Читать книгу Processing of Ceramics - Группа авторов - Страница 13

Since single crystal materials are widely applied in various industrial fields and it is difficult to describe the technical problems of these single crystals together at one time, firstly we will focus on laser crystals that require the highest quality. In 1960, laser oscillation was firstly demonstrated by Cr doped Sapphire by Maiman, and then, laser oscillation at room temperature using Nd‐doped YAG (Y₃Al₅O₁₂) single crystal by Guesic in 1964 was a trigger for the birth of solid‐state laser. After laser oscillation by YAG single crystal, unique laser performance of various types of laser gain media, such as YVO₄, Cr:Forsterite (Mg₂SiO₄) KGW (KGd(WO₄)₂), Ti:Sapphire (Al₂O₃), Cr:ZnSe, and Cr:ZnS, has been reported. However, new laser gain materials, which can exceed the YAG, have not been found in total performance including the quality of materials, and even now, the mainstream of solid‐state lasers is YAG and it is still unchanged. YAG materials are applied in most of solid‐state laser as gain medium, and almost all of them are single crystals grown by the Czochralski (CZ) method. A transition metal element such as Cr, Ti, or a lanthanide element such as Nd, Yb, Er, Tm, and Ho is added to the YAG single crystal as a laser (active) element. Among these active elements, Nd is belonging to the four‐level system and it is easy to create a population inversion (a state in which number of electrons in the upper state is higher than the lower state) at the f‐f electron transition of Nd by external excitation. The laser oscillation is relatively simple in this system, and it has a fluorescent line with a narrow spectral line width and high quantum efficiency; hence, it is considered to be the most important laser active element in YAG host crystal. In recent years, however, LD (laser diode) excitation system became typical in these days and strong excitation became possible as well; therefore, Yb with three‐level system with higher quantum efficiency of 91% has also been used as laser gain material. The CZ method shown in Figure 1.2 is generally used to grow the YAG single crystal. The starting materials are Y₂O₃, Al₂O_3, and Nd₂O₃ powders with high purity above 4 N (99.99 mass%) grade, and these raw materials are weighed in YAG composition (not strict stoichiometric composition) and mixed. Then the mixture is molded and calcined. A relatively dense sintered body obtained by sintering is used as a raw material for melting. It is filled in an iridium (Ir) crucible. The Ir crucible is heated by high frequency induction and melted at temperature higher than the melting point of YAG (1950 °C).

Figure 1.2 Schematic diagram of YAG single crystal grown by CZ method.

Generally, YAG seed crystals with <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.