# Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE
## 1 Introduction
### Atomic Diffusion Bonding (ADB)  
Critical components for thin-disk laser development are high-quality, efficiently cooled thin disks. Since the disks themselves are very thin and fragile, they are carried by a heatsink with high thermal conductivity. The bonding layer between the gain medium and the heatsink must be highly thermally conductive, as well as mechanically rigid in order to withstand high operating temperatures and mechanical stresses caused by thermal gradients during the laser operation. Thin disks are often bonded to a metallic heatsink by indium-based metallic solder. Such a connection is not perfectly homogeneous; the thermal conductivity of such a contact is insufficient for kW class lasers, and creates a bottleneck for efficient heat extraction. Soldering also generates residual mechanical stresses in the gain medium. Therefore, the usual approach is to use a special epoxy bonding to a diamond heatsink. We employed another technique for the first time, so-called Atomic Diffusion Bonding (ADB) [36], which is an epoxy-free process based on the recrystallization of the contacting layers, which thus allows the formation of a perfect thermal bridge [37].  
The first demonstration of the ADB process was realized by the bonding of Yb:YAG to an Al heatsink (Figure 3). Nanocrystalline Au films were deposited on both Yb:YAG and Al heatsink surfaces with Ti underlayers, as shown in Figure 3. The Ti underlayers enhanced the adhesion strength of the deposited films on the bonded surface and controlled the crystallographic orientation of the deposited films. Au-layer-coated surfaces were bonded at room temperature under high vacuum environment and pressure application. During this procedure, the Au film recrystallizes and creates a single metallic layer. ADB with extremely thin Au films ensures better thermal management, mechanical tolerances, and prevention of the out-gassing of materials in a vacuum or high temperature environment. The strength of the connection depends on film thickness, surface cleanliness, and the quality of surface polishing. Since the thickness of the nanocrystalline layer can be controlled within a certain percentage of variation, such a uniform bonding interface offers the fabrication of a laser active medium with tight tolerances on thickness. Unlike thermal diffusion bonding or soldering, no annealing or curing at high temperatures is required, making it possible to work with materials of different coefficients of thermal expansion.  
We experimentally compared the potential of commercially available indium-soldered Yb:YAG thin disks and ADB bonded Yb:YAG/YAG composite ceramics thin disks [38]. It was proven that the peak surface temperature of ADB disk pumped at 280 W (4.5 kW cm\({}^{-2}\) for the incident pump beam) is 57 \({}^{\circ}\)C lower than the temperature of the soldered disk under identical operating conditions. By a thin-disk surface deformation measurement setup developed at HiLASE [37], we also demonstrated the different mechanical behaviors of the disk when pump radiation is applied. The CW output power reached in the proof of principle experiment was 100 W and 177 W from the soldered and the ADB  
Figure 3: Process of Atomic Diffusion Bonding (ADB): **(a)** Thin nanocrystalline Au films are deposited on the precisely polished and coated Yb:YAG and the Al heatsink surfaces at room temperature by the vacuum sputtering process. The bonding is accomplished in air by contacting the two Au surfaces and pressing. Recrystallization occurs at the Au–Au contact interface and the two surfaces can be bonded together without the use of adhesives or heat; **(b)** Schematic diagram of Yb:YAG/YAG composite ceramic thin disk on the Al heatsink fixed by the ADB technique **(left)** and commercially available Yb:YAG thin disk on the CuW heatsink fixed by soldering **(right)**[36].  
disk, respectively. A new series of ADB disks with optimized gain medium and bonding procedure parameters with metallic and dielectric heatsinks for kW lasers is now under preparation.