| Summary: | The recently developed achromatic metalenses have exhibited a significant potential to replace the conventional bulky lenses. However, the numerical aperture (NA) of current broadband achromatic metalenses (BAMs) operated in long-wavelength infrared (LWIR) range is still insufficient for the applications of microscopy or high-resolution imaging. To address the restriction, a BAM with high NA for sub-diffraction focusing in LWIR waves is proposed. Specifically, the multipole decomposition is performed to avoid the undesirable resonances in the operation wavelength from 9 to 11 μm, which ensures the great linearity of the phase profiles from meta-atoms. After elaborately arranging the meta-atoms to compose a high-NA BAM, numerical results demonstrate that the chromatic aberration is well corrected in the operation wavelength and the maximum deviation is only 3.95%. Moreover, the actual NA of proposed BAM reaches up to 0.79 and the transverse sizes of focal spots are beyond the Abbe diffraction limit. The results indicate that the proposed high-NA BAM may have promising potential in the broadband applications of high-resolution imaging and microscopy.
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