A Tunable Semiconductor-Based Transmissive Metasurface: Dynamic Phase Control with High Transmission Level

Herein, a tunable semiconductor-based metasurface, based on electro-optical modulation of an array of geometrically-fixed silicon (Si) nanobars, sandwiched between two distributed Bragg reflectors (DBRs) is proposed. A flat-top transmission spectrum with a steep phase profile, which turns out to be essential to realize a highly-efficient spatial light modulator in transmission mode, is formed by excitation of two spectrally close Fabry-Perot-type and guided-mode resonances. The refractive-index of Si nanobars is electrically modulated by the injection of electrons and holes using a P-I-N junction configuration, considered along each nanobar. It is theoretically demonstrated that wide phase agility of 215 degrees and transmission amplitude higher than 0.6 (with an average of approximate to 0.83) can be accomplished at the operating wavelength of approximate to 1.55 mu m. This wide-range tunability is realized by introducing free electron and hole carrier densities of increment N = increment P = 5 x 10(18) cm(-3) accompanied with Si refractive-index-change of increment n(Si) = 0.01. The transmission phase of each unit cell can be separately controlled, which in turn allows to design a tunable meta-array with real-time beam control. As a proof of concept, a dynamic focusing metalens with an adjustable focal length is designed and numerically investigated.