Shangda Xia

Optical properties of nanocrystalline Y2O3:Eu depending on its odd structure

The structure of nanocrystalline Y2O3:Eu prepared by a combustion reaction was analyzed by XRD and high-resolution electron microscopy. Compared with a large-scale particles, 5-nm Y2O3:Eu particles presented as distorted crystallite and rough surfaces. Luminescent and absorption properties of nano-Y2O3:Eu showed remarkably particle size effects. At Y2O3:Eu particle sizes smaller than 10 nm some new results were observed: (a) a red shift of the charge-transfer-state absorption; (b) new emission bands of Eu3+ in the 5D0 --> 7F2 region; (c) luminescent decay of energy level 5D0 of Eu3+ turning to a two-step exponential; and (d) a pronounced increase in quenching concentration and much lower phonon density compared with those of the bulk material. All these phenomena can be attributed to the effect of the softened lattice and surface state of the nanomaterial. The latter was confirmed by stronger excitation by the host absorption after the surface modification.

Preparation and size effect on concentration quenching of nanocrystalline Y2SiO5:Eu

Abstract Nanocrystalline Y2SiO5:Eu was prepared by the sol-gel technique. The photoluminescence spectra and quenching concentration of nanocrystalline Y2SiO5:Eu and normal Y2SiO5:Eu synthesized by the high-sintering method were measured and compared. Higher quenching concentration and stronger luminescent intensity in nanocrystalline Y2SiO5:Eu than in normal Y2SiO5:Eu was observed. This is ascribed to the influence of a confinement effect on resonant energy transfer in nanosized particles. These properties show that nanocrystalline Y2SiO5:Eu has important potential applications.

Luminescence concentration quenching of 1D2 state in YPO4:Pr3+

Under selective excitation of the 3P0 level of the Pr3+ ion in YPO4, the emission spectra and fluorescence decay curves were measured at different concentrations and temperatures. The origin of the spectral line located at 16 318 cm-1 (612.8 nm) was discussed and attributed to the 1D2→ 3H4 transition. The process of concentration quenching for the 1D2 state was also studied. Using the Inokuti-Hirayama model, the non-exponential fluorescence decay curves of the 1D2 level were fitted. The result shows that dipole-quadrupole interaction between Pr3+ ions, which causes 1D2→ 1G4 and 3H4→ 3F4 cross-relaxation, results in the quenching of 1D2 emissions.

A model analysis of 4fN-4fN-15d transitions of rare-earth ions in crystals

Abstract The 4f N ↔4f N −1 5d transition model proposed by Duan et al. [Phys. Rev. B 66 (2002) 155108] and Duan and Reid [J. Solid State Chem. 171 (2003) 299] is presented in detail and extended to allow explicit calculation of line strengths of transitions between any two multiplet states by using Racah–Wigner algebra. This extended model is used to simulate the 4f 2 5d→4f 3 emission spectrum of Nd 3+ in YPO 4 crystal.

Extraction of crystal-field parameters for lanthanide ions from quantum-chemical calculations

A simple method for constructing effective Hamiltonians for the 4f(N) and 4f(N - 1)5d energy levels of lanthanide ions in crystals from quantum-chemical calculations is presented. The method is demonstrated by deriving crystal-field and spin-orbit parameters for Ce(3 + ) ions doped in LiYF(4), Cs(2)NaYCl(6), CaF(2), KY(3)F(10) and YAG host crystals from quantum-chemical calculations based on the DV-Xα method. Good agreement between calculated and fitted values of the crystal-field parameters is obtained. The method can be used to calculate parameters even for low-symmetry sites where there are more parameters than energy levels.

Spectroscopy of High-Energy States of Lanthanide Ions

We discuss recent progress and future prospects for the analysis of the 4f N-1 5d excited states of lanthanide ions in host materials. We demonstrate how ab initio calculations for Ce 3+ in LiYF 4 may be used to estimate crystal-field and spin-orbit parameters for the 4f 1 and 5d 1 configurations. We show how excited-state absorption may be used to probe the electronic and geometric structure of the 4f N-1 5d excited states in more detail and we illustrate the possibilities with calculations for Yb 2+ ions in SrCl 2 .

