Collinear Pulse Train PLD: Fabrication of High-Refractive-Index-Difference TiO2/ZnO Multilayers with Multifunctional Applications

Pulsed laser deposition (PLD) is widely used for functional film fabrication, but traditional nanosecond-laser-induced thermal effects and interface roughness severely limit the quality of multilayer structures. To address this critical challenge, a picosecond pulsed laser with collinear pulse train output was adopted for TiO2/ZnO multilayer preparation, achieving dual advantages of thermal diffusion suppression and roughness reduction. A systematic investigation was conducted on the properties of TiO2 and ZnO films, establishing a “constant-deposition-rate multi-pulse regulation” strategy that yielded low roughness (4.43 nm for TiO2, 3.27 nm for ZnO) and optimized refractive index matching. Through 500 °C oxygen annealing, TiO2’s refractive index was enhanced to 2.6, forming a large refractive index difference (Δn = 0.77) with ZnO (~1.83) for efficient photonic band gap (PBG) regulation. Integral annealing was identified as the optimal post-treatment, enabling the four-layer TiO2/ZnO multilayer to reach a maximum reflectance of 75% with excellent structural uniformity. The multifunctional applications of the multilayers exhibit excellent ability in photocatalytic degradation of tetracycline hydrochloride (TCH) and fluorescence enhancement of CdSe quantum dots (QDs). This work pioneers a high-quality PLD-based multilayer fabrication route and opens new avenues for its application in environmental remediation and optoelectronic devices.