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I worked for a refractory company specializing in steelmaking and was planning to retire as a refractory engineer after graduating from the university (master's course) in 1983. In 1991, my boss, the research director, said to me, “I want you to develop anything good new technology.” New technology for refractory companies generally means “development of refractories useful for steel smelting.” However, as a young and motivated engineer at that time, “anything good” was interpreted by me as “new development in any area of expertise is okay!” I was very interested in ceramics at that time, but I was quite an amateur, so if I have to do anything new, I chose a research that is the most challenging in the world and in which no one has succeeded until now. I had read a variety of literature and judged that “laser oscillation with polycrystalline ceramics would be the most difficult technology.” At that time I focused on the article “Polycrystalline Ceramic Lasers, J. Appl. Phys. (1973)” by C. Greskovich and J. P. Chernoch, but their results were distinctly different from laser oscillation. Besides, similar research was not reported by other researchers. I suddenly understood that this must be a certainly difficult technology. I also understood that even single crystals cannot oscillate laser with high efficiency in the case of lamp excitation system. I thus interviewed several Japanese laser specialists and scientists regarding the possibility of developing ceramic lasers, but the only answer I received was “even glasses and single crystal laser gain media have optical problems; ceramics with lots of scattering sources aimed for laser gain media is absolutely meaningless.” The same question was asked to material scientists as well, and their answers were also similar that “translucent ceramics has been developed, but its optical quality is low quality that cannot be compared with single crystal. So, you should quit this foolish idea to develop a ceramic laser.” Even from the viewpoint of scattering theory, it seemed considerably impracticable, so I presumed that “this must be a new technology,” and it became the starting point for my new research topic i.e. the development of ceramic lasers.

However, since I was merely a refractory engineer with not much expertise in ceramics or laser, I did not know what the fundamental problems are and how to approach the development of ceramic lasers, except that I had to create the material from scrap. The only thing I could think was “first of all I just have to completely eliminate the scattering sources and then the last remaining problem is the existence of the grain boundary,” and I believed that “my challenge will succeed if there is no optical problem with the grain boundary.” The kamikaze challenge began in the summer of 1991, in autumn I succeeded in making a transparent YAG (Y₃Al₅O₁₂) sintered body, and in December a transparent YAG ceramics doped with laser active element Nd was successfully produced. Although I knew it was still of insufficient optical quality, I requested the research institution and companies for laser oscillation test using the samples, but they all rejected it because of the only reason being that it was a “polycrystalline material.” Finally, in late December, I brought my ceramic samples to Osaka University, Laser Fusion Research Center, which is the only university in Japan interested in ceramic lasers. Prof. K. Yoshida also tested it with uncertainty, but the next day he was able to oscillate the CW laser using my samples at room temperature. This was the birth of the world's first ceramic laser. However, one year after the first laser oscillation, research and development was interrupted because I worked in a refractory company where research and development of refractories is the main focus. In 1994 I first presented the possibility of laser oscillation by ceramics at a conference in Japan, but most researchers ignored the results. For the purpose of summarizing my research results, I submitted a paper titled “Fabrication and Optical Properties of High‐Performance Polycrystalline Nd:YAG Ceramics for Solid‐State Lasers” to the journal of American Ceramic Society in 1995, after which the study was completely stopped. In order to continue the development of ceramic lasers, I retired from the refractory company in 1996, joined a private company, and later joined a research institution; however, “ceramic laser” was not well recognized in Japan. Eventually, there was no way but to establish my own company in 2005 and continue the research and development of ceramic laser. The research was suspended for 14 years from the birth of ceramic lasers. Meanwhile, research on ceramic lasers, which is more promising than single crystals for their higher performance and high output power, has been active abroad and received a lot of funding. I have attempted to resume ceramic laser development for 14 years, and fortunately, since 2007 I have been able to resume research on “ceramic laser” with support from AOARD/AFOSR (Asian Office of Aerospace Research and Development/Air Force Office of Scientific Research), which are part of AFRL (Air Force Research Laboratory). Bringing out truly new science and technology on the basis of our understanding of the surroundings is not an easy one, and in my case it took me a lot of endurance and perseverance.

Incidentally, ceramic lasers have grown into an important technology that can replace a single crystal laser gain medium. Today, ceramic laser has become a paradigm of solid‐state laser especially in generation of a 100 kW class laser by a large medium that is difficult to manufacture with a single crystal and high functionality by forming composite gain medium (such as Nd:YAG‐Cr⁴⁺:YAG composite that can simultaneously provide laser generation and switching function); generation of megawatt/cm³ class power density by a small medium that is resistant to laser damage; laser generation by a new type of material such as Y₂O₃, Lu₂O₃, etc., which is difficult to manufacture with a single crystal; and so on. The existence of super high‐quality single crystals capable of optical amplification and the fact that ceramics comparable to that single crystal was successfully developed have initiated the development of other optical ceramics. One example is development of Pr:LuAG (Lu₃Al₅O₁₂) or Ce:LuAG ceramics as a high‐speed scintillator for PET (positive electron tomography) or high‐energy physics, which requires high transparency and gamma ray shielding function. Although still under development, they are expected to have wide applications in the future. Recently, the industrial application of 1 μm band fiber laser has advanced, and the demand for isolators has also increased accordingly. Currently, the main material for this application is TGG (Tb₃Ga₅O₁₂) single crystal, but in recent years, the same TGG ceramic has also been developed and commercialized. Since this material has insufficient Verdet constant and tends to generate thermal lenses, recently ceramic materials such as TYO (Tb₂O₃‐Y₂O₃) and TAG (Tb₃Al₅O₁₂), which are difficult to grow by single crystal growth techniques, have been reported, and in the near future market share of ceramics is expected to increase. Iron garnet ceramic isolators have also been reported for telecommunication (1.3–1.5 μm band), indicating the possibility of clearing the technical and economic problems of current single crystal isolators. Development and practical application of blue‐violet LED and LD are advancing. In addition, LED lighting that can convert blue‐violet light into white light was also commercialized by using organic–inorganic composite phosphors in which Ce:YAG powder was dispersed in silicone resin. However, the development of all‐ceramic phosphors (Ce:YAG ceramics as a representative example) has been carried out in response to the demand for long durability and high power application, and some applications have begun to be applied in automobile head lamps and projectors. In addition, for military applications, the development of high strength and highly transparent ceramic dome and ceramic armor to replace sapphire single crystal has also been advanced, and the basic technology of laser ceramics will be applied in various fields in the near future. It has been thought that grain boundary scattering (Rayleigh scattering) cannot be avoided even if scattering sources, excluding grain boundaries, are completely eliminated in ceramics. It should be noted that Rayleigh scattering is the theory of the scattering phenomenon in the atmosphere and is not premised on scattering in real substances (ceramics). Of course the theory is correct, but it seemed that we, the material scientists, wrongly imagined the limits of material development just by assumption, without confirming the truth of natural science. Polycrystalline ceramics with optical properties superior to high‐quality single crystals have been developed in the wavelength range from ultraviolet to visible to infrared recently, and the concept of optical material development based on the conventional theory has been completely overturned. This “technological innovation” has caused the trend in optics to move from single crystal to polycrystalline ceramics.

This book “Breakthroughs in Optical Materials” not only introduces research and development examples starting from the development of ceramic lasers that broke through conventional common sense, but also mentions historical background, theory, manufacturing process, and applications. This book is a compilation of the transparent ceramics revolution that I started working on since 1991.

Akio Ikesue