Joining of transparent material along free-form interfaces

The applications of ultrashort laser pulses are versatile and numerous. One current research field is the stable bonding of different transparent materials.

Fig.1: Bonding of transparent materials.

By using a high repetition rate laser system, heat accumulation of successive pulses leads to melting around the focal region of the ultrashort laser pulses. Thus the laser acts here as a three-dimensionally localized heat source. This may be used to weld two samples, if the focus is placed at the interface. Due to the localized and defined heat deposition, the induced thermal load and stress is minimized and bonding of dissimilar materials becomes possible. Furthermore, this versatile technique enables tailored bonding along three dimensional geometries. With this technique, we could achieve bonding stabilities which are comparable to the ultimate strength of the bulk material.
In comparison with conventional bonding technologies, which require different fabrications steps (for example the use of adhesives or the annealing at high bonding temperatures), the laser assisted bonding offers several advantages:
- minimal processing time
- minimal induced stress
- bonding along free form interfaces
- maximum bonding stability
- bonding of dissimilar materials
- gas proofed sealing
- no outgassing of adhesives etc.

Microscopic front view at ultrashort pulse induced bonds between different glass samples
Fig.2: Microscopic front view at ultrashort pulse induced bonds between different glass samples.

Summary of the achieved breaking stabilities
Fig.3: Summary of the achieved breaking stabilities: The gray bars identify the breaking strength of
          the unmodified bulk materials. The blue bars represent the optimal welding results for
          homogeneous material combinations and the orange bars depict different material combinations.

Bonding of BK7 on silicon
Fig.4: Bonding of BK7 on silicon: Within the microscope image (left) the laser modified glass
         (retraced by dashed line) and molten silicon can be seen. The Raman signal of the silicon-
         peak (520 cm-1, left image) indicates that the molten silicon flows into the glass, forming
        stable bonds between the materials.

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