All-optical broadband terahertz modulator based on NiO/Si heterojunction and interface photoconductivity analysis

All-optical THz heterojunction modulators have gained significant attention to meet the demanding requirements of next-generation wireless communication technologies. Self-powered heterojunction photodetectors have the high separation and utilization of photoinduced charge carriers even without external power sources. This high efficiency separation is attributed to the built-in electric field between heterojunction layers, a crucial factor for all-optical THz heterojunction modulators. Consequently, NiO/Si heterojunctions are anticipated to function as high-performance all-optical THz modulators. In this work, NiO/Si heterojunctions were fabricated, and their modulation properties were characterized using THz-time-domain spectroscopy (THz-TDS). The results demonstrate that the NiO/Si heterojunction exhibits broadband modulation (0.4–1.6 THz) with a high modulation depth of 85% (at 2 W/cm2 of 532 nm laser irradiation), representing a three-fold enhancement compared to bare silicon. Simple optical switching experiments further underscore the potential of these modulators for encoding information onto THz transmission waves. The modulation mechanism was elucidated through band theory and photoconductivity measurements. The photoinduced carrier mobility was estimated to be as high as 8285 cm2/(V s) under 532 nm laser irradiation, surpassing the values of other typical high-carrier-mobility materials. The results suggest that core heterojunction materials of self-powered photodetectors can serve as a promising foundation for high-performance all-optical THz modulators. In addition, THz-TDS emerges as a sensitive and non-contact technique for evaluating the performance of photodetectors.