From Chinese Academy of Sciences [中国科学院] (CN) : “Researchers Discover Origin of Near Ultraviolet and Visible Absorption Characteristics of Ti: sapphire Laser Crystals”

From Chinese Academy of Sciences [中国科学院] (CN)

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Fig. 1. (a) The supercell structure of Al2O3, (b) the interstitial Ti3+, Al vacancy and substitutional Ti3+ models, and their transformation process, (c) the line-contact Ti3+-Ti3+ ion pair model, (d) the face-contact Ti3+-Ti3+ ion pair model, (e) the point-contact Ti4+-Ti3+ ion pair model (Al vacancy is considered as the charge compensation mechanism of Ti4+). (Image by SIOM)

Recently, a research group from the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) carried out a theoretical research on the origin of Ti: sapphire laser crystal in near ultraviolet and visible regions using the first principles method based on density functional theory. Related research results have been published in Materials Today Communications on June 4.

Ti: sapphire, also known as Ti-doped α-Al2O3 single crystal, is a very important laser crystal material. At present, it is also one of the key materials in a class of super-intense, ultrafast, and tunable laser devices. Since the laser properties of it was reported in 1982, the origin of some suspicious absorption phenomena in the optical absorption band of Ti: sapphire has been one of the focuses of attention and research.

According to the wavelength distribution, these questionable absorption bands can be roughly divided into three regions: the near ultraviolet absorption band with a peak at 390 nm, the visible absorption band with multi-peak configuration and small bumps, and the residual infrared absorption band overlapped with the laser emission band.

In this study, the researchers performed a systematic theoretical study on the suspicious absorption phenomenon of Ti: sapphire in near ultraviolet and visible regions.

Through the analysis of the crystal structure of alumina and the calculation of the electronic and optical properties of the possible single Ti doping defect models and Ti ion pair defect models in Ti: sapphire, they pointed out that when there is an Al vacancy near the interstitial Ti3+, the interstitial Ti3+ will enter the Al vacancy through structural relaxation, and finally form defect equivalent to the substitutional Ti3+.

The charge transfer transition of substitutional Ti3+ ion’s 3d electron from Ti 3d orbital to Al 3s3p orbital is the main reason for the near ultraviolet absorption band, and the calculated absorption spectra are in good agreement with the experimental spectra.

Moreover, the multi-peak configuration and bumps of the visible absorption band are mainly caused by the contribution of the line-contact Ti3+-Ti3+, face-contact Ti3+-Ti3+, and point-contact Ti4+-Ti3+ ion pairs.

In addition, the researchers provided a more comprehensive understanding of the multi-peak configuration and bumps of visible absorption bands from the perspective of ligand field theory and thermal activation.

This study not only reveals the origin of the suspicious absorption characteristics in Ti-doped Al2O3 crystal but also provides ideas for the study of defects and properties of similar transition metal ions doped oxides having corundum structure.

This work was supported by the Strategic Priority Research Program of CAS, the National Key R&D Program of China, the National Natural Science Foundation of China, etc.

See the full article here .

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The Chinese Academy of Sciences [中国科学院] (CN) is the linchpin of China’s drive to explore and harness high technology and the natural sciences for the benefit of China and the world. Comprising a comprehensive research and development network, a merit-based learned society and a system of higher education, CAS brings together scientists and engineers from China and around the world to address both theoretical and applied problems using world-class scientific and management approaches.

Since its founding, CAS has fulfilled multiple roles — as a national team and a locomotive driving national technological innovation, a pioneer in supporting nationwide S&T development, a think tank delivering S&T advice and a community for training young S&T talent.

Now, as it responds to a nationwide call to put innovation at the heart of China’s development, CAS has further defined its development strategy by emphasizing greater reliance on democratic management, openness and talent in the promotion of innovative research. With the adoption of its Innovation 2020 programme in 2011, the academy has committed to delivering breakthrough science and technology, higher caliber talent and superior scientific advice. As part of the programme, CAS has also requested that each of its institutes define its “strategic niche” — based on an overall analysis of the scientific progress and trends in their own fields both in China and abroad — in order to deploy resources more efficiently and innovate more collectively.

As it builds on its proud record, CAS aims for a bright future as one of the world’s top S&T research and development organizations.