Two-step sintering process improves the performance of planar waveguide----Shanghai Institute of Optics and Fine Mechanics

The SSCG (solid-state single crystal growth) technique is a novel PWG (planar waveguide) preparation method, which involves using a single crystal seed to induce absorption of grains inside the ceramic interior accompanied by appropriate heat treatment. The interface between ceramics and seed crystal disappears as the seed crystal grows inside the ceramic matrix, forming an integrated composite structure. The PWG prepared by SSCG method can completely restrict the rare earth ions inside the core layer, forming a step change of refractive index and making the incident light completely reflect in the core layer, thus improving the light output quality. Hence, SSCG method emerges as an effective method for producing high quality PWG. However, during the SSCG process, pores remain at the original interface between the ceramics matrix and seed crystal due to the thermal etching, which remains a challenge for further application.
Recently, a research team, led by Professor ZHANG Long from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, has successfully fabricated high transmittance YAG (yttrium aluminum garnet) PWG composite structure without pores on the bonding interface by two-step sintering process. The work was published on Journal of the European Ceramic Society.
In their experiment, the YAG raw ceramics were vacuum presintered at 1600 ℃ for 5 h under a vacuum of 10^-3 Pa, and the relative density of ceramics reached more than 97.5%. The high relative density made the shrinkage of ceramics in the subsequent sintering process very small, not easy to crack. The presintering YAG ceramics were polished on two sides to make them optically bond with the YAG single crystal, and then the composite sample were sintered at 1770 ℃ for several hours under a vacuum of 10^-3 Pa to induce the SSCG process. In the end, the PWG structure without pores on the bonding interface was obtained.
As we all know, the smaller the value of grain size is, the larger the free energy difference between the seed crystal and ceramic grain is, and the lower the temperature required for the SSCG process is. When the average grain size of the ceramic presintered under 1600 ℃ is only about 2 μm, the SSCG process of the corresponding composite sample begins to occur at a temperature lower than that of thermal etching. That means the seed single crystal starts to absorb the small grains of the ceramics at a lower temperature, and the original bonding surface disappears earlier. In this process, the thermal etching will not occur, and the pores will not be introduced into the bonding interface.

The significantly improved optical properties of PWG indicates that the use of low-temperature presintering ceramics in SSCG method can effectively improve the optical quality of composite samples, which makes the sample more suitable for application in high-power solid lasers.

SEM micrographs of the surface of the presintering ceramic.

Optical microscope micrographs of the composite samples growth area and bonding surface from top view.

Article website:https://doi.org/10.1016/j.jeurceramsoc.2019.04.019

Contact:
Mr. Cao Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn