Plasma behaviors and welding property control in non-optical contact metal-glass laser welding.

Reliable femtosecond laser bonding of glass and metal is urgently required for extreme-service optical components, yet severe thermal expansion mismatch and roughness-induced interfacial air gaps severely degrade joint performance. This work systematically investigates plasma behaviors and welding property control in metal-glass laser welding under non-optical-contact conditions based on the regulation of metal surface roughness. Four gradient-roughness specimens are prepared via grinding and opal polishing suspension (OPS) polishing, with Sa ranging from 439.33 to 13.42 nm. High-speed imaging verifies that microgrooves on ground steel trigger lateral plasma escape and insufficient interfacial fusion, yielding a shear strength below 0.56 MPa. By contrast, OPS nanopolishing eliminates interfacial gaps to realize optical contact, confining plasma vertically and boosting joint strength to 16.73 MPa. Scanning electron microscope (SEM) characterizations confirm wider Si-Fe-O interdiffusion zones and cohesive fracture inside fused silica for polished samples, where longitudinally constrained high-density plasma provides persistent vertical action to drive sufficient bidirectional melt interpenetration at the interface. This study reveals the full correlation among surface topography, plasma dynamics, elemental diffusion, and mechanical performance, offering a pretreatment strategy for high-strength transparent-metal joints.