Yanyun Wang

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.

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.

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.

A new analysis method to characterize the S-band luminescence decay of porous Si

Abstract Analysis of the non-exponential luminescence decay and extraction of additional physical information from it are important issues for the spectroscopic study of porous Si. In this work a new approach to extract the decay time distribution function is proposed, based on the fact that the non-exponential luminescence decay of porous Si is a multiple exponential. It was found that the integrated intensities of different exponential components could be fitted by a Gaussian distribution function, characterized by τ 0 , the decay time with maximum probability, and d , the width of the Gaussian distribution function. The parameters τ 0 and d were found to vary with emission wavelengths. τ 0 decreases exponentially with an increase of the emission energy and d has a smaller value in the middle of the emission wavelengths.

Up-converted energy transfer from Er3+ to Cr3+ in LiNbO3 single crystal fibers

In this paper, site occupations of Er3+ and Cr3+ in LiNbO3 (LNO) single crystal fibers are discussed. A pairing structure of Er-Cr tends to provide a geometric advantage for cooperative processes. Efficient up-converted energy transfer from Er to Cr was observed by pumping the 4I92 state in the near infrared. Downward energy transfer from Cr3+ to Er3+ was studied by pumping the 4T2 state of Cr3+. The up-converted and the downward energy transfer can be used to develop diode-pumped tunable lasers or to sensitize 1.5 μm Er lasers, respectively.

Effects of uniaxial stress on the metastable level in Cr4+: Y2SiO5

Abstract When applying uniaxial stress on Cr4+: Y2SiO5 a pseudo-splitting of the electronic origin at 8709 cm-1 is observed. Furthermore, the electronic origin and the broad emission band were very sensitive to stress. In contrast, the splitting of the excited state doublet remained nearly unchanged under stress.

Formation of small polaron and excitation transfer in niobate crystals

Abstract Using a ps-YAG laser operated at 532 nm as a light source for luminescence and degenerate-four-wave-mixing (DFWM) measurements on niobate crystals such as S0.61B0.39Nb2O6 (SBN), KNaSBN, KTN family (KTa1−xNbxO3)and KNbO3, a broad emission at 400–700 nm with lifetime of ~ 10 ns was observed. It is due to the charge carriers produced in conduction band by two-photon absorption (TPA), trapped at Nb5+ center to form Nb4+ − O− small polarons. In Ce3+-doped niobate crystals the DFWM experiment shows that the excitation of niobate host can transfer to cerium, resulting in Ce+3 emission which would otherwise not be able to be observed by direct excitation of cerium at 532 nm.

Excitation dynamics of cerium in niobate crystals

Abstract Transient gratings have been observed in Ce 3+ -doped SBN(Sr x Ba 1− x Nb 2 O 6 ) and KNSBN (potassium–sodium SBN) niobate crystals by picosecond-pulse, two-photon excitation using degenerate four-wave-mixing (DFWM) in the case where the C -axis is perpendicular to the grating vector. The charge-carriers produced in the conduction band are responsible for the observed coherent signal associated with the third-order nonlinearity, while a post-coherent, slow signal is due to the temporal lattice distortion associated with formation of Nb 4+ -hole polarons. Following excitation of the niobate host, energy transfer between host and rare-earth ions occurred, resulting in the excited state of cerium ions being populated and cross-luminescence from host to Ce 3+ being observed. This excitation mechanism was also verified by a fluorescence study.