Broadband midinfrared antireflection metasurfaces based on silicon

As one of the fundamental phenomena in optics, reflection always occurs for the refractive index contrast between different materials for the impedance mismatch. In many applications, such as solar cells or photodetectors, reflection is unwanted and the reduction of reflection is highly desirable. Metasurfaces have attracted intensive attention recently for their ability to efficiently reshape electromagnetic waves in desired manners on a flat and ultrathin platform. Numerous new concepts, effects, and applications have been intensely studied in recent years. As some of the most important applications, metasurfaces exhibit superior capabilities to enhance absorption, antireflection, and transmission. Here we demonstrate a silicon metasurface with significantly enhanced antireflection over a broad spectrum from 1 to 5 mu m. Over the more than two-octaves bandwidth, the transmittance is all above 78% with an average value as high as 95%. The proposed metasurface is a silicon layer on top of an InAs layer on a GaSb substrate and exhibits polarization-insensitive transmission enhancement for the symmetry of the geometry. This structure can be potentially used for thermal targets detection, imaging, sensing, and biochemical analyses.