Controllable multi-focal terahertz metalens based on vanadium dioxide

We designed a C-shaped ring metalens featuring dynamically switchable focal points, based on vanadium dioxide. The metalens is segmented into inner and outer rings, utilizing the phase change properties of vanadium dioxide to enable different regions to transition from a metallic state to a dielectric state. This design facilitates the switching of the metalens among three configurations: bifocal, single short focus, and single long focus. The maximum focusing efficiency achieving 19.91%, with a diffraction limit as low as 0.42λ and a numerical aperture reaching up to 0.83. This metalens effectively separates and switches multiple focal points, which will enhance the multifunctional applications of phase change materials in terahertz technologies such as imaging and tomography.