Researchers from the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences demonstrated a broadband achromatic metalens with a numerical aperture of 0.32. The demonstration stemmed from a general method, proposed by the same team, to implement a broadband achromatic metalens in the longwave infrared (LWIR) band based on the combination of the dynamic and geometric phases. The researchers said that compared to state-of-the-art chromatic-aberration-restricted LWIR metasurfaces, the achromatic metasurfaces yielded in their work bring the field a step closer to practical applications. The LWIR band is essential for applications such as thermal imaging and optical communications. However, conventional LWIR optics are bulky and expensive, which hinders the development of integrated LWIR optics. Metasurfaces, which are composed of subwavelength meta-atoms, have powerful light manipulation capabilities and are promising platforms for integrated optics. However, conventional metasurfaces are highly chromatic, despite comprising weakly dispersive materials. Generally, the dynamic phase can be tuned by manipulating the resonance mode supported by the meta-atom through tuning the in-plane geometric parameters. The geometric phase is only related to local rotation angle of the meta-atom and does not change the dispersion characteristics of the meta-atom. Therefore, by combining dynamic phase and geometric phase, the phase and dispersion can be decoupled, the researchers explained. They chose germanium as the base material of the meta-atom in the work; germanium has a refractive index and low intrinsic loss properties in the LWIR band. For the archetypes of the meta-atoms, the researchers chose combinations of nanofins. This is due to the coupled waveguide mode, supported by the adjacent nanofins, allowing users to control the dispersion more precisely. Tests showed that the focal length of the fabricated achromatic metalens shifted only 0.65% relative to mean focal length from 9.6 to 11.6 μm. The focal length of a chromatic metalens shifted 20% relatively. Further, the Strehl ratio of the achromatic metalens is above 0.96 for the entire operating band, indicating that the broadband diffraction-limited focusing is achieved. The average efficiency of the broadband achromatic metalens was 31%, which is comparable to the efficiency of monochromatic metalens in LWIR. The Strehl ratio is the ratio of peak diffraction intensities of an aberrated versus perfect wavefront. This measure, of the quality of an image formation, indicates the level of image quality in the presence of wavefront aberrations and can define the highest acceptable level of wavefront aberration for general observation in or with an optic. According to the researchers, their proposed method worked equally well with other types of achromatic metasurfaces. They demonstrated a broadband achromatic metasurface grating with constant deflection angle of 30° from 9.6 to 11.6 μm. The research was published in Nanomaterials (www.doi.org/10.3390/nano11102760).
