1.19a is a setup diagram using a He–Ne laser having a wavelength of 633 nm. In order to observe the scattering sources existing inside the material, a laser is irradiated to the surface‐polished material, and the scattering state and the intensity of the scattering are measured from a direction perpendicular to the laser irradiation direction using a CCD camera and a power meter. For this material, the laser‐irradiated surface was AR‐coated, and the internal loss of each material was measured from the intensity of the initial beam and the intensity of the laser emitted through the material.
As shown in Figure 1.19b, the optical loss of spinel ceramics shows extremely low scattering of 0.07%/cm, which is much lower than that of the same single crystal produced by the Verneuil or Czochralski method. In Figure 1.19c, the scattering state inside the material was observed with a CCD camera installed perpendicular to the laser irradiation direction. For any materials, the scattering state could not be detected as an image by the laser tomography. Figure 1.19d shows the measurement of weak scattering using a light‐receiving element instead of a CCD camera. The detected scattered light was normalized by setting the scattering intensity from the crystal by the Czochralski method to 100. As a result, the scattering intensity becomes smaller in the order of Verneuil crystal → Czochralski crystal → polycrystalline spinel ceramics by sintering method. Since only one residual pore (approximately 1 μm) is observed in the spinel ceramics, the porosity is at the level of 10−13, and it can be considered that there is no Mie scattering in this material. The residual pore can be detected by a transmission microscope, and when it is irradiated with a He–Ne laser (from the left side of the image), the light is scattered from the vicinity of the interface between the base material and the residual pores (the portion where the refractive index fluctuation exists). In this state, when the light of the transmission microscope is turned off and the observation is continued while irradiating the He–Ne laser, circular Mie scattering can be clearly confirmed scattered from the interface of residual pores and the base material. This scattering conditions are shown in Figure 1.19e. Ceramic materials have numerous grain boundaries, and even though the He–Ne laser passes through the grain boundaries, the scattered light cannot be detected by a normal tomography or a tomography using an optical microscope. This means that scattering from the grain boundaries is very insignificant or almost nonexistent (not limited to the spinel ceramics). Therefore, it is still necessary to seriously discuss grain boundary scattering phenomenon in ceramics, which has been doubted to be used as an optical material.
Figure 1.19 (a) Optical loss at 1064 nm and laser tomography at 633 nm of polycrystalline and single crystal, (b) residual pore and Mie scattering from pore by He–Ne laser.
By the way, Rayleigh scattering is expressed as follows.
where θ is the scattering angle, I0 is the light intensity before transmission, n is the refractive index; R is the distance between the measurement point and the scattering source, d is the scatterer size, and λ is the measuring wavelength. Basically, light scattering increases in proportion to the power of the scatterer size to the sixth power, the reciprocal of the measurement wavelength to the fourth power. In common sense which is well recognized by almost all material scientists until now, “There are many dislocations at the grain boundaries in the ceramics and their dislocations become scattering sources causing grain boundary scattering, so the transmittance of the ceramics increases as the wavelength becomes shorter.” However, the obtained result is opposite to the conventional common sense that “polycrystalline ceramics having grain boundaries are superior in optical properties to single crystals, and in particular, in the short wavelength region, they show a significant difference in optical properties.”
It is important that the optical material must be “extremely low scattering,” but optical uniformity is also a very important parameter. Laser beam patterns after passing through the spinel ceramic (25 mm‐thick), Czochralski and Verneuil single crystals irradiated with a He–Ne laser having a Gaussian mode are observed with a beam profiler, and these results are summarized in Figure 1.20. When laser irradiation is performed, no scattering line is detected inside the spinel ceramic material (see Figure 1.20a), and only Fresnel scattering (surface scattering due to the difference in refractive index between air and the base material) is observed at the input and output surface of laser irradiation. As a reference, the original beam pattern is shown in Figure 1.20b‐1. The beam that has passed through the Verneuil spinel single crystal with significant optical inhomogeneity showed the greatest distortion (see Figure 1.20b‐2). The beam that has passed through the spinel single crystal by the Czochralski method is also deformed into an elliptical (vertical) shape (see Figure 1.20b‐3). Only the beam that has passed through the ceramic maintains a concentric shape similar to the original beam (see Figure 1.20b‐4) (because the material surface is not AR‐coated, the laser beam that has passed through any material is attenuated by about 24% compared to the original beam due to surface reflection). Beam quality is a critical parameter for optical materials, and the superiority of ceramics, which is different from the conventional understanding, has been proved. The fact that spinel ceramics exhibit extremely low scattering and high beam quality is closely related to the microstructure of the material. As can be seen from the laser tomography shown in above Figure 1.19, a clean grain boundary in which no grain boundary phase exists reduces Rayleigh scattering. In addition, since there is almost no residual pore as a main scattering source, Mie scattering can be almost completely eliminated.
Figure 1.20 (a) He–Ne laser irradiation test and (b‐1) original and (b‐2‐4) changing of beam pattern via various specimens.
The following results clearly indicate that high beam quality, one of the lifelines of optical materials, can be guaranteed. The internal optical stress of the spinel single crystals prepared by the Verneuil method and the Czochralski method was observed using a polarizer. In addition, the uniformity of the refractive index inside these crystalline materials was observed with a Schlieren imaging system. These observation results are summarized in Figure 1.21b‐1, b‐2, c‐1 and c‐2. The Verneuil single crystal shows significant optical inhomogeneity as well as significant optical stress. Since the single crystal of the Czochralski method has a core at the center of the grown crystal ingot,
