3D Terahertz Confocal Imaging with Chromatic Metasurface

Abstract

Terahertz confocal imaging allows 3D see-through of a non-metallic object with high resolution. Conventional methods acquiring 3D images of thick objects suffer from limited depth-of-field, constrained depth resolution, and/or inconsistent spatial resolution at different depths. To address these limitations, the intrinsic chromatic aberration of a typical focusing metasurface is exploited to achieve frequency-dependent focal lengths. An object located within this extended focal range can be readily 3D inspected by performing 2D raster scans. A rigorous analysis reveals that the focal spot maintains a constant waist diameter of 2.4 mm (equivalent to 2.2${\lambda }_0$ at 275 GHz) and migrates 68.1 mm (equivalent to 62.4${\lambda }_0$, or 16.4 times of Rayleigh length, or 1.4-fold of the designed focal length at 275 GHz) from 175 to 525 GHz, and thus achieving a consistent spatial resolution and a large depth-of-field for 3D imaging. Importantly, this large depth-of-field is achieved with a relatively high numerical aperture of around 0.42. Measurements conducted between 220 and 330 GHz exhibit close agreement with the calculation. To demonstrate its imaging functionality, two stacked papers with different texts, a mobile phone, and earphones concealed in a charging case are imaged, where a short-time Fourier transform is implemented in the time-domain terahertz images to enhance image contrast. The presented metasurface is technologically significant for imaging systems to rapidly inspect objects in 3D with exceptional resolutions. Its potential applications include in-situ defect detection and object identification in security screening.