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A Terahertz Metalens for Reversible Conversion Between Linear and Arbitrary Circular Polarization

In a new publication from Opto-Electronic Advances, researchers from Tianjin University, China and Capital Normal University, China, discuss dynamic phase assembled terahertz metalens for reversible conversion between linear polarization and arbitrary circular polarization.

Metalens is a class of metasurface devices with beam focusing function. If metalens integrates the function of single-handed circular polarization (CP) wave conversion, the focusing lens together with CP lens in traditional cameras may be replaced by a metalens. Several techniques have achieved CP conversion and focusing, for example, introducing chiral optics and spin-decoupling mechanisms into metalens. However, the physics of these techniques do not allow for a clean and efficient conversion of linear polarization (LP) to single-handed CP waves.

The metalens must depend on several different unit cells those phase delay can cover 0-2p. Traditionally, the principle of Pancharatnam-Berry (PB) phase is in whole or in part introduced in the phase design of the metalens, even for those metalenses containing chiral optics and spin decoupling mechanisms. For instance, the general operation is that several unit cells with different structural parameters are firstly designed to meet the phase change of 0-p, and then the phases of the remaining p-2p are obtained by rotating these structures at 90o.

Although an unit cell can realize the conversion of LP to single-handed CP, the helix of CP will be opposite direction after this cell being rotated by 90o due to the spin dependence of PB phase. Therefore, the output CP wave contains non-negligible left-handed and right-handed components, so that the LP wave component synthesized by the two CP components is also non-negligible, which is not applicable to advanced cameras.

Meanwhile, the polarization conversion of the existing metalenses is usually not reversible. Although a LP wave through those metalenses is converted into other polarization states and focused, wave with other polarization states can no longer be converted into the original LP wave and focused. These shortcomings also limit the multifunctional applications of the metalens to some extent.

The authors of this article propose an all-dielectric silicon-based terahertz metalens, which is assembled using dynamic phase and getting rid of the PB phase. The problems considered in this work are: 1) metalens integrated with single-handed CP wave conversion have practical application value in terahertz (THz) cameras, and how metalens converts LP THz wave into arbitrary single-handed THz wave as perfectly as possible; 2) how THz metalens achieves reversible conversion between LP wave and arbitrary single-handed CP wave to expand their application scope as much as possible.

Article reference: Li JT, Wang GC, Yue Z, Liu JY, Li J et al. Dynamic phase assembled terahertz metalens for reversible conversion between linear polarization and arbitrary circular polarization. Opto-Electron Adv 5, 210062 (2022) . doi: 10.29026/oea.2022.210062 

Keywords: terahertz metalens / metamaterials / metasurfaces / dynamic phase / single-handed circular polarization / reversible conversion


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