Ultrafast laser direct welding of sapphire: interfacial microstructure, joining mechanism and optical performance optimization

Abstract

Sapphire is critically important for ultraviolet photodetectors, high-temperature sensors and optical windows. Traditional brazing and diffusion bonding suffer from integral heating, structural deformation and optical performance degradation. In this work, an innovative interlayer-free ultrafast laser joining method was developed to realize localized low-thermal-damage direct joining of sapphire. The influences of laser power, defocus distance and scanning speed on the microstructure and mechanical properties of joints were systematically investigated. Under the optimal parameters of 150 mW laser power, −75 μm defocus distance and 4 mm/s scanning speed, the joint achieves the maximum shear strength of 36.2 MPa. The joint formation mechanism was further revealed, the upper sapphire produces plasma under ultrafast laser irradiation, followed by ultrafast cooling to form a robust joint. The transient high-temperature and high-pressure effect combined with ultrafast cooling promotes the formation and retention of metastable phases and amorphous structures at the interface. Furthermore, the proposed joining design with a reserved optical window effectively alleviates light attenuation caused by micro-defects and well maintains the intrinsic broadband optical transmittance of sapphire. This study provides a high-precision, low-damage and reliable joining strategy for sapphire, and offers technical and theoretical support for the packaging of high-end optoelectronic devices.