Luminescence Properties In Er～(3+): YAG And Er～(3+): YAlO₃ Under Ultraviolet Excitation

The trivalent erbium (Er³⁺) ion is an attractive optical activator that can emit fluorescence in ultraviolet, visible and infrared range due to its rich energy level structures. In the last decade, there has been an intense interest in studying erbium doped laser crystal. Especially YAG and YAlO₃ crystals are most extensively investigated due to their superior hardness, thermal conductivity and optical properties. It is important for realizing lasing emission to study the properties of fluorescence decay. Most studies about the properties of fluorescence decay were focused on infrared and visible range. The properties of fluorescence decay under violet excitation remain to be explored. In this paper, the time-resolved laser-induced fluorescence technique was used to investigate and measure the fluorescence lifetimes and luminescence characteristics of the three ultraviolet multiplets ²P_3/2,²G_9/2 and ⁴S_3/2 levels for three crystals Er³⁺ (1at%):YAlO₃,Er³⁺(10.5at%):YAG and Er³⁺ (8.4at%):YAG. These crystals have the same sizes: 1.0×1.0×1.0 cm³ and all the experiments were carried out at room temperature (300 K). In order to eliminate the affection of cascade transitions, the selective excitation method was applied for the above six interested energy levels. A tunable OPO laser system (Continuum Sunlite EX) with 8ns duration at a repetition of 10Hz provided the required pulse laser, and frequency doubling of the OPO laser output used a BBO crystal. All emission spectra were taken photographically on a monochrometer with a resolution of 1 ?, which was equipped with a photomultiplier tube (Hamamatsu R3896) at the exit slit. For fluorescence signal collection and recording, a Tektronix TDS 620B 500MHz digital real-time oscilloscope was used; while a boxcar integrator (Stanford Research Systems SR250) was used for fluorescence and excitation spectra measurements. The luminescence lifetimes of the three energy levels were obtained by an exponential fitting of fluorescence decay curves, In the crystal of Er3+(1at%):YAlO3, they are 47.1 μs, 587.9 ns, 108.2 μs, respectively. In the crystal of Er3+(10.5at%): YAG, they are 1.86 μs,132.2 ns,158 ns, respectively. And In the crystal of Er3+(8.4at%):YAG, they are 12.3 μs,155.3 ns,5.98 μs, respectively. According to the formula describing the relation between the spectral intensity parameters ? and concentration X, we calculated the total radiative probabilities for 4S3/2 in Er3+:YAG crystals with different Er3+ concentrations by the Judd-Ofelt theory, and we can find how the theoretical lifetimes and measured lifetimes and the probabilities of cross relaxation change along with the Er3+ concentration in Er3+:YAG crystal. When the Er3+ ions were excited by 408.6nm OPO laser into the 2G9/2 multiplet, the fluorescence spectra and the kinetics of fluorescence decay of the 2G9/2, 4S3/2 states were recorded. In this paper, by analyzing time-resolved signals we can distinguish the stronger emission of violet light (402 nm-422 nm) and green light (540 nm-573 nm) from the transition (2G9/2-4I15/2),( 4S3/2-4I15/2)and (2G9/2-4I13/2)+( 4S3/2-4I15/2) respectively. We will provide the detailed discussion in the next subsection. The Stark splits of energy levels were obvious from the fluorescence spectra. Sequentially we recorded the decay curves at 541.9 nm, 543.2 nm and 545.9 nm. Fig.4 shows the decay curves in a larger scale time. The front parts of these curves are not pure exponential decay. These curves exhibit a clear rise time about 560 ns. We considered the increasing lifetime and the rise time originated from the multiphonon and cross relaxation process. The influence of multiphonon and cross relaxation on the fluorescence decay was analyzed. The calculated rate of population decay was given. The varieties of the decay curves...