Weiyi Jia

Phosphorescent dynamics in SrAl2O4: Eu2 +, Dy3 + single crystal fibers

Abstract We report for the first time the growth of activated alkaline earth aluminate single crystals with long persistent phosphorescence. Measurements of the phosphorescence and decay dynamics on samples of SrAl2O4 crystals doped with Eu2 + only and with Eu2 + and Dy3 + indicate that the trapping rate of Dy ions is very fast; the efficiency of trapping is estimated to be about 40% at room temperature. The decay curve of the after-glow is nonexponential, and cannot be explained simply by thermally activated detrapping processes. Hole transport may be an important factor in persistent phosphorescence.

Performance of a Cr:YAG laser

We report on the performance of a Cr/sup 4+/:YAG laser, tunable over the range from 1.32 to 1.53 mu m, under different operating conditions. Laser action has been achieved by pumping with a Nd:YAG laser in the Q-switched mode and cw mode and with a Cr:Nd:GSGG laser in the long pulse mode. Slope efficiencies up to 22% in the Q-switched mode have been achieved. The lowest threshold was 30 mJ for the long pulse pumped mode. Excited state absorption seems to limit the tuning range and the output characteristics. >

Irradiation-induced luminescence enhancement effect of ZnS:Mn2+ nanoparticles in polymer films

The effects of irradiation-induced luminescence enhancement by ZnS:Mn(2+) nanoparticles in poly(vinyl butyral) films are reported. The luminescence intensity increases several times when fresh samples are irradiated by a 248-nm excimer laser. The decay time also increases with exposure time. The increase in the initial intensity of the slow component of luminescence makes the main contribution to the enhancement effect. A tentative model is proposed in which the efficiency of the energy transfer to Mn(2+) ions increases with exposure. States at interfaces are expected to play an important role in this process.

Hot luminescence of Mn2+ in ZnS nanocrystals

Abstract Time resolved emission spectra of Mn2+ in ZnS nanocrystals were studied. Two split-pair bands at high- and low-energy sides of the peak position of the conventional Mn2+ emission band were observed at short delay times. They emerge gradually into the conventional emission band of Mn2+. These two bands were assumed to be hot luminescence which comes from the vibronic transitions from the vibrational states of the excited state of Mn2+ to its ground state. The population and energy relaxation times of hot electrons were deduced.

Near infrared luminescence properties of the laser material Cr: Y2SiO5

Abstract We report spectroscopic investigations of the near infrared (NIR) laser center in chromium doped Y 2 SiO 5 (YSO). A comprehensive study including polarized absorption and emission spectra, and the temperature dependence of the fluorescence decay time and the integrated emission intensity is presented. In addition, we performed high resolution excitation and emission measurements. The results are most consistent with the NIR center being attributed to Cr 4+ in a distorted tetrahedral site. The assignment of the energy levels and the identification of the metastable level responsible for the NIR laser action will be discussed in detail. The measurements further show that Cr: YSO does not contain octahedrally coordinated Cr 3+ which is in contrast to Cr : YAG and Cr : forsterite. In this sense, Cr: YSO can be regarded as a model system for studying the optical properties of the new NIR center.

Optical properties of ZnS : Mn2+ nanoparticles in polymer films

Abstract In this paper we report the abnormal phenomena observed in ZnS : Mn2+ nanoparticles embedded in polymer films. Irradiation-induced luminescence enhancement (IILE) effect was observed. The luminescence intensity of fresh samples under irradiation by 248 nm excimer laser light increases several folds with exposure time. After exposure, the luminescence intensity slightly decreases within hours, then reaches a stable value. Another abnormal property of ZnS: Mn2+ nanoparticles in PVB film is the thermal behavior of luminescence intensity. Luminescence intensity of Mn2+ nanoparticles in PVB film increases several times with temperature from 8 to 275 K. then decreases with temperature. A tentative model has been proposed, in which efficiency of energy transfer to Mn2+ ions increases with exposure. The electron states at the interface are expected to play an important role. The energy transfer to Mn2+ in ZnS nanoparticles is assumed to be phonon-assisted. The experimental data arc fitted by the calculated curves.

Stress effects on the fluorescence spectra of tetravalent chromium in some crystalline hosts

Abstract We have investigated the effects of uniaxial stress on the fluorescence spectra of Cr-doped forsterite, YAG and Y2SiO5. Large spectral shifts were found in all three crystals. In addition, pseudo-splittings of the emission spectra were observed. The large spectral shifts allow us to conclude that the lasing emission in these crystals originates from the 3T2 state but not the 1E state. The stress effects also provide onformation which helps us to identify the nature of the site being occupied by tetravalent chromium ions in these crystalline hosts.

Charge transfer vibronic excitons in ferroelectric oxides : experiment and theory

Abstract Review of the charge transfer vibronic excitons (CTVE) study on the basis of optical response investigations in ferroelectric oxides is presented. CTVE is the correlated pair of electron and hole small polarons whose stability is controlled by a pair pseudo-Jahn–Teller effect. The study is focused on the CTVE-induced critical dependence near the ferroelectric phase transition. The critical effect in absolute diffraction efficiency (ADE) formation of the laser-induced transient gratings near ferroelectric phase transition in KTa 1− x Nb x O 3 (KTN) is considered as an example. Critical concentration dependence for slow as well as for instantaneous (fast) optical response was detected. The peak positions for these x – dependences were different. The CTVE-model can explain these experiments taking into account not only the soft TO phonon mode contribution but also that of soft tunnel quasilocal modes of clusters of the second component.

Vibrational dynamic of the Raman-active mode in barium nitrate crystal

The relaxation time, 1047 cm−1, of the Raman-active mode in barium nitrate crystal has been measured as a function of temperature by a time-resolved coherent anti-Stokes probe scattering technique. The temperature dependence is consistent with the combination of two relaxation mechanisms involving four and six phonons that are dominant at low and high temperatures, respectively. It was found that the Raman mode has a rather long decay time, resulting in attractive stimulated Raman scattering features of barium nitrate crystal.

Up-conversion of multi-site Er in LiNbO3 single-crystal fibers

Abstract In this work, the near infrared to visible up-conversion process was studied in Er3+ doped LiNbO3 single-crystal fibers. The absorption spectra of the 4I 9 2 multiplet and site-selective excitation of up-converted emission from the 4S 3 2 multiplet were measured at 10 K. Multiple sites for Er3+ were found to exist in the fiber samples. They fall into two groups which may be related to the substitution of Li and Nb by Er. Near-infrared excitation spectra of the up-conversion and transient dynamics have shown that either excited state absorption (ESA) or energy transfer up-conversion (ETU) processes play dominant roles at certain pump wavelengths. In addition, laser-heating-induced up-conversion or photon avalanche up-conversion may also take place at some pump wavelengths.