Broadband Achromatic Sub-Diffraction Focusing by an Amplitude-Modulated Terahertz Metalens

Recently, there are growing interests in developing super-resolution metalenses for applications of focusing and imaging. On one hand, various sub-diffraction metalenses have been demonstrated; however, most of them only work at a single wavelength or multiple discrete wavelengths. On the other hand, the previously reported broadband achromatic metalenses are diffraction-limited, or their focal spots are larger than the corresponding Abbe diffraction limit, 0.5 lambda/NA, where lambda and NA are the lens working wavelength and numerical aperture. In the present work, an approach of realizing broadband achromatic sub-diffraction focusing is proposed utilizing both dispersion engineering and wave front manipulating through amplitude modulation. To verify this concept, based on high-index dielectric meta-atoms, a broadband achromatic sub-diffraction focusing metalens is designed with a radius of 5.11 mm, a focal length of 40.39 mm, and NA of 0.125 in the terahertz frequency range of 2.29-2.70 THz by simultaneously controlling the distributions group of the delay, frequency-dependent phase, and amplitude transmittance. Both theoretical and experimental results demonstrate the performance of achromatic sub-diffraction focusing within the bandwidth. The proposed approach provides a simple way to achieve broadband achromatic super-resolution metalenses, which may have promising potentials in various applications of super-resolution focusing and super-resolution imaging.