Absorption enhancement of thin layer black phosphorous in the mid-infrared with an all-dielectric metasurface

Infrared (IR) light photodetection based on two dimensional (2D) materials has attracted increasing attention. However, the weak IR absorption in 2D materials due to their inherent atomically thin thickness degrades their performance when used as IR photodetectors. Dielectric metasurfaces have represented fantastic functionalities in realizing high-efficiency wavefront control in the optical and infrared ranges. In this work, we design an all-dielectric phase gradient metasurface in the mid-infrared to improve the limited absorption of black phosphorous (BP). With the elaborately designed metasurface the incident infrared light in the form of plane wave is converted into surface wave, and it is guided to concentrate at the very center of the structure. Enhanced electric field is achieved at the center of the structure. With placement of thin layer BP of 5 µm×5 µm size, the absorption can be enhanced to about 3.77% at 3.6 µm, which is about 20 times larger than that of bare thin layer BP. Different sizes and thickness of the BP are also investigated to demonstrate the effectiveness and flexibility of our design. Owing to the lossless and absorption enhancement properties of our structure, our design may find applications in the upcoming highly efficient 2D photodetectors.