Ultrafast All‐Optical Switching and Active Sub‐Cycle Waveform Control via Time‐Variant Photodoping of Terahertz Metasurfaces

The development of high-speed and high-performance optical switches has been a long-standing issue in the field of photonics. This paper introduces a pioneering time-resolved spectroscopy-based approach for realizing photon-induced ultrafast terahertz (THz) modulation within an electrical split-ring resonator (SRR) via photoexcitation, rather than relaxation dynamics, in a silicon-based indirect-bandgap material. Two competitive effects (shorting of LC circuit and metallization of substrate) occur during photon-induced THz modulation. The tradeoff between these two effects enables high-speed optical switching via different time scales of the photoexcitation processes-THz-optical cooperative effect and phonon-assisted electron transition. THz-optical cooperative photoexcitation, causing a shorting effect within the LC circuit, has been observed in the SRR gap, whose size typically exceeds that facilitating impact ionization (IMI). Notably, a remarkably short THz switching time of 1.3 ps has been achieved via only photoexcitation and with a high-performance transmission intensity modulation depth of over 500%. In addition, active temporal waveform control down to a sub-cycle pulse has been successfully demonstrated. The proposed approach suggests a new route for constructing high-speed and efficient THz dynamic photonic devices with potential applications in temporal waveform control.