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
1 Introduction
Akio Ikesue and Yan Lin Aung
World‐Lab Co., Ltd. Mutsuno, Atsutaku, Nagoya, Japan
1.1 Introduction
Human beings have used ceramics symbolized by tableware from ancient times, but the modernization of ceramics and the “ceramic science” based on sintering began since the middle of the twentieth century. In recent years, engineering ceramics used for bearing parts, milling media, surface plate for semiconductor steppers, minor parts of automobile engines, pyroelectric materials for infrared detection, PTC (positive temperature coefficient), NTC (negative temperature coefficient) thermistors as temperature sensors, inkjet printer, touch panel, moreover, sonar for fish finder in fishery and military application, piezoelectric material as ultrasonic diagnosis in medical field, ionic conductors for air‐fuel ratio control in automobile and gas sensor for oxygen detection in molten steel in steel production, magnetic materials used in general motor and servomotor, translucent ceramics as functional materials used in high‐pressure sodium discharge lamps and optical shutters, and so on. Without ceramics, the economic activity of modern society is impossible now.
The development of the translucent ceramics mentioned above was initiated by the development of translucent alumina ceramics by Dr. Coble in the 1950s and its application to high‐pressure sodium lamps [1]. Principally, ceramics has been considered to be opaque, but he controlled the microstructure of ceramics (that is, by controlling the migration rate of pores and grain boundaries in the process of sintering alumina), and was able to reduce the volume of residual pores, characteristic light scattering sources in ceramics, and finally, he succeeded in developing alumina ceramics which can transmit visible light for the first time in the world. By applying this idea, PLZT (lead lanthanum zirconate titanate) ceramics for flash protection, MgF2 ceramics for infrared ray transmission, scintillation optical ceramics such as Pr,Tb:GOS (Gd2O2S) ceramics and Eu:(GdY)2O3 ceramics for X‐ray CT (computed tomography) etc. were successively developed, and some of them are already applied in industrial field. About 50 years have passed since the invention of translucent alumina ceramics by Dr. Coble, and it has been applied to high‐pressure sodium discharge lamp until today (2018).
Figure 1.1 Schematic design and transmission mechanism of sodium lamp using translucent Al2O3 ceramic tube.
As shown in Figure 1.1, the high‐pressure sodium lamp is illuminated by discharging the metal sodium (Na) inside the alumina tube by applying a high voltage and radiating the luminescent line to the outside of the discharge tube. The radiation efficiency of high‐pressure sodium lamp is about 14–20% which is very high compared to 1–3% of the normal light bulb, has high power, and has a long life (8–9000 hours) so it is used for lighting in tunnels and express highway. The wall thickness of this alumina tube is at most about 1 mm, and the in‐line transmittance is only about 20–30%, and the other discharge lights are radiated from the inside to the outside by diffuse transmission.
Radiated light certainly generates Fresnel loss of about 7% inside the discharge tube, and it is also radiated while repeating transmission and reflection inside the tube. The total transmittance (i.e., amount of light radiated to the outside of the discharge tube/total radiation amount) of the alumina discharge tube is about 97–95%, and the light energy of 3–5% is lost. That is, if calculated simply, optical loss of 3–5%/mm (discharge tube thickness) is generated. The optical quality of these alumina tubes is not a major problem for discharge lamp application even if the in‐line transmission and scattering characteristics are not sufficient. Because it is sufficient as long as the total transmission (total of in‐line & diffuse transmission) is high. Even in the translucent alumina ceramics that has been continuously improved, there are still many scattering sources inside; for example, residual pores, pinning agents such as MgO, Y2O3 situated at the grain boundary portions, and also grain boundary phases (generally spinel and YAG [Y3Al5O12]) due to the reaction between the pinning agents and the host material. The optical loss of a laser gain medium which
