Processing of Ceramics. Группа авторов
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 1.4 together with the transmittance curve of Sapphire single crystal as a reference. In the case of alumina whose crystal structure belongs to hexagonal system, even the thickness of samples is as thin as 1 mm, their transmittances are very different to each other. This means that there will be a significant difference in transmittance when the thickness becomes larger. Optical loss is an important factor for knowing the transparency of the material. Even for materials with cubic crystal system such as MgO and Spinel (MgAl2O4), the optical loss of these transparent ceramics is remarkably larger than that of their single crystal counterparts. The optical performance of transparent ceramics was very poor in the material synthesis technology before laser ceramics was reported, and therefore, industrial utilization of translucent ceramics was extremely limited.
Figure 1.4 Relationship between wavelength and in‐line transmittance for commercial sapphire single crystal, Al2O3 ceramics by hot pressing, and Al2O3 ceramics by pressureless sintering.
Let us consider the relationship between microstructure of polycrystalline ceramics and its transmission characteristics. An image of light scattering caused