Yongfa Kong

Luminescence and energy transfer of a color tunable phosphor: Dy3+-, Tm3+-, and Eu3+-coactivated KSr4(BO3)3 for warm white UV LEDs

A series of Dy3+-, Tm3+-, Eu3+-coactivated KSr4(BO3)3 phosphors were synthesized via a standard solid-state reaction under normal ambient air, and the emission colors could be tuned from blue to yellow and then to red, including almost all the white light region, through tuning the energy transfer. It was discovered that the energy is transferred from Tm3+ to Dy3+ by: directly observing overlap of the excitation spectrum of Dy3+ and the emission spectrum of Tm3+; the systematic relative decline and growth of emission bands of Tm3+ and Dy3+, respectively; and faster decay times of the blue emissions from energy donors. The resonance-type energy transfer from Tm3+ to Dy3+ was demonstrated to be via the dipole–quadrupole mechanism and the critical distance of energy transfer was calculated to be 20.7 A. Rietveld refinements of the crystal structures of the products obtained from powder X-ray diffraction (XRD) elucidated a preferable occupancy in the crystal unit for the doped rare-earth cations, which well explained the formation of the Tm3+–Dy3+ close pair. By utilizing the principle of energy transfer, we have demonstrated that with appropriate tuning of activator content, KSr4(BO3)3:Dy3+,Tm3+,Eu3+ phosphors exhibit great potential for use as single-component phosphors for warm white ultraviolet light-emitting diodes (UV LEDs).

New doped lithium niobate crystal with high resistance to photorefraction—LiNbO3:In

Highly indium‐doped lithium niobate crystals have been grown. It was found that a LiNbO3:In (5 mol %) crystal had a similar high resistance to photorefraction as a LiNbO3:Zn (7.5 mol %) crystal. The result of x‐ray fluorescence showed that the doped concentration of In in LiNbO3:In (5 mol % in the melt) exceeds the concentration threshold of trivalant elements (3.0 mol % in the crystal). The LiNbO3:In (5 mol %) crystal is another doped LiNbO3 crystal with high resistance to light‐induced refractive index damage.

Highly optical damage resistant crystal : Zirconium-oxide-doped lithium niobate

Usage of lithium niobate in nonlinear optics is seriously hampered by optical damage, in particular, where high intensity is needed. Doping with magnesium can improve its resistance against optical damage. However, since a rather large dopant concentration is required (more than 4.6mol% MgO) and since the distribution coefficient is unfavorable, it is difficult to grow crystals of high optical quality. The authors show that by doping with zirconium, one can obtain at the same time a higher resistance against optical damage, a lower doping threshold (only 2.0mol% ZrO2), a distribution coefficient near 1.0, and a low coercive field that is only one-third of that of congruent LiNbO3. These properties suggest that zirconium-doped lithium niobate is an excellent choice for nonlinear optical applications.

The optical damage resistance and absorption spectra of LiNbO3:Hf crystals

Highly HfO2 doped lithium niobate crystals have been grown. The experimental results indicate that LiNbO3:Hf (up to 4?mol%) can withstand the same light intensity, of 5 ? 105?W?cm?2, as LiNbO3:Mg (6.5?mol%). And the OH? absorption bands of these LiNbO3:Hf crystals shift to 3487?cm?1 from the 3484?cm?1 for congruent pure LiNbO3. The difference spectra and fitting treatments show that the OH? absorption peak corresponding to (HfNb4+)??OH? is located at 3500?cm?1.

Structure and photoluminescence properties of KSr 4 (BO 3 ) 3 :Eu 3+ red-emitting phosphor

We have synthesized a Eu3+-activated KSr4(BO3)3 red phosphor by solid state reactions. Rietveld refinement on X-ray diffraction data indicates that Eu3+ ions are inclined to occupy Sr(2) (8c) site in the structure of KSr4(BO3)3. The composition-optimized KSr4(BO3)3:Eu3+ exhibits a dominant emission peak at 612 nm (5D0-7F2) with CIE coordinates of (0.64, 0.35) under the excitation at 394 nm. By codoping M+ ions (M = Li, Na, and K) in KSr4(BO3)3:Eu3+ to compensate the charge unbalance, the intensities of emission spectra at 612 nm can be increased greatly, but the CIE coordinates will not be changed. The red-emitting KSr4(BO3)3:Eu3+ phosphor may be potential candidate in the fabrication of white light-emitting diodes (LEDs).

Enhanced photorefractive properties of LiNbO3:Fe crystals by HfO2 codoping

Photorefractive properties of congruent lithium niobate crystals codoped with HfO2 and Fe2O3 were investigated and it was found that Fe ions are still located at Li sites as photorefractive centers when the doping concentration of HfO2 goes above the threshold value. As a result, their photorefractive response speed and sensitivity are significantly enhanced. Meanwhile, the high saturation diffraction efficiency is still maintained. Experimental results definitely show that Hf is now the most effective doping element for LiNbO3:Fe crystal to improve its photorefractive properties.

Enhancement of ultraviolet photorefraction in highly magnesium-doped lithium niobate crystals.

We investigate UV photorefraction in Mg-doped LiNbO(3) crystals. Strong UV photorefraction is achieved in highly Mg-doped LiNbO(3) crystals with high two-wave mixing gain, fast response, and low noise. It is also demonstrated experimentally that so-called damage-resistant dopants such as Mg are damage resistant only in the visible and that they will enhance photorefraction in the UV.

Increased optical-damage resistance in tin-doped lithium niobate

Applications of lithium niobate in nonlinear optics at high light intensities are seriously hampered by optical damage. Recent investigations have shown that Hf(4+) and Zr(4+) ions have some advantages in suppressing optical damage of LiNbO(3) with respect to Mg(2+). Here we present Sn-doped LiNbO(3) (Sn:LN). Experimental results indicate that Sn:LN