title: Hydroxide catalysed bonding applied to new materials for photonics applications
Creators: name: Lacaille, Gregoire
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Hydroxide catalysed bonding, adhesive-free bonding, phosphate glass, YAG, strength test, reflectivity, light induced damage threshold, surface activation.

description: In the recent years, bonded components have seen a rise in popularity in solid state laser gain media as a way to mitigate thermal issues which are detrimental to their performances and limit power scaling. The bonding technique is currently selected based on a trade-off between cost and performance. Hydroxide catalysed bonding is a rather young technique coming from the field of gravitational research which has not really made an impact in photonics applications. Yet it is believed that thanks to its low cost and overall good performance, there is a non-negligible share of applications for which it could prove as the most advantageous process. In this work phosphate glass and YAG are selected as materials of interest due to their common use as host matrices for solid-state lasers gain media. Both these materials have a different chemistry from the materials of interest in previous studies on hydroxide catalysed bonding and the bonding mechanisms are to a large extent unknown. Therefore the work has to be carried out from first principles. The highlighted properties to study are the reflectivity and the Light Induced Damage Threshold (LIDT) of the bond which are both of interest when the interface has to be crossed by a laser beam. The mechanical strength is added as a way to check the robustness of the component, to obtain an insight into the curing kinetics and to address concerns associated with manufacturability. Results on how the parameters of the solution used affect both the optical and mechanical properties of the bonded component, and which solution is the most suitable for photonics applications, are presented. The beginning of this work largely draws on a presentation of the science case and a review of the relevant literature, from the description of the heat generation phenomenon in laser operation, to designs and techniques that allows mitigation of this heating. Initial trials were carried out on phosphate glass. The parameters for the aqueous solution used in hydroxide catalysed bonding are not standardized and therefore various solutions were used to estimate the range of properties that can be obtained with the technique. Successful bonding of phosphate glass with either pure hydroxide (HCB) or silicate-enriched hydroxide (SB) was achieved. The strength of the bond was tested using 4-point flexure technique and strengths of about 13 MPa and 3 MPa were obtained for SB and HCB, respectively, from the first week of curing and with no significant variation with more time. The reflectivity of the bond at 532 nm was measured using an in-house built setup fully described in this work and the values obtained were of the order of magnitude of 0.01 % (HCB) and 0.1 % (SB). Since the lower strength can be a limitation to handle and manufacture components, it was decided to focus on SB samples for the following trials. The parameters of the solutions were changed and as a result the reflectivity was reduced to values between 0.01 % and 0.1 % depending on parameters. The reflectivity trials also allowed an estimation of the bond thickness and refractive index in a non-destructive way using an optical model based on thin-layer interference described in this work. The kinetics of the change of these properties with curing time was studied. These gave valuable insight into the bonding mechanisms, and hypotheses were made on the chemistry occurring. The LIDT trials were outsourced and values obtained were about 1.75 GW/cm2 in the near infrared long pulses regime with no significant influence from the parameters of the solution used. The attention was then shifted towards YAG again with a major focus on the reflectivity of the bond and improvement of the existing setup. Mechanical strengths of about 24 MPa (SB) after 2 months of curing were obtained. The reflectance obtained for the interface was about 0.2 % which makes it borderline for photonics applications without further work. However this value is relatively low compared to the ones obtained in similar studies on sapphire, and further investigations on the chemistry were carried out using x-ray analysis to understand why. From the results, bonding mechanisms that differ from the ones presented until then are suggested – mechanisms that form a hybrid layer of bonding chemical and substrate material - and the outlooks of the use of the techniques for new materials are discussed. Finally various studies are presented on parameters that could affect the transfer of the process from a laboratory to a production environment such as that at Gooch and Housego. Reduction of curing time by applying elevated temperature would allow decreasing the lead time of the process. Control over the settling time of the bond would allow better yield on parts by making the operation easier. Last but not least, a UV/Ozone cleaning process, as an alternative to the current abrasion process used, would open the technique to a new range of samples designs while increasing the yield for larger batches.