Upconversion fluorescence of Nd3+ ions in K2YF5 single crystal

Abstract Upconversion fluorescence of Nd 3+ ions in K 2 YF 5 single crystal was studied under 785-nm (near-infrared), 514.5-nm (green) and 325-nm (ultraviolet) laser excitation. With laser excitation set at 785 nm, a near-infrared upconversion emission band centered at 745 nm with a peculiar linear power dependence was observed and explained successfully by using a multiphonon assisted upconversion mechanism. With 514.5-nm laser excitation, five upconversion emission bands centered at 358, 386, 417, 451 and 496 nm were recorded. The power dependence of these emission bands was also studied and the upconversion mechanism was attributed to the excited state absorption from 4 F 3/2 state. In addition, the ultraviolet upconversion fluorescence which peaked at 287 nm under 325-nm laser excitation was also presented and analyzed.

Energy transfer processes of Er3+ in Y AlO3

In this study the upconversion of the 4I13/2 and 4I11/2 multiplets, and the cross-relaxation (CR) processes of 4S3/2 and 4I9/2 of Er3+ in Y AlO3, which are related to its 3 µm laser operation, have been investigated. The dominant energy transfer (ET) pathways are discussed and the ET rates of the contributing multipolar interaction mechanisms have been estimated, based upon the Holstein–Lyo–Orbach or the Miyakawa and Dexter formalisms for phonon-assisted ET. The Kushida formalism has been employed for the investigation of migration processes, and the matrix elements of Kaminskii for the relevant electronic transitions have been used in our calculations. The major results are that at room temperature, the contributions of 2H11/2 (particularly from electric quadrupole–electric quadrupole interactions) to the CR and migration of 4S3/2, and to the upconversion of 4I11/2, are important, and are even the dominant ones in some cases.

Spectroscopic properties and blue to ultraviolet upconversion for Ho3+ ions in Cs2NaGdCl6 crystals

Abstract A spectroscopic study of Ho 3+ ions in Cs 2 NaGdCl 6 is reported and four strong emission bands (peaked at 493, 545, 587 and 658 nm, respectively) are observed. The 587-nm emission band is assigned to 5 G 4 → 5 I 6 transition, in accordance with the assignment made by previous works. The other three emission bands are easily assigned to 5 F 3 → 5 I 8 , ( 5 F 4 , 5 S 2 )→ 5 I 8 , and 5 F 5 → 5 I 8 transitions, respectively. The emission intensity dependence on Ho 3+ concentration is also presented. It is found that the first three emission bands are quenched almost completely when the concentration x of Ho 3+ ions in Cs 2 NaGd 1− x Ho x Cl 6 is larger than 0.5, but that of 658-nm emission band always increases with concentration. The two efficient cross-relaxations 5 F 3 + 5 I 8 → 5 F 5 + 5 I 7 and 5 G 4 + 5 I 8 → 5 F 3 + 5 I 7 are responsible for the phenomenon. In addition, an exploration of the visible to ultraviolet energy upconversion in Cs 2 NaGd 1− x Ho x Cl 6 is carried out. Upon blue ( λ exc =457.9 nm) laser excitation, a quite strong ultraviolet upconversion emission band, ranging between 320 and 340 nm, is observed in Cs 2 NaGd 1− x Ho x Cl 6 ( x =0.05 to 1.0) for the first time . This upconversion emission is attributed to the 5 D 4 ′→ 5 I 5 transition and the excited-state absorption (ESA) upconversion is presumed to be the main mechanism.

Theoretical analysis of the two-photon absorption spectrum of Tb3+ in Cs2NaTbCl6

Eighteen selected two-photon absorption (TPA) transition line strengths with polarization angles θ = 0° and 45°, spanning several orders of magnitude, have been calculated for the Tb3+ ion in the cubic host Cs2NaTbCl6. The results are in reasonable agreement with experimental results in the literature. The calculation utilized the crystal field (CF) wavefunctions for the initial and final states of the 4f8 configuration, and utilized free ion or CF wavefunctions (with the corresponding energies) for 4f7 core states of the whole intermediate 4f7 5d configuration comprising 34 320 states. The intensities of certain transitions were found to be very sensitive to the inclusion of the CF interaction within the 4f7 core. In contrast to previous fourth- or third-order calculations of the TPA transition line strength of the strong transition (7F 6)A1g → (5D 4)A1g using pure Russell–Saunders (RS) wavefunctions for the |7F 6 initial and 5D 4 | final states, our second-order direct calculation shows that the admixed RS wavefunctions |[7F 6 ] and [5D 4 ]| must be used to account for its high intensity. The effects of CF interactions within the 4f7 core, i.e. J-mixing and CF energy level splitting, upon the (7F 6)A1g → (5D 4)Eg TPA transition line strength have been separated, and the latter effect is shown to be more important for the transition investigated.