Chen Xueyuan

Judd-Ofelt parameter analysis of rare earth anisotropic crystals by three perpendicular unpolarized absorption measurements

Abstract The expressions to calculate integrated absorbances for uniaxial crystals were analyzed mathematically and their direction dependence was formulated and shown by curves. Based on this analysis, it was proposed that to measure the line strengths for the calculation of effective Judd-Ofelt parameters, it is appropriate to measure the absorption spectra in three perpendicular directions and average the results (referred to as TPM method). The relative errors of the measured integrated absorbances for two assumed polarized spectra were calculated. After a short discussion, it is pointed out that the method proposed can also be used in the calculation for biaxial crystals and an example of Nd 3+ :KGd(WO 4 ) 2 crystal was presented.

Recent Progress on Spectroscopy of Lanthanide Ions Incorporated in Semiconductor Nanocrystals

Doping luminescent lanthanide ions into semiconductor nanocrystals is an ideal approach for developing nanodevices for various applications. Quantum confinement effects are expected for lanthanide ions doped in small semiconductor nanocrystals. The most recent progress on the synthesis and spectroscopy of lanthanide ions in various semiconductor nanocry stals such as II-VI, III-V and IV-VI families were systematically reviewed, focusing on our recent findings on the optical spectroscopy of Eu(3+) doped in ZnO and TiO(2), nanocrystals by wet chemical synthesis. The energy transfer from the band-gap excitation to lanthanides further confirmed that lanthanide ions could be successfully incorporated into the lattice sites in spite of the mismatch in ionic radii.

Structure and luminescence of Ca2Si5N8:Eu2+ phosphor for warm white light-emitting diodes

We have synthesized Ca2Si5N8:Eu2+ phosphor through a solid-state reaction and investigated its structural and luminescent properties. Our Rietveld refinement of the crystal structure of Ca1.9Eu0.1Si5N8 reveals that Eu atoms substituting for Ca atoms occupy two crystallographic positions. Between 10 K and 300 K, Ca2Si5N8:Eu2+ phosphor shows a broad red emission band centred at ~1.97 eV–2.01 eV. The gravity centre of the excitation band is located at 3.0 eV–3.31 eV. The centroid shift of the 5d levels of Eu2+ is determined to be ~1.17 eV, and the red-shift of the lowest absorption band to be ~0.54 eV due to the crystal field splitting. We have analysed the temperature dependence of PL by using a configuration coordinate model. The Huang–Rhys parameter S = 6.0, the phonon energy hv = 52 meV, and the Stokes shift ΔS = 0.57 eV are obtained. The emission intensity maximum occurring at ~200 K can be explained by a trapping effect. Both photoluminescence (PL) emission intensity and decay time decrease with temperature increasing beyond 200 K due to the non-radiative process.

Crystal-field analysis of the energy levels and spectroscopic characteristics of in crystal

On the basis of the analysis of the group-chain scheme ), the crystal-field-level fitting has been carried out for , in which the ions occupy site symmetry positions. The RMS value of the energy-level fitting is and the group character of each Stark level compares well to the experimental assignment on the whole. The wavefunctions obtained were used in the study of the Zeeman interaction and the fluorescence spectra. The calculated g-factors obey Karayianiss partial g-sum rule, and the g-factor of the ground state of is predicted. According to the Judd-Ofelt theory, the odd crystal-field parameters have been quantitatively determined by the fit to the experimental transition rates from . The line-to-line transition rates and thus the fluorescence spectra from at room temperature have been calculated, which are in agreement with the experiment by and large.

Crystal-field analysis of the spectroscopic characteristics and magnetic properties of in crystal

On the basis of the analysis of a group-chain scheme and with the use of the constraint condition determined by the ratios of crystal-field parameters calculated using the point-charge model, a crystal-field-level fitting has been carried out for , in which the ions are assumed to occupy positions with point symmetry . The RMS value of the energy-level fitting is . The wavefunctions obtained were used in the study of the magnetic, thermal, and spectroscopic properties of the crystal. The calculated g-factors obey Karayianiss partial g-sum rule. The temperature dependences of the Schottky specific heat and magnetic susceptibility agree well with data published by others. The line shown by calculation to be the strongest is -polarized at 450.4 nm, which corresponds to blue upconversion lasing in the experiment. The method proposed turns out to be effective in the study of the spectroscopic and magnetic properties of localized centres in crystals.

Modelling of the Self Sum-Frequency-Mixing Laser

A theoretical model of the self sum-frequency-mixing (SFM) laser generated by a single crystal is proposed, in which spatial distribution of the pump and circulating fundamental lasers with arbitrary beam waists are taken into account. The model is then applied to two kinds of crystals of current interest, Nd:YAl3(BO3)4 and Nd:Ca4GdO(BO3)3. Numerical analyses of the self-SFM laser properties predict and confirm some experimental results. The model proposed is not limited to self-SFM lasers and may be applied to general analyses of the fundamental or nonlinear laser generation with Gaussian beams.

High Efficient Laser Operation of the Nd:KGd(WO4)2 Crystal Grown by Flux Method

The laser performance of Neodymium-doped potassium gadolinium tungstate (Nd:KGd(WO4)2) crystal was studied by using Ti:Sapphire laser as the pump source. The maximum optical-to-optical efficiencies for the 1.067 and 1.3 μm laser outputs were measured to be 60% and 32.3%, respectively. The internal loss coefficient of the crystal for 1.067 μm laser was estimated to be as low as 0.004 cm-1. The oscillation thresholds at 1.067 and 1.3 μm for different output transmittances are also given.

The effect of mass difference between host ions on the thermal shift and broadening of the spectral lines of the active ions in crystals

It is shown in this letter that the mass difference between ions of the host crystals has an effect on the thermal broadening and shift of the spectral lines of active ions in the crystals. The effect is expressed as a multiplication factor D2 for the thermal broadening and shift caused by the one-phonon absorption and emission mechanism, and a multiplication factor D4 for the thermal broadening caused by the Raman scattering process. The expressions for the D-factor are given for crystals constituted of two and three kinds of ion respectively.

Group-chain scheme analysis of the energy levels and magnetic properties of Nd3+ in LiYF4 crystal

Based on the analysis of group-chain scheme, the crystal-field-level fitting of Nd3+:LiYF4 has been carried out, in which the Nd3+ ions occupy positions with site symmetry S4. The RMS deviation of energy-level fitting is 12.8 cm-1. Using the obtained wave functions, g-factors of the ground state are calculated, which are g||?=?2.067 and g??=?2.631, in good agreement with the experimental values (g|| = 1.987 and g??=?2.554). The method proposed turns out to be effective in the study of spectral properties of localized centres in laser crystals.