A Dynamically Modulated All-Dielectric Metasurface Doublet for Directional Harmonic Generation and Manipulation in Transmission

In this paper, an electrically tunable all-dielectric metasurface doublet is proposed operating at mid-infrared frequency regime with dynamic 2 pi phase span in transmission mode. Each layer of the metasurface consists of a periodic array of silicon nanobars configured into p-i-n junctions in which the double carrier injection into the intrinsic region under forward bias allows for tuning of the silicon refractive index. The physical mechanism is based on the spectral overlap of high quality factor guided mode resonances supported by each constituent layer establishing an extreme Huygens' operation regime of nearly reflectionless transmission with steep phase spectrum. The short response time of field-driven carrier injection in p-i-n junctions allows for modulation of refractive index with radio-frequency biasing signals, yielding a stable time-modulated all-dielectric metasurface with directional frequency conversion and harmonic generation in transmission. A systematic approach is adopted to control the output spectrum of generated frequency harmonics through engineering the temporally modulated phase of transmitted light via adjusting the modulation waveform. Specifically, pure frequency mixing and concurrent dual-frequency generation in transmission mode are demonstrated. Unique applications of the time-modulated all-dielectric metasurface are presented including wavelength-multiplexed metalens for dual-polarity focusing/diffusing and frequency diverse transmitarray for ultrafast spatiotemporal beam-scanning.