of the spinel single crystal is analyzed, MgO/Al2O3 (M/A Ratio) was 1.00, and the average composition of the material is almost stoichiometric.
It is considered that the cause of the nonuniform refractive index domain structure observed by Schlieren observation is that the M/A ratio in each domain is different. When materials having different spinel M/A ratios are formed, cation or anion defect structures are induced to form inside the material, and these defect structures certainly affect the band gap. Therefore, it is considered that the absorption edge of the single crystal shifted to the longer wavelength side. Since the refractive index domain structure could not be detected in the spinel ceramics, the shift of the absorption edge due to defects inside the material was small, and this result was similar to the transmission spectrum behavior by computational science. This result breaks the conventional outline that the optical uniformity of single crystals is absolutely superior to ceramics.
The optical uniformity of ceramics reported to date (2020) is far superior to single crystals regarding different types of materials, and the superiority of optical quality of single crystal material which has been regarded as common sense began to collapse. It is extremely difficult to explain by classical theory about the optical properties of ceramics produced by modern science. Rayleigh's scattering theory was built in the nineteenth century, but it is a mathematical expression of the atmospheric scattering phenomena by calculation. The author thinks that the Rayleigh's scattering theory is still correct in the sense of its theory, but when applying the theory by computational science to a practical material, we should consider again the contradictions existing in theory and in reality, etc., and therefore, it is essential to predict the future of material science in this way.
Some technological examples which changed the material science will be described in Chapter 4. It is difficult to produce TAG (Tb3Al5O12) and YIG (Y3Fe5O12) single crystals by FZ method, and it is not easy to synthesize high‐quality crystals because these single crystals are incongruent materials. The optical quality of ceramics prepared by sintering method for these materials is equal to or superior to those of single crystals, demonstrating the scale‐up and high productivity of optical performance and size that cannot be realized with single crystals. For optical isolators used in laser processing in the 1 μm band (especially for fiber lasers), TO (Tb2O3) and TYO (Tb2O3‐Y2O3) ceramics could be expected to become Faraday rotation elements having the largest Verdet constant, but their melting point is as high as around 2400 °C and also have phase transition point just below the melting point; therefore, the synthesis of single crystals is virtually impossible. For this reason, the representative of the Faraday rotation element is TGG (Tb3Ga5O12) crystal. But the TO and TYO ceramics which the author first demonstrated in the world show Verdet constants which are nearly four times that of the TGG single crystal, and this fundamental performance makes the isolator device much smaller. An isolator device using this material has already been put to practical use and contributes to the high reliability of a fiber laser for processing.
1.5 Conclusions
As mentioned above, it is a translucent ceramic pioneered by Coble in the 1950s, but the technical potential since 1995 has significantly improved since the development of ceramic laser materials. Indeed, ceramics became a paradigm of optical materials, and in the future industrial application, it is beginning to change the era from “optical single crystal to optical ceramics.” There are some technical issues, but one of them is that the crystal structure of current ceramic (polycrystalline) optics is limited to cubic system. Since the anisotropic material has birefringence, there is no possibility as an optical material from the viewpoint of the current science and technology. However, if it is possible to create true nano‐sized materials without birefringence which has never been experienced, there is the possibility of initiating the same technological innovation as cubic system ceramics. On the contrary, if it is possible to create a single crystal from compositionally homogeneous fine grains, there is also the possibility of an ideal crystal that can breakthrough an anisotropic single crystal synthesized from the conventional melt solidification method. The synthesis of laser grade ceramics by sintering method suggests the development of infinite material science in the future.
Finally, what the scientist should not forget is “What is the truth and what should we do for the future?” the authors themselves will continue to try new challenges based on this philosophy.
References
1 1 Coble, R.L. (1959). Am. Ceram. Soc. Bull. 38 (10): 501.
2 2 Ikesue, A., Kinoshita, T., Kamata, K., and Yoshida, K. (1995). J. Am. Ceram. Soc. 78 (4): 1033–1040.
3 3 Coble, R. L. (1962). Transparent alumina and method of preparation, U.S. Patent 3026210, 20 March 1962.
4 4 Adachi, G., Shibayama, K., and Minami, T. (1987). New Technology for Advanced Materials, 1830. Kyoto: Kagaku‐Dojin.
5 5 R. L. Coble. U.S. Patent 3026210, 1962.
6 6 (a) Greskovich, C. and Woods, K.N. (1973). Ceram. Bull. 52 (5): 473–478. (b) Greskovich, C. and Chernoch, J.P. (1973). J. Appl. Phys. 44 (10): 4599–4605.
7 7 Greskovich, C. and Chernoch, J.P. (1974). J. Appl. Phys. 45 (10): 4495–4502.
2 Ceramic Laser/Solid‐State Laser
Akio Ikesue and Yan Lin Aung
World‐Lab Co., Ltd. Mutsuno, Atsutaku, Nagoya, Japan
2.1 Background
Lasers are generally classified into (i) semiconductor lasers, (ii) gas lasers, and (iii) solid‐state lasers and applied to various fields.
Regarding solid‐state lasers, research, and development on various lasers have been carried out with the discovery of ruby laser by Dr. Maiman in 1960 as a trigger. Among them, continuous‐wave laser oscillation at room temperature using Nd:YAG single crystal by Dr. Guseic in 1964 and tunable ultra‐short pulse laser using Ti:Sapphire single crystal by Dr. P. F. Moulton in 1982 are technically excellent, and it is still applied industrially.
Solid‐state laser gain medium is used as a so‐called “coherence converter of light” that converts white light into coherent light; hence, the obtained yield (conversion efficiency) of coherent light is obtained with respect to excitation light energy decreases. However, the important thing is the nature of coherence light. As the energy density is remarkably high when collimated, and even small energy can be applied to high‐precision processing technologies such as laser marking and cutting. Since the laser amplifies the coherent light inside the gain medium while injecting the excitation light, very low optical scattering and very good optical uniformity are required as a laser gain medium. For this reason, glass or single crystals have been used conventionally. Since the former can synthesize a large laser gain medium, it is generally used for drivers in laser fusion, but because of its low thermal conductivity, it is difficult to repeat lasing and continuous oscillation of the laser. Single crystal materials are excellent in thermo‐mechanical properties and can be easily used for industrial laser because they can oscillate in continuous‐wave and pulsed laser mode. However, it is difficult to manufacture gain media on large scale, and it is also difficult to generate large output power.
When a transition metal element such as Cr, Ti, etc. or a lanthanide rare earth element such as Nd, Yb, Er, etc. is added to a laser gain material, these laser‐active elements absorb light energy of a specific wavelength and then emit fluorescence. Strong monochromatic light obtained by stimulated emission of the fluorescent rays inside the optical resonator is laser. Conventionally, white light such as a flash lamp or an arc lamp was used as an excitation source, but the absorption wavelength