Terahertz Switchable Focusing Planar Lens With a Nanoscale Vanadium Dioxide Integrated Metasurface

Tunable focusing planar lens possesses more flexible characteristics to facilitate the expansion of its function and application. This study presents a planar meta-lens based on the phase gradient vanadium dioxide composite metasurface controlled by an external thermal stimulus that enables the switchable focusing. By utilizing the insulator-metal phase transition characteristics of the vanadium dioxide components, the composite metasurface can show a different phase gradient distribution that could lead to various focuses. Both simulated and experimental results demonstrate that without the external stimulus, the vanadium dioxide components display dielectric characteristics that are a phase-transparent response to THz wave. While applying an external thermal stimulus, the phase transition leads to the metallization of vanadium dioxide components that could induce the redistribution of the phase gradient of the metasurface to realize a tunable focusing. The measurements show that the results are in line with our expectations and achieve three times the tunable focal length variation from 2.5 mm with a numerical aperture (NA) of 0.78 to 7.5 mm with an NA of 0.39 at 0.65 THz, which was reversible. The proposed switchable planar metalens is expected to deliver a promising new design for electromagnetic wave dynamic manipulation and enable active applications, such as dynamic imaging or other optical systems at THz frequencies.