Continuously varifocal metalens for broadband achromatic focusing of terahertz waves

The correction of chromatic aberration is essential for promoting broadband imaging systems, which can be accomplished by recently proposed broadband achromatic metalenses (BAMs) in a more compact and efficient way compared to that of traditional optical lenses. However, current BAMs are limited to the fixed functionality that severely hinders their potential applications. In order to address such limitation, a continuously varifocal and broadband achromatic metalens (CVBAM) for converging THz waves is proposed by employing thermally tunable material of indium antimonide. At initial temperature, numerical results manifest that the chromatic aberration over 1.8–2.2 THz can be well corrected by CVBAM and the transverse sizes of corresponding focal spots are beyond the Abbe diffraction limit. Moreover, the numerical aperture (NA) and average focusing efficiency of CVBAM and up to 0.62 and 73.3%, respectively. In addition, the average focal length of CVBAM has a tunable range from 736.25 μm (NA = 0.62) to 861.02 μm (NA = 0.56) by altering operation temperature. This work may offer an advanced scheme for designing tunable BAMs, which can remarkably facilitate the development of THz near-field imaging and spectroscopy systems.