A high-performance, low-cost terahertz modulator is fabricated by spin-coating CdS nanowires onto silicon. This all-optical modulator achieves broadband modulation (0.4–1.6 THz) with a significant depth of 85% at a low power density (2 W cm⁻²), exceeding bare silicon by fourfold. THz-TDS confirms its terahertz amplitude modulation capability. Tests of the simple photonic switch further illustrate the modulator’s potential for carrying information on a terahertz transmission wave. The modulation mechanism is explained through energy band theory and photoconductivity data. This work also presents a novel method for probing heterostructure formation using THz-TDS with laser irradiation.

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.

This study synthesizes and comparatively investigates two squaric acid-based phthalocyanine-like dyes, SNF and its long-chain alkylated derivative LNF, to systematically elucidate the influence of peripheral hydrophobic groups on their third-order nonlinear optical (NLO) properties. The NLO characteristics were comprehensively characterized using femtosecond Z-scan and I-scan techniques at both 800 nm and 900 nm. Both dyes exhibited strong saturable absorption (SA), confirming their potential as saturable absorbers. Critically, the comparative analysis revealed that SNF exhibits a significantly greater nonlinear absorption coefficient (β) compared to LNF under identical conditions. For instance, at 800 nm, the β of SNF was approximately 3–5 times larger than that of LNF. This result conclusively demonstrates that the introduction of long hydrophobic alkyl chains attenuates the NLO response. Furthermore, I-scan measurements revealed excellent SA performance, with high modulation depths (e.g., LNF: 43.0% at 900 nm) and low saturation intensities. This work not only clarifies the structure–property relationship in these D-A-D dyes but also presents a clear strategy for modulating the NLO properties of organic chromophores for applications in near-infrared pulsed lasers.