All-optical active THz metasurfaces for ultrafast polarization switching and dynamic beam splitting

Miniaturized ultrafast switchable optical components with an extremely compact size and a high-speed response will be the core of next-generation all-optical devices instead of traditional integrated circuits, which are approaching the bottleneck of Moore’s Law. Metasurfaces have emerged as fascinating subwavelength flat optical components and devices for light focusing and holography applications. However, these devices exhibit a severe limitation due to their natural passive response. Here we introduce an active hybrid metasurface integrated with patterned semiconductor inclusions for all-optical active control of terahertz waves. Ultrafast modulation of polarization states and the beam splitting ratio are experimentally demonstrated on a time scale of 667 ps. This scheme of hybrid metasurfaces could also be extended to the design of various free-space all-optical active devices, such as varifocal planar lenses, switchable vector beam generators, and components for holography in ultrafast imaging, display, and high-fidelity terahertz wireless communication systems. Active metasurfaces: Terahertz control All-optical control and manipulation of terahertz waves is now possible thanks to the development of custom-designed, dynamic reconfigurable metasurfaces. Realized by Longqing Cong and coworkers from Nanyang Technological University in Singapore and Tianjin University in China, the metasurfaces enable ultrafast (sub-nanosecond) polarization switching and beam splitting. They consist of an array of miniature aluminum-coated silicon split-ring resonators on a sapphire substrate. Illumination with short pulses of an infrared “pump beam” causes charge carriers in the silicon patch of the microring to transition from the valence to the conduction band, temporarily changing the photoconductivity and thus switching the transmission of the metasurface between “off” and “on” states for a polarized terahertz wave. Such high-speed switching of polarization is expected to be useful for applications such as data-encoding and multiplexing in a terahertz communications system as well as holography.