Energy transfer in YAG ceramics codoped with Nd³⁺ and Cr³⁺

Abstract

Recently, ceramic lasers have been attractive since 1995 [1,2] because ceramics can be produced in large volumes , made into composite media with complicated structures, and heavily and homogeneously doped with laser-active ions. The ceramic lasers with high power density and high resistance to laser damage are very useful for energy conversion system from inexhaustible solar energy to other optical energy. Absorption spectra of Cr 3 + ions in ionic crystals, for example , Y 3AIsOl2 (YAG) cover the visible range (400-700 nm). YAG ceramics codoped with Nd 3 + and Cr 3 + ions are renewed as a laser medium with excitation of solar energy because of highly efficient energy transfer from Cr 3 + to Nd 3 + ions [3,4].YAG ceramics codoped with lat.% Nd 3 + and O.lat.% Cr3+ were sintered by Konoshima Chemical Company. Fluorescence spectra and decay curves were measured in the temperature range of 100-500 K using excitation of tunable laser light from an OPO (Lotis-TI , LT2215), a near infrared InGaAs detector (Thorlabs, PDA400), and a digital oscilloscope (Yokogawa, DLl740). The decay curves of the 1064 nm Nd 3 + fluorescence consist of rising and falling components.The curves represent nonexponential functions (fast and slow decay components). Figure I shows the temperature dependence of rise times, short and long lifetimes, and the integrated intensities estimated from the decay curves observed with excitation of 432 nm. The short lifetimes (0.45 ms) are almost constant in the range of 100-500 K, whereas the long lifetimes (5 ms at 77 K) gradually decrease up to 200 K, rapidly decrease in the range of200-350 K and almost constant above 400 K. The rising component appears above 200 K where the fast decay component decreases . The rise time rapidly decreases up to 500 K. The fast and slow decay components are due to the 1064 nm fluorescence emitted directly from Nd 3 + ions and through energy transfer from Cr 3 + to Nd3+, respectively. The decrease of the rise time suggests an enhancement of the energy transfer rate from Cr3+ to Nd3+. As a consequence, the integrated intensity of the Nd fluorescence observed at 500 K is about twice that at 77 K. Figure 2 shows schematically the energy levels of Cr 3 + and Nd 3 + in YAG and the energy transfer from Cr 3 + to Nd 3 +.