Investigation of Striae Patterns in Laser Crystals

Abstract

This research involves studying the concentration variations of dopant ions in the striae patterns observed in crystals. These patterns may be due to the temperature variations during growth process. Materials to be investigated are 2 µm laser materials which are of great interest for atmospheric remote sensing measurements. Trivalent Thulium (Tm3+) sensitized trivalent holmium (Ho3+) doped crystals are used for 2 µm lasers. Tm is used to enhance Ho emission in the 2 µm wavelength region. Tm absorbs well at the wavelength ~780 nm which is suitable for diode laser pumping. The energy transfer process called “The Cross Relaxation Process” which takes place among Tm ions is described as follows: An excited Tm ion in the 3H4 state decays to the 3F4 state by exciting another Tm ion in the 3H6 ground state to the 3F4 state. Tm in the 3F4 state then transfers its energy to the upper laser level 5I7 of Ho enhancing the 2 µm emission from this level. The cross relaxation process has been found to be highly concentration dependent. However, even when high concentrations of Tm are examined, the decay of the 3H4 manifold has always been found to have a nonexponential curve. The noneven concentration distribution in the striae patterns could contribute to this behavior of the Tm decay curve. Absorption, emission and lifetime measurements are being conducted on 5 to 11% at. Tm doped crystals using an excitation source with a small beam diameter (smaller than the width of striae).