With the rapid development of portable equipment, the demand for the miniaturization of optical elements is increasing. Metasurfaces are considered as potential planar analogs of conventional devices due to their small size and their extraordinary ability to modulate light. As one of the most important branches of metasurfaces, metalenses attract much attention and are fascinating to develop compact, miniature optical imaging devices. However, the highly chromatic characteristic limits their further applications. Therefore, achromatic metalenses with constant focal lengths over a broad bandwidth are highly desirable. Here, we demonstrate a diffraction-limited achromatic metalens with an octave-wide bandwidth in the infrared. Unlike typical metalenses with periodic unit cells, the proposed metalens comprises well-designed an aperiodic array of elements based on the Pancharatnam-Berry phase. The proposed metalens with a numerical aperture of 0.49 achieves an average focusing efficiency of 37% in a wavelength range from 1.15 to 2.3 μm, which is one octave, and maintains a near-constant focal length of 25 μm. It paves the way for miniaturized and broadband imaging applications.
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 μm. 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.
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