Lizhu Lu

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.

Coloration of chromium‐doped yttrium aluminum garnet single‐crystal fibers using a divalent codopant

We have grown single‐crystal fibers of Cr:YAG and Cr,Ca:YAG under oxidizing and reducing conditions by the laser‐heated‐pedestal‐growth method. The Cr:YAG crystals were light green due to Cr3+ in octahedral sites, while the Cr,Ca:YAG crystals were brown. The presence of the divalent codopant was the dominant factor determining the coloration in these single‐crystal fibers, while the oxidizing power of the growth atmosphere had little effect on the coloration. The Cr,Ca:YAG had a broad absorption band centered at 1.03 μm and fluoresced from 1.1 to 1.7 μm, with a room‐temperature lifetime of 3.5 μs. The presence of both chromium and a divalent codopant were necessary to create the optically‐active center which produces the near‐infrared emission. Doping with only Ca2+ created a different coloration with absorption in the blue and ultraviolet. The coloration in the Cr,Ca:YAG is attributed to Cr4+ and is produced in as‐grown crystals without irradiation or annealing, as has been necessary in previous work.

The Long‐Persistent Photoconductivity of SrAl2 O 4 : Eu2 + , Dy3 + Single Crystals

Single crystals of SrAl 2 O 4 :Eu 2+ , Dy 3+ with extremely strong afterglow were investigated with photoconductivity experiments. The photocurrent excitation spectra were measured at 80 and 300 K and compared with photoluminescence excitation spectra. Experiments on single crystals without doping Dy 3+ were also performed for comparison. The results are found to support a hole-trapping model which has been used to explain the strong afterglow from this material.

Erbium energy levels relative to the band gap of gadolinium oxide

Abstract Undoped and Er 3+ doped Gd 2 O 3 single crystal fibers were prepared by the laser heated pedestal growth (LHPG) method. The absorption and emission spectra of the samples were studied. The band gap of Gd 2 O 3 crystal is found at 5.2 eV. A two-step photoconductivity measurement was designed to determine the Er 3+ energy level position relative to the host band gap. The lowest Er 3+ 4f–5d transition energy (5.93 eV) exceeds the host band gap. By measuring the photoconductivity induced from an excited state to the 5d bands, the Er 3+ ground state is found to be 0.7±0.2 eV below the valence band.

Energy transfer in Pr3+- and Er3+-codoped CaAl12O19 crystal

Abstract We have studied the energy transfer processes in Pr 3+ - and Er 3+ -codoped CaAl 12 O 19 crystal from 12 to 290 K. We observed energy transfer from Pr 3+ to Er 3+ ions in this system. The transfer can partially convert the 1 S 0 UV fluorescence of Pr 3+ into the characteristic green emission of Er 3+ . The efficiency of the energy transfer has been estimated based on the spectroscopic data and is of the order of 25% at room temperature. The temperature dependence on the transfer is also discussed.

Measurement of quantum efficiency in Pr3+-doped CaAl4O7 and SrAl4O7 crystals

We demonstrated a method for measuring the quantum efficiency of deep UV emission using the two-step excitation and the difference of the integrated spectral intensities in Pr3+-doped CaAl4O7 and SrAl4O7 crystals. The quantum efficiencies of the emission from the lowest 4f5d state in the two systems have been estimated to be less than 10% at room temperature. The effect of excited-state absorption of the lowest 4f5d state is analyzed for the quantum efficiency measurement. The actual efficiency may be higher when this effect is taken into account, especially under the condition of strong excitation.

Growth and spectroscopic analysis of single-crystal fibers

Abstract We have constructed an apparatus which uses the CO 2 -laser-heated pedestal growth method to pull single-crystal fibers. Spectroscopic techniques are described for determining dopant concentrations, homogeneity, and crystal quality. Preliminary results on several fibers of rare earth- and transition metal-doped insulators are described.

Comparison of the spectra and dynamics of Er3+:Y2−xScxO3 (x = 0, 1, 2)

We have studied the spectra and lifetimes of Er3+ in the isostructural series, Y2O3, YScO3. Addition of Sc3+ to Er3+:Y2O3 shifted the energy level positions, changed the inhomogeneous broadening and shortened the fluorescence lifetimes for Er3+ in the C2 site. Going from Er3+:Y2O3 to Er3+: Sc2O3 increased the barycenter of the four lowest J-multiples, decreased the barycenter of the 4F92 and higher multiplets, and increased the crystal-field splitting within each J-multiplet. The inhomogenous line widths in the Er3+:YScO3 spectra were much larger than in Er3+:Y2O3 or Er3+: Sc2O3 due to random substitutional disorder in the mixed system. The random disorder in Er3+:YScO3 removed the inversion symmetry of the C31 lattice site and allowed observation of transitions from Er3+ ions in this site. All fluorescence lifetimes decreased with addition of Sc3+ indicating an increase in the nonradiative relaxation.

Photoluminescence of Mn2+‐Doped ZnGa2 O 4 Single‐Crystal Fibers

Manganese-doped zinc gallate is a new promising cathode ray phosphor material with low excitation voltage. On excitation of the host, we observed strong emission from both the self-activated (SA) host material and tetrahedrally coordinated Mn 2+ ions in single-crystal fibers. The efficient host-Mn 2+ energy transfer makes this material an attractive candidate for display technology. The experimental results show that the SA center is not an intermediate involved in the host-Mn 2+ energy-transfer process

A novel luminescent center in LiNbO3 : Cr : Mg crystals

We report on the observation of novel luminescent center in LiNbO 3 : Cr : Mg. This center is characterized by a set of narrow emission lines located in the 770 - 850 nm region which occur in addition to the well-known broad 4 T 2 emission of the major low-field Cr 3+ site. The set of narrow emission lines can be readily observed in samples containing more than 4% Mg. All lines exhibit the same relaxation time and are strongly π polarized. Substitution of Mg by Zn produces the same set of lines, indicating that the Mg ions are not directly incorporated in the novel center, but play an essential charge compensating role. Based on our absorption, excitation, luminescence, and fluorescent line-narrowing experiments, we associate the novel center with an exchange coupled Cr-center.