<110> orientation which has the smallest surface‐free energy is used but <110> or <100> oriented seed crystals are also applied in some cases. This seed crystal is immersed in the molten YAG, and then, crystal growth is continuously carried out at a rotation speed of 10 ~ 30 rpm and a pulling rate of about 0.2 mm/hour. In the case of Nd‐doped YAG crystal, Nd ions substitute Y sites in YAG lattice. The ionic radius of Nd ion is too large compared with that of the Y ion, so it is well known that it is not easy to dissolve the Nd ions in the YAG host crystal as a solid solution. The segregation coefficient of Nd ion to YAG host crystal is very small (that is, the concentration ratio of Nd in crystal to Nd in the melt is very small). According to the literature, since the segregation coefficient of Nd to YAG crystal is about 0.2 [4], normally the concentration of Nd ions in the melt is set to be several times higher than the target Nd concentration (that is, the concentration of Nd doping in the YAG sinter body which is used as starting material for melting is prepared with higher than the target composition). Even if the concentration of Nd in the melt is prepared with very high, the concentration of Nd in the YAG crystal may not be automatically increased homogeneously and simply in proportion to its concentration. When the concentration of Nd in the YAG crystal is increased to higher than 1 at.%, many precipitates (scatterers) are generated in the crystal and it is difficult to utilize it as a laser gain medium. Even in the case of the commonly used 1 at.% Nd:YAG, (211) facets tend to be formed from the pulling axis of the crystal (ingot) toward the outer periphery [4], and thus, only the outer periphery of the columnar crystals can be used as a laser gain medium. Also, when pulling YAG single crystal, the Nd concentration in the growth crystal is significantly lower than that in the melt, so the Nd concentration in the melt increases as the crystal grows. For this reason, the concentration of Nd in the melt at the initial stage of growth differs from the concentration of Nd at the middle to end stage of the growth. So, the grown crystal also suffers this influence, resulting in a gradient concentration change of Nd in the crystal growth direction. Due to this drawback, each end face of the laser rod is influenced by the composition variation accompanying the Nd concentration change. Therefore, only crystals with nonuniform refractive index are produced. This is a disadvantage in the principle of crystal growth.
Figure 1.3a shows an image of the optical quality of a YAG crystal (ingot) doped with about 1 at.% Nd. In the center part of the ingot, there are a core (strong birefringence part), a large number of facets (consecutive layers with different Nd concentrations = growth striation) from inside the material to the outside peripheral part, and almost no optically homogeneous part. Figure 1.3b shows the appearance of a commercially available Nd:YAG slab and the photograph of the same material observed under a polarizing plate (crossed nicol). Commercially available Nd:YAG single crystals have very high transparency, and they appear optically very uniform in the naked eye observation. However, when observing through the polarizing plate, the layered facets can be detected at irregular intervals in the direction crossing the length direction of the slab. It is not necessarily optically uniform that the optical quality of the single crystal of the highest level at the present time that is commercially available in the market. If an optically inhomogeneous part is remained in the laser gain medium, the optical amplification efficiency and beam quality will be extremely lowered. Therefore, required characteristics for optical quality of single crystal for use of laser gain media are set very strictly. The quality control at the actual single crystal production is carried out in the following procedures. First of all, positions with less thermal distortion and refractive index change in the Nd:YAG ingot are searched by observing through a polarizing plate or interferometer. Next, using a laser light, the concentration of the scatterers existing in that part is inspected, and then, a portion with good quality is taken out by boring and it can be used as a laser gain medium. However, even if there are many nonuniform portions, it is not a big problem when the laser emission direction is perpendicular to the facet. The optical loss of the highest quality Nd:YAG single crystal at present is 0.1%/cm level or less, and this quality is remarkably higher than the same single crystal synthesized by the Verneuil method in the 1964s. However, single crystalline technology is already technically limited, and it is principally difficult to obtain higher quality due to its technological limitation. To grow a YAG single crystal, it requires not only a very expensive Ir crucible and growing equipment, but also its growth speed is very slow, and it takes about one month to grow an ingot having a diameter of 4–6 in. and a length of 8 in. In addition, the initial cost (very expensive growth equipment) and the running cost (electricity, crucible recovery cost, etc.) are very high, and the yield of the laser gain medium is very low, which is disadvantageous in economy. On the other hand, it is difficult to obtain a large‐sized laser rod or slab even from a technical point of view, thus leaving technological problems such as difficulty in generating high output laser.
Figure 1.3 (a) Optical quality image of Nd:YAG single crystal ingot and (b) appearance of commercial Nd:YAG crystal slab and its observation under polarizer and crossed nicol.
Source: Akio Ikesue, Yan Lin Aung, Voicu Lupei (2013), Ceramic Lasers, Cambridge University Press. https://doi.org/10.1017/CBO9780511978043.
YAG laser material has superior overall characteristics as compared with other lasers but the Nd:YAG single crystal which is the most critical part in the solid‐state laser system has economical (including productivity) and technological problems as described above. It is the actual condition that there are many unsolved problems. It is difficult in principle to break through the current problems with the conventional single crystal growth method, and hence, the creation of new innovation is indispensable.
1.3 Problem of Conventional Translucent and Transparent Ceramics
As mentioned above, regarding translucent ceramics, Dr. R. L. Coble developed translucent alumina in 1959 [1], and GE applied it to arc tube for high‐pressure sodium lamp in the 1960s [5]. Although polycrystalline ceramics has been considered to be opaque up to now, it was experimentally proved that light can be transmitted (diffuse transmission in case of alumina) after reducing residual pores and sintering until high density. After that, purity, particle size, and homogeneity of the starting material were well controlled, and the sintering process based on the sintering theory was improved to produce sintered body with a high purity and high density, in which the microstructure of the ceramics was controlled. Many studies on synthesis of various translucent ceramics have been conducted under such technical background, and some of them were applied in practical applications such as Gd2O2S:Pr and (YGd)2O3:Eu as scintillators for X‐ray CT (computed tomography), Ce:YAG ceramic phosphors for whitening the GaN‐based blue‐violet LED (light emitted diode), and LD (laser diode), and so on. But, these materials are also not transparent, and they are just translucent quality. There are many scattering sources in these translucent ceramics.
However, the translucent ceramics developed in the past only showed “translucency or transparency” in appearance only when the sample is thin, and there were almost no ceramics with high optical quality. Very few studies have been reported about the optical constants of transparent ceramics that have been successfully synthesized. In the previous reports up to now, since the optical properties of ceramics with grain boundaries are significantly inferior to those of single crystals, only photographs of sample with small thickness are shown in their reports to convince that their ceramics apparently have high optical quality.
Transmittance curves of translucent alumina ceramics prepared by hot pressed process and by normal pressure sintering are shown in Figure
