Dynamic switchable multi-functional Janus metasurface for THz-beam focusing

Abstract Recently, the development of beam-focusing lenses based on metasurfaces has advanced rapidly towards higher levels of versatility and integration. By utilizing the phase transition of VO 2 , we demonstrate a multi-functional Janus metasurface that is switchable between transmissive and reflective modes. This metasurface is composed of a designed bilayer hybrid structure integrating VO 2 and silicon dielectric. The two dielectric layers modulate the incident beam four times via their geometric and propagation phases, enabling flexible control of both its direction and polarization. In transmission mode, the metasurface acts as a polarization-selective metalens, focusing left-circularly polarized and right-circularly polarized terahertz waves at distinct focal points according to their polarization. The average focusing efficiency is approximately 48%. In reflection mode, the metasurface operates as a polarization-insensitive metalens, effectively reflecting and focusing incident waves based only on propagation direction. The average focusing efficiency is about 38%. This design combines geometric phase and propagation phase characteristics, enabling four-channel multiplexing functionality in both reflection and transmission modes. The resulting architecture enhances the metalens’s functionality while expanding its application potential—opening new avenues for developing multifunctional, compact novel terahertz devices based on metasurfaces.