 to 0.9 THz. Submillimeter features separated by approximately 0.2 mm can be resolved with high precision, such as glass fabric patterns within FR4 panels and fibrous tissue on leaves, with a field of view (FOV) of 90°. Our approach offers a feasible and cost-effective means to implement THz super-resolution imaging, which holds great potential in non-destructive testing and biomedical imaging.

Three-dimensional (3-D) printing, also known as additive manufacturing, provides a novel and cost-effective approach for implementing microwave devices. With the rapid advancement and improved manufacturing resolution of the 3-D printing technology, additive manufacturing has enabled the design and fabrication of electronic devices in higher terahertz (THz) frequency bands, contributing to bridging the gap between microwaves and photonics. Simultaneously, metalenses have garnered significant attention due to their ability to shape electromagnetic (EM) wavefronts. Metalens technology offers a promising solution for wave focusing, surpassing traditional dielectric lenses with advantages such as reduced weight and low loss, particularly at THz frequencies. In this article, we present an overview of the development of 3-D-printed THz metalenses, ranging from single metalenses to dual-layer and trilayer configurations. The functionality of the metalenses becomes more powerful, from replacing conventional light-focusing dielectric lenses for single-layer metalenses, achieving 2-D beam scanning, holographic imaging, and reconfigurable orbital angular momentum (OAM) for dual-layer metalenses, to enabling 3-D focus scanning for trilayer metalenses. We also discuss practical measurement technologies for THz metalenses and briefly outline the prospective to propel the 3-D-printed metalens technology forward.