Broadband Terahertz Response Enhancement in Zinc Telluride via Nanostructuring-Induced Near-Infrared Local Field Effects

Zinc telluride (ZnTe) is a benchmark electro-optic crystal for terahertz time-domain spectroscopy and an infrared window material. However, the high refractive index of ZnTe causes approximately 20% efficiency loss due to Fresnel reflection, while conventional antireflection coatings suffer from poor adhesion. In this study, a versatile strategy combining the Langmuir–Blodgett method with reactive ion etching is proposed to fabricate centimeter-scale nanostructure arrays on a ZnTe crystal surface, enabling simultaneous performance enhancement in the near-infrared (NIR) and THz bands. Well-defined and 100% covered nanostructures with characteristic dimensions ranging from 100 to 450 nm were obtained by precisely modulating the mask dimensions and etching parameters. The effective refractive-index gradient formed by nanostructures with diameters of 300–450 nm suppresses Fresnel reflection across 800–2500 nm, yielding a transmittance enhancement of 18.5%. Furthermore, finite-difference time-domain simulations were employed, revealing the strong resonance modes surrounding 164 nm diameter units, which enhance the local power density of 800 nm probe light up to 5.6-fold. Consequently, the THz detection sensitivity was enhanced by 47.1% over a broadband range of 0.1–2.5 THz, highly consistent with the simulation results of 45%. This work develops a scalable method for fabricating large-area, controllable nanostructures on ZnTe to achieve dual-band enhancement, which alleviates the intrinsic efficiency limitations of systems based on high refractive-index crystals.