​Optical properties of sapphire crystals (3)

Optical properties of sapphire crystals (3)

Luminescence in sapphire is caused by impurities and lattice defects. Several weak luminescence bands are found in sapphire. Their intensities are related to the type of impurity and the history of the crystal. The luminescence is related to lattice defects and impurity ions, respectively. In particular, the emission region of chromium ions is at 650-774 nm; the reflection region of manganese ions is at 680 nm; the emission at 620 nm is due to vanadium ions and the emission at 480, 530 and 600 nm is The emission line corresponds to the trivalent cobalt ion. The most extensive luminescent impurities are chromium and titanium. The Cr:Al2O3 crystal has a strong RL band mainly located at 695nm (1.78eV), a prolonged afterglow (over 10h) at room temperature, and is quite rich Collected light, and a typical TSL peak at 300 °C, the intensity of the peak is related to the chromium content.

The comparison of sapphire grown by different methods [54] shows that the crystals obtained by the hydrothermal method have the smallest light enrichment, weak afterglow and fast afterglow decay under laser, X-ray, and electron excitation. At the same time, these crystals have the lowest Radiation resistance. In general, the emission of crystals and the increase in temperature increase the afterglow intensity of some bands or change the ratio of their intensities. Only in Ti3+:Al2O3 crystals does the luminescence intensity at nitrogen temperature exceed that at room temperature [55]. Figure 2.46 gives a typical example of the intensity proportional behavior of the afterglow band [56]

The intensity of the low-energy part of the luminescence spectrum suddenly decreases at liquid helium temperature, because the phonon part of the luminescence is frozen out. Many types of luminescence (eg, cathodoluminescence and photoluminescence) have the same properties in unactivated crystals . So in some papers reported that no photoluminescence in cathodoluminescence can be attributed to the difference in excitation density.

The TSL of unactivated sapphire can be identified by the presence of thermal peaks at 408 K. In UV-irradiated Ti:Al2O3, TSL is characterized by the largest thermal peaks at 423 and 495-500 K. The TSL spectrum consists of 290 and 410 nm band composition. The intensity and spectrum of TSL are determined by the amount of titanium, the oxidation-reduction potential of the crystal and the annealing atmosphere. To explain this determination, three models of activated vacancy centers and centers have been proposed [57].

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