Metasurfaces for High Contrast Imaging

The light incidents onto the “EDGE” shaped object, then passes through the metasurface at the Fourier plane, and finally its edge information is obtained at the image plane. (Source: Science China Press)

Over the past decades, image processing has evolved into a significant technique for science and engi­neering societies. One such is edge detection, which is a process that attempts to capture the signi­ficant features of objects in the image. There are two basic approaches for edge detection achieve­ment. One of them is digital computation, the other one is analog compu­tation. Compared with digital compu­tation, the analog computation can deal with real time and continuous image processing with power-saving advantages. Therefore, researchers develop various methods for analog edge detection.

Especially, as the meta­material and meta­surface development, engineered nano­photonic materials have been widely studied for optical analog image processing. Recently, several theo­retical works propose the approach to perform mathe­matical operations with complex material and fabri­cation process. Furthermore, spatial differen­tiation has been extended to experimental demons­tration using various approaches, including surface plasmonics, photonic crystals, the photonic spin Hall effect and the Pancharatnam-Berry phase metasurface. However, all of these current methods are subject to their own limi­tations, and experi­mental demons­tration of a highly efficient, compact, two-dimensional spatial differen­tiation device working for broadband frequencies is still missing.

In response to this challenge, recently, inspired by the radial phase gradient of Pancharatnam-Berry phase meta­surface, for the first time, the optical research team led by Hailu Luo from the Hunan Univer­sity and Zhaowei Liu from Uni­versity of California, San Diego, proposed a new design to impart 2D spatial differen­tiation on the impinging wavefront, based on the dielectric meta­surface in trans­mission mode, providing the advan­tages of high efficiency, broadband and high-contrast. The dielectric metasurface based on geo­metric phase without any resonance structure ensures the operation at broadband working wavelength (whole visible range), enabling differen­tiation of color images. The high trans­mission mode of the compact metasurface sample make it easier to be aligned or inte­grated with the rest of the optical system, which is important for imaging-processing appli­cations.

The designed system has the ability to filter the edge information of one object. The proposed method can work as a 2D differen­tiator effi­ciently, with the 2D edge detection covering all the visible frequencies. Different imaging techniques are used to evaluate the proposed edge detection approach for the intensity object. Compared with dark field and phase contrast technique, the proposed method exhibits clear and strong signals at the cell edges indicating excep­tionally high sensi­tivity and precision to detect the trans­parent bio­logical specimens. The exploration of the highly efficient meta­surface performing a basic optical differen­tiation operation opens new oppor­tunities in appli­cations of fast, compactible and power-efficient ultrathin devices for data processing and biological imaging. (Science China Press)

Reference: J. Zhou et al.: Two-dimensional optical spatial differentiation and high-contrast imaging, Nat. Sc. Rev. nwaa176 (2020); DOI: 10.1093/nsr/nwaa176

Link: Dept. of Nano Engineering, University of California San Diego, La Jolla, USA

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