A Dual-Wavelength Bifocal Microwave Metalens Based on Bi-Layer Plasmonic Metasurface

Metasurfaces (or metalenses) provide an excellent platform to manipulate incident waves across wide spectrums. Compared to traditional microwave lenses based on phase accumulation along trajectory such as Luneburg lens, meta-lenses can greatly reduce the device thickness simultaneously with more degrees of freedom. In this paper, a dual-wavelength microwave metalens with a single-layer plasmonic metasurface with two different phase gradient profile via spatial multiplexing is designed and simulated numerically. It is shown that the designed flat metalens can produce a dual-focus spot with two different focal length at two distinct wavelength of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{1}=10.5\text{GHz}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{2}=14.5\text{GHz}$</tex>, respectively. Besides, the focusing performances at these two wavelengths are also analyzed and the spot size is diffraction-limited with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.48\lambda_{1}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.87\lambda_{2}$</tex> in the far field, respectively. Moreover, the focusing efficiency is about 45% and 42% for low frequency and high frequency operation, respectively. The proposed scheme and studies provide a method to develop dual-band and multi-functional microwave metalens and meta-devices by integrating different types of meta-atom in a single-layer metasurface.