Metasurface-Based Dual-Mode Bright-Field and Spiral Phase Contrast THz Imaging With Enhanced Focal Depth

Bright-field imaging and spiral phase contrast imaging are the two foremost imaging modes in microscopy, that provide entire and edge-enhanced information about target objects, respectively. However, conventional microscopes usually suffer from limited depth-of-focus, making them support high-resolution imaging only at a fixed distance. In order to solve the problem, we present a versatile imaging system that allows for simultaneous bright-field imaging and edge-enhanced imaging within the long focal depth range at 0.1 THz. The system is based on a 3D printed dielectric metasurface of which a phase profile is a sum of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$l=+1$</tex-math></inline-formula> spiral phase, axicon phase, and deflection phase. Owing to powerful ability in manipulating electromagnetic waves of the multifunctional metasurface, the metasurface-based imaging system is compact, without resorting to the complex and bulky <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4f$</tex-math></inline-formula> Fourier filtering system. Experimental results show that the system can present both bright-field image and edge-enhanced image of the object in the same field of view, with a resolution of up to 4 mm. In addition, the system is capable of dual-mode imaging over an extended depth-of-focus range. Capitalizing on compactness and high-resolution characteristics, it is believed that the scheme may shed new light on optical imaging and information processing, and open new doors to machine vision and microscopy.