Longwave infrared (LWIR) corresponds to one of the atmospheric windows and is essential for a wide range of applications such as thermal imaging and optical communication. 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. 

In a study published in Nanomaterials, Prof. SONG Naitao from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) proposed a general method to implement broadband achromatic metalens in LWIR based on combined phase.  

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. 

Using this method, the researchers demonstrated a broadband achromatic metalens with numerical aperture of 0.32. Germanium is chosen to be the base material of the meta-atom for its high refractive index and low intrinsic loss properties in LWIR. Combinations of nanofins are chosen to be archetype of meta-atom because the coupled waveguide mode supported by the adjacent nanofins allows people to control the dispersion more precisely.  

The focal length of achromatic metalens shifted only 0.65% relative to mean focal length from 9.6μm to 11.6μm, and the focal length of chromatic metalens shifted relatively 20%. The Strehl ratio of achromatic metalens is all above 0.96 for the entire operating band indicate the broadband diffraction-limited focusing is achieved. The average efficiency of the broadband achromatic metalens is 31%, which is comparable to the efficiency of monochromatic metalens in LWIR. 

The method works equally well with other types of achromatic metasurfaces. The researchers demonstrated a broadband achromatic metasurface grating with constant deflection angle of 30° from 9.6 μm to11.6 μm.

Compared with state-of-the-art chromatic-aberration-restricted LWIR metasurfaces, achromatic metasurfaces demonstrated in this work shows a substantial advance and brings the field a step closer to practical applications.