Broadband polarization-independent achromatic varifocal metalens in the terahertz region

Lenses with tunable focal lengths are quite critical to various optical sensors, imaging, and detection systems. A traditional varifocal system is usually composed of multiple conventional refractors and needs to mechanically adjust the axial distance among them, inevitably leading to complex operation and bulky volume. Recently, metasurfaces have provided an alternative for flexibly shaping the electromagnetic field. However, simultaneously realizing continuous zoom and achromatic function remains a challenge. This paper proposes an achromatic continuous varifocal metalens consisting of two coaxial metasurfaces working in the terahertz range. The underlying mechanism depending on the superimposed phase distribution of these two metasurfaces is similar to a spherical lens, and the focal length can be continuously varied by changing the mutual rotation angle. The tuning range of focal length is continuous from 3.08 mm to 11.52 mm, corresponding to the numerical aperture from 0.58 to 0.19. Meanwhile, careful dispersion engineering based on the particle swarm optimization algorithm has also achieved achromatic bandwidth ranging from 0.9 THz to 1.2 THz. The maximum deviation of the focus length is below 8%, and the coefficient variations of the focal lengths among the entire bandwidth are under the allowed scope (5%) of the international standard of chromatic aberration, while a focusing efficiency of 32% can be obtained simultaneously. This scheme will provide promising opportunities and possibilities for future display technology and integrated optical imaging systems.