Ultrafast laser direct welding of sapphire and Invar under non-optical contact conditions with white-light-interferometric gap measurement

Robust welding of transparent optoelectrical windows and metal frames depends on the precise regulation of the interfacial gap. In this study, we presented a white-light-interferometer (WLI)-based method to measure the interfacial gap of ultrafast-laser-welded samples, thereby establishing a direct correlation between interfacial gap thickness, joint morphology, and mechanical performance. By systematically varying the interfacial gap from intimate contact to non-optical contact, we identify distinct bonding mechanisms: sapphire melting with smooth interface at small gaps, spallation-induced metal eruption and rough interface at medium gaps, crack-rich joint at large gaps. Fracture and cross-sectional analyses of SEM and EDS results confirm that the interfacial eruptions originate from laser-induced spallation, providing new insight into the bonding mechanism. The welded joints achieved shear strengths up to ∼291 MPa, representing 73% of the intrinsic strength of sapphire, and the large-scale components welding was successfully demonstrated. These results lay the foundation for advanced optoelectronic packaging and integration.