Terahertz (THz) technology holds promise for future communication systems, but the development of high-performance THz modulators is hampered by the lack of suitable materials and efficient active regulation. Two-dimensional (2D) materials, with their strong light-matter interactions, atomic thinness, and fast carrier recombination, offer a promising platform. While 2D-material-based THz modulators have gained attention, challenges remain: achieving a balance between modulation depth and speed, managing insertion losses and bandwidth. Conventional semiconductors offer high modulation depth but slow speed, while existing 2D material modulators suffer from low modulation depth or require high pump fluence. Tellurium (Te), a mono-elemental 2D material with a unique helical chain structure, offers advantages such as layer-dependent bandgap, high carrier mobility, strong optical response, and good air stability. Heterojunctions, formed by stacking different 2D materials, overcome lattice-matching limitations of bulk semiconductor heterojunctions, creating atomically sharp interfaces with desirable properties. Type II band alignment facilitates photocarrier separation and transfer, crucial for light-electric interconversion in optoelectronic applications. While Te-based heterojunctions have shown promise in solar cells and infrared photodetectors, their application in high-performance THz modulators remains largely unexplored. This study investigates all-2D Te-based heterojunctions to understand the van der Waals (vdW) interlayer coupling and carrier dynamics, aiming to optimize modulation performance and clarify the working principle of these novel devices. The researchers propose a strategy using Te-based nanofilms, substrate engineering, and vdW heterojunctions to improve modulator performance.

The paper reviews existing challenges in developing high-performance terahertz modulators, highlighting the trade-off between modulation depth and speed. It discusses the potential of 2D materials and existing limitations of conventional semiconductors and other 2D materials used for THz modulation. The literature review emphasizes the unique properties of tellurium (Te) and its suitability for this application, citing prior research on its use in solar cells and infrared photodetectors. The authors point out the lack of research on Te-based heterojunctions for THz modulation, setting the stage for their investigation. The advantages of using van der Waals heterojunctions in overcoming lattice matching constraints are also discussed, highlighting the potential for improved device performance.

High-quality, large-area Te nanofilms were deposited using electron beam evaporation onto fused silica substrates. The structural and crystalline nature was examined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. High-resolution transmission electron microscopy (HRTEM) confirmed the crystal structure. The thickness and optical bandgap of the Te films were determined. An optical pump-THz probe technique was used to investigate the active modulation response of Te-based THz devices and Te-based heterojunctions with graphene and germanium. Transient THz dynamics were measured for Te nanofilms of varying thicknesses and under different pump fluences and wavelengths. The data were fitted to biexponential functions to extract relaxation times. Complex photoconductivity spectra were calculated and fitted to a modified Drude-Smith conductivity model. The study also included investigations of Te/graphene and Te/germanium heterojunctions, comparing the modulation depth and speed in different stacking orders. Density functional theory (DFT) calculations were performed to understand the substrate-induced electric field and its influence on charge transfer. The photoinduced dynamics of excitons were analyzed by scanning the change of the THz amplitude at a zero-crossing point to exclude the contribution of the photoinduced amplitude change.

The study found that Te (100 nm) nanofilms exhibited optimal modulation characteristics, achieving a modulation depth (MD) of 14.5% at an extremely low fluence of 2.6 µJ cm⁻² and 65.3% at 260 µJ cm⁻², with a low insertion loss of -0.33 dB. The carrier lifetime was independent of pump fluence, indicating a consistent modulation speed. Analysis of complex photoconductivity revealed a decrease in carrier scattering time and an increase in diffusion time with increased pump fluence. Heterojunctions of Te with graphene (Gr) demonstrated enhanced MD compared to individual materials, with Gr/Te exhibiting a higher MD than Te/Gr, emphasizing the importance of stacking order. Te/germanium (Ge) heterojunctions showed even more significant improvements, particularly the Ge/Te configuration, which achieved an MD of 87.6% at 260 µJ cm⁻² and 27.8% at 2.6 µJ cm⁻². The faster relaxation times in heterojunctions were attributed to efficient charge transfer across the interface and subsequent interlayer exciton decay. DFT calculations confirmed the presence of a substrate-induced electric field influencing the charge transfer process. The study concludes that Te-based all-2D vdW heterojunctions, particularly Ge/Te, with substrate engineering offer a promising approach to realize high-performance, optically controlled terahertz modulators.

The findings address the challenge of balancing modulation depth and speed in THz modulators by demonstrating that Te-based heterojunctions, especially Ge/Te, offer a significant improvement. The high modulation depths achieved at both low and high pump fluences are particularly noteworthy, indicating versatility for various applications. The substrate-induced electric field plays a crucial role in enhancing device performance, highlighting the importance of material selection and device architecture. The study's results are significant for the field of THz optoelectronics, providing a new path toward developing high-performance modulators for next-generation communication systems. The ultrafast carrier dynamics and efficient charge transfer observed in the heterojunctions demonstrate their suitability for high-speed applications. The results validate the theoretical understanding of interlayer interactions and carrier dynamics in 2D materials and contribute to the fundamental knowledge in this area.

This research demonstrates that Te-based all-2D vdW heterojunctions, particularly Ge/Te, offer a promising approach to creating high-performance, optically controlled terahertz modulators. Substrate engineering plays a critical role in optimizing device performance. The high modulation depths achieved, along with the ultrafast carrier dynamics, highlight the potential for next-generation communication technology. Future work could explore different 2D material combinations within the heterojunction structure to further optimize performance and investigate the impact of different substrate materials on device properties.

The study focuses on specific 2D material combinations (Te/graphene and Te/germanium) and a limited range of thicknesses. Further research is needed to explore other material combinations and optimize device parameters. The study primarily uses optical pump-THz probe technology; additional characterization techniques could provide a more comprehensive understanding of the device physics. The scalability and manufacturability of the devices, crucial for practical applications, were not addressed.