High-Performance All‑Optical Terahertz Modulator Using Graphene/TiO₂/p‑Si Trilayer Heterojunctions

Abstract

We present a trilayer terahertz (THz) modulator that integrates a p‑type silicon (p‑Si) substrate, a thin TiO₂ interlayer, and a monolayer graphene sheet. The Si/TiO₂ interface establishes a built‑in electric field that drives photo‑generated electrons from Si into TiO₂, and subsequently injects them into graphene. This electron transfer raises the graphene Fermi level, increases its conductivity, and consequently attenuates transmitted THz radiation. The device achieves broadband modulation across 0.3–1.7 THz with a maximum depth of 88 % under 808‑nm laser excitation. These results demonstrate the graphene/TiO₂/p‑Si heterostructure’s promise for next‑generation terahertz imaging and communication systems.

Introduction

Terahertz (THz) imaging and communication are rapidly evolving fields that rely on efficient modulators to control wave transmission and reflection. Traditional semiconductor modulators based on Si or Ge often fall short in modulation depth, prompting exploration of novel materials such as metamaterials and phase‑change media like VO₂. While metamaterial‑based devices offer high speed, their narrow bandwidth and complex fabrication limit practicality. VO₂ modulators, driven by temperature or voltage‑induced phase transitions, suffer from slow thermal dynamics.

Graphene, with its exceptional carrier mobility, tunable conductivity, and mechanical robustness, has emerged as a versatile platform for THz modulation. Early electrically controlled graphene modulators combined with metamaterials achieved up to 47 % amplitude and 32 % phase modulation. Field‑effect transistor‑based graphene modulators, however, were limited by modest carrier injection. Recent graphene/n‑Si devices reached near‑unity (99 %) modulation under femtosecond laser pumping, yet required external electric fields for optimal performance.

TiO₂, a low‑cost, chemically stable oxide, has proven effective in enhancing charge separation at semiconductor interfaces, as demonstrated in MoS₂/TiO₂ and perovskite/TiO₂/Si photodetectors. Leveraging TiO₂’s built‑in field to improve carrier extraction, we fabricated a graphene/TiO₂/p‑Si heterostructure that delivers an 88 % broadband THz modulation depth from 0.3 to 1.7 THz using a simple 808‑nm semiconductor laser.

Methods

Five‑hundred‑micron‑thick p‑Si wafers (ρ ≈ 1–10 Ω·cm) were sequentially sonicated in acetone, ethanol, and deionized water, then etched in 4.6 M HF for 10 min to remove native oxide. A 10‑nm TiO₂ film was grown by immersing the cleaned Si in 0.1 M TiCl₄ at 343 K for 1 h. Monolayer graphene was synthesized on copper via chemical vapor deposition and transferred onto TiO₂ by a wet‑etch method, forming a 1 cm² graphene/TiO₂/p‑Si stack. Raman spectroscopy confirmed monolayer quality (G at ~1580 cm⁻¹, 2D at ~2681 cm⁻¹). UV–vis absorption and ultraviolet photoemission spectroscopy (UPS) characterized optical gaps and band alignment. Static THz modulation was measured with a Fico THz time‑domain system (Zomega Terahertz Corp.).

Results and Discussion

Figure 1a illustrates the device geometry: a 808‑nm semiconductor laser (spot ~5 mm) and a ~3 mm THz beam illuminate the graphene side simultaneously. Laser power was varied from 0 to 1.4 kW cm⁻². Raman spectra (Fig. 1b) show minor G and 2D peak shifts when graphene rests on TiO₂, indicating slight strain but retaining monolayer integrity.

Figure 2a–c displays THz transmittance of bare Si, graphene/Si, and graphene/TiO₂/Si under increasing laser power. Without photoexcitation, all samples transmit ~55 % of THz energy; TiO₂ and graphene introduce negligible loss. Upon illumination, transmittance drops across 0.3–1.7 THz due to enhanced conductivity from photo‑generated carriers. The graphene/Si device shows a steeper decline than Si alone because carriers diffusing into high‑mobility graphene amplify conductivity changes. The graphene/TiO₂/Si structure exhibits the most pronounced attenuation: at 1400 mW, transmittance falls to ~10 %, yielding an 88 % modulation depth. Modulation depth (ΔT = (T₀ – T)/T₀) versus laser power is plotted in Fig. 2d, confirming that the TiO₂ layer amplifies carrier separation and injection into graphene.

Band‑edge measurements (Fig. 3) reveal Si and TiO₂ gaps of 1.19 and 2.98 eV, respectively. UPS data give work functions of 5.85 eV (Si), 5.21 eV (TiO₂), and 5.75 eV (graphene), and valence band maxima at –6.35 eV (Si) and –7.09 eV (TiO₂). The resulting band diagram (Fig. 4) shows a fully depleted 10‑nm TiO₂ layer that forms an n‑type conductive channel under illumination, driving electrons into graphene and shifting its Fermi level upward. This process enhances graphene’s conductivity and increases THz absorption.

Overall, the trilayer heterostructure achieves high‑speed, broadband THz modulation with a simple all‑optical scheme, avoiding complex electrode deposition and femtosecond laser pumping.

Conclusions

We have fabricated a low‑cost, all‑optical terahertz modulator based on a graphene/TiO₂/p‑Si trilayer. The device offers 88 % modulation depth over 0.3–1.7 THz, driven by a 808‑nm semiconductor laser. The TiO₂ interlayer creates a p‑Si junction that enhances carrier separation; photo‑electrons migrate to TiO₂ and then into graphene, raising its Fermi level and conductivity, thereby attenuating THz transmission. The simple wet‑etch fabrication and absence of electrodes make this architecture attractive for scalable THz imaging and communication applications.

Abbreviations

p‑Si:

P‑type silicon

THz:

Terahertz

UPS:

Ultraviolet photoemission spectroscopy