High-performance terahertz modulators induced by substrate field in Te-based all-2D heterojunctions

Terahertz (THz) modulators are critical for next-generation communication systems but face a tradeoff between modulation depth and speed, along with insertion loss and bandwidth constraints. Two-dimensional (2D) materials offer strong light–matter interactions, ultrathin profiles, and fast carrier recombination, making them promising for optoelectronics. Existing all-optical THz modulators based on conventional semiconductors provide high modulation depth but slow speed, while many 2D materials show ultrafast response but low modulation depth or require high pump fluence. Mono-elemental 2D tellurium (Te), with a unique helical chain structure, layer-dependent bandgap, high carrier mobility, strong optical response, and air stability, is a promising candidate. Van der Waals 2D heterojunctions can form atomically sharp interfaces without lattice matching constraints, and type-II band alignment promotes efficient carrier separation and transfer. Prior Te-based heterojunctions have shown efficient solar energy conversion and fast, sensitive infrared photodetection, but their use in high-performance THz modulators is underexplored. This work proposes Te nanofilms as optically controlled THz modulators and employs substrate engineering and all-2D heterostructures (e.g., with graphene and Ge) to optimize modulation depth and speed, investigate stacking-order effects, and elucidate carrier dynamics and substrate-induced fields.

The paper reviews challenges in THz modulators: the depth–speed tradeoff, insertion losses, and bandwidth limitations. Conventional semiconductors achieve high modulation depth but have relatively slow response, limiting ultrafast applications. 2D materials enable optical, magnetic, and electric control of THz waves; among these, optically driven control offers rapid, non-destructive tuning. However, many 2D-based modulators either exhibit low modulation depth or require high pump fluence. The literature highlights advantages of 2D Te (layer-dependent bandgap, high mobility, strong optical response, air stability) and the promise of 2D van der Waals heterojunctions, which avoid lattice matching constraints and can exhibit type-II band alignment favorable for carrier separation and transfer. Te-based heterojunctions have enabled highly efficient solar cells and infrared photodetectors with high detectivity and fast response, yet few studies have targeted high-performance THz modulators using Te-based heterojunctions. The study positions THz spectroscopy as a powerful nondestructive probe of complex photoconductivity and carrier/exciton dynamics to understand interlayer coupling and substrate effects.

- Materials synthesis: High-quality, large-area Te nanofilms were deposited on fused silica substrates by electron beam evaporation. The nominal thicknesses studied included 50 nm, 100 nm, and 300 nm (100 nm optimized). - Structural and compositional characterization: XRD showed Te (101) and (110) reflections (PDF#36-1452). XPS exhibited Te 3d5/2 at 573.2 eV and Te 3d3/2 at 583.9 eV. Raman spectra showed modes at ~94, 121, and 142 cm−1 (E1, A1, E2). HRTEM confirmed trigonal Te with 0.32 nm lattice spacing for (101) planes. Optical bandgap ~0.37 ± 0.01 eV; film thickness ~100 nm. - Device configurations: All-2D van der Waals heterojunctions were fabricated by stacking Te (100 nm) with monolayer graphene (Gr) and with Ge (100 nm) in both stacking orders (Te/Gr vs Gr/Te; Te/Ge vs Ge/Te). Comparative measurements were also performed on single-component Te, Gr, and Ge films; and supporting comparisons included Ge/Gr and Gr/Ge. - THz spectroscopy: Optical pump–THz probe measurements were performed (schematic provided). Excitation wavelengths included 800 nm (primary) and 400 nm (supplementary). Pump fluence ranged from 2.6 to 260 µJ cm−2. Transient change in THz transmission was recorded, with −ΔT/T0 defined using peak THz amplitude; dynamics were fitted with bi-exponential decays to extract fast (τ1) and slow (τ2) components. Zero-crossing analysis of the THz waveform was used to isolate exciton dynamics. - Frequency-domain analysis: Broadband THz transmission spectra at 0 ps delay were measured; insertion loss derived from 20lg(Esam/Esub). Complex photoconductivity spectra (σ1, σ2) were extracted and fitted with a modified Drude–Smith model σ(ω) = (ne²/m)τ/(1 − iωτd), yielding total scattering time τf and diffusion time τd as functions of pump fluence. - Theoretical calculations: First-principles density functional theory (DFT) calculations of differential charge density were carried out to determine the presence and direction of substrate-induced electric fields and to elucidate their influence on interlayer charge transfer in the photoexcited, non-equilibrium state. - Performance metrics: Modulation depth (MD = ΔT/T0max), carrier lifetimes, exciton lifetimes (from zero-crossing analysis), insertion loss, and fluence dependences were quantified.

- Te nanofilms (100 nm) act as low-loss, broadband THz modulators with ultrafast response: - High baseline transmission: 96.3% without pump; insertion loss −0.33 dB. - Modulation depth (MD): 14.5% at 2.6 µJ cm−2; up to 65.3% at 260 µJ cm−2. - Transient dynamics: biexponential decay with τ1 ~ fast component ~2 ps and τ2 ~ slow component ~8 ps; both largely pump-fluence independent (also unchanged under 400 nm excitation), indicating electron–phonon scattering (τ1) and combined Auger/defect trapping processes (τ2) dominate. - Drude–Smith analysis: with increasing pump fluence, total scattering time τf decreases from 750 fs to 375 fs, while diffusion time τd increases from 21 fs to 53 fs, consistent with increased carrier concentration. - Thickness dependence: Te (300 nm) exhibits much longer carrier lifetimes; Te (50 nm) shows lower MD, identifying Te (100 nm) as optimal for performance. - Te/graphene heterojunctions (Te/Gr, Gr/Te): - Graphene alone shows negative THz photoconductivity and low MD (~2.8% saturation). - Heterojunctions retain positive photoconductivity and slightly reduce carrier lifetimes compared to Te, consistent with interlayer exciton contributions. - Extracted times (800 nm, 26 µJ cm−2): Te τ1 = 1.95 ± 0.12 ps, τ2 = 8.00 ± 0.25 ps; Te/Gr τ1 = 1.85 ± 0.15 ps, τ2 = 7.80 ± 0.20 ps; Gr/Te τ1 = 1.80 ± 0.15 ps, τ2 = 7.90 ± 0.25 ps. - MD enhancement, especially at low fluence: at 2.6 µJ cm−2, Gr/Te MD = 23.5% vs Te = 14.5%; at 260 µJ cm−2, Gr/Te ~69%, similar to Te (limited further enhancement at high fluence). - Te/Ge all-2D heterojunctions deliver substantial performance gains: - Ge alone exhibits weak optical response with negligible exciton contribution and small MD (up to 8.3% at high fluence). - Heterojunctions show stronger dynamics and enhanced exciton components vs Te/Gr and Gr/Te. - Extracted times (800 nm, 26 µJ cm−2): Te/Ge τ1 = 1.60 ± 0.10 ps, τ2 = 7.70 ± 0.20 ps; Ge/Te τ1 = 1.50 ± 0.10 ps, τ2 = 7.75 ± 0.25 ps, indicating ultrafast interlayer charge transfer and interlayer exciton decay; overall relaxation times remain sub-8 ps. - MD at 26 µJ cm−2: Te/Ge = 55.4%; Ge/Te = 69.5%. - Fluence-dependent MD: Ge/Te achieves 27.8% at 2.6 µJ cm−2 and up to 87.6% at 260 µJ cm−2; Te/Ge also markedly enhanced relative to individual layers. The 87.6% MD ranks among the highest for all-2D broadband modulators reported. - Stacking-order and substrate effects: - Electrical behavior differs: Te/Ge shows ohmic-like contact; Ge/Te shows good rectifying characteristics, implying stacking order alters band structures via substrate-induced fields. - DFT differential charge density calculations validate the existence and direction of a substrate-induced electric field, clarifying its role in driving interlayer charge transfer post photoexcitation. - Mechanism: Enhanced modulation arises from efficient interlayer charge transfer and subsequent interlayer exciton recombination at the vdW interface, with substrate-induced fields and stacking order critically shaping the dynamics.

The study addresses the central challenge of achieving both high modulation depth and fast response in all-optical THz modulators. Te nanofilms provide inherently low insertion loss, broadband response, and picosecond carrier dynamics, enabling substantial modulation at low fluence. Forming all-2D vdW heterojunctions with graphene and especially Ge further enhances performance by facilitating efficient interlayer charge separation and transfer, followed by interlayer exciton recombination that expedites recovery within sub-8-ps timescales. The observed dependence on stacking order (e.g., ohmic-like vs rectifying behavior for Te/Ge vs Ge/Te) highlights the pivotal role of substrate-induced electric fields in tuning band alignment and carrier transfer pathways. DFT results corroborate the presence and direction of this field, tying the heterostructure–substrate interplay directly to the improved modulation metrics. Collectively, these findings show that substrate engineering and interface design in Te-based all-2D heterostructures simultaneously optimize modulation depth, speed, and insertion loss, advancing practical THz optoelectronic device development.

This work introduces 2D Te nanofilms as a new class of optically controlled THz modulators that combine low insertion loss, broadband operation, high modulation depth, and ultrafast dynamics. Through all-2D vdW heterojunction engineering—particularly Ge/Te—and deliberate control of stacking order, the devices achieve MDs up to 87.6% at moderate fluence with sub-8-ps relaxation times. Photoresponse and exciton analyses, together with DFT charge-density calculations, reveal that efficient interlayer charge transfer and interlayer exciton recombination, modulated by substrate-induced electric fields, underlie the enhanced performance. These insights provide a pathway for the design and optimization of high-efficiency, ultrafast THz modulators and motivate further exploration of Te-based and other 2D heterostructures, as well as systematic substrate and interface engineering, for next-generation THz optoelectronics.