Direct laser bonding of transparent materials using ultrashort laser pulses at high repetition rates
Transparent materials surround us everywhere in our daily life. Especially glass has a dominant role due to its excellent optical, mechanical and chemical properties. For the processing of glass numerous techniques are well known, some of them are more than several hundred years old. In contrast, the reliable and stable bonding of two different glasses is still a demanding problem. Most of the developed glass bonding techniques are adapted from well known silicon waver techniques [1, 2]. All the established methods e.g. optical contacting, direct bonding or anodic bonding exhibit certain disadvantages as these methods were not designed for the bonding of glasses [3, 4]. For example, optical contacting - were two ultraclean glass samples are pressed together and adhere due to the van der Waals forces between their surface atoms - yields only weak bonds. Alternatively, laser pulses can be used to weld the glass samples together. Commonly high average power cw-lasers are used for laser welding. However for the absorption of cwlaser radiation an opaque material is required. Alternatively, ultrashort laser pulses have proven to be a powerful tool for the processing of transparent materials within the last two decades. The extremely short pulse duration allow nonlinear processes that fundamentally differ from traditional light-matter interaction mechanisms. Nonlinear absorption induced by ultrashort pulses leads to an extremely non-equilibrium state in a confined volume resulting in a variety of different material modifications [5, 6]. Due to the highly localized energy deposition machining of sub micron features and full three dimensional processing of transparent materials become feasible [5, 7]. In fused silica three different types of modifications can be induced: isotropic and anisotropic index changes [5, 6, 8, 9, 10] and the generation of small cavities [11, 12]. Promising applications include the realization of photonic circuits [13, 14, 15, 16], birefringent elements [17, 18], microfluidic channels  or data storage devices .