Ultrafast Terahertz Frequency and Phase Tuning by All-Optical Molecularization of Metasurfaces

摘要

The integration of photoactive semiconductors exhibiting strong light-matter interactions into functional unit meta-atoms facilitates effective approaches to dynamically manipulate terahertz (THz) waves. Here, a new metaphotonic modulator is proposed and comprehensively studied, which demonstrates extensive tunability of the resonant frequency and phase with the merit of ultrafast photoswitching. Specifically, parallel silicon (Si) bridges are embedded in metasurfaces to reinforce the connection ability, achieving ultrafast optical molecularization from a magnetic quadrupole into an electric dipole. Under femtosecond pulse excitation, the demonstrated resonant frequency tuning range is as high as 40% (from 1.16 to 0.7 THz) and can be further promoted up to 48% (from 1.56 to 0.81 THz) by varying the Si bridge length. Meanwhile, the phase delay at given frequencies can be controlled up to 53.3 degrees without significantly changing the high transmission. Furthermore, the transient frequency switching and phase shifting dynamics are systematically investigated for the first time, showing a full recovery time within 2 ns. By optically molecularizing metasurfaces, extended tuning ranges with regard to the resonant frequency and phase, as well as an ultrafast switching speed, are simultaneously acquired in the proposed metamodulator, which provides deeper insight into the multifunctional active-tuning systems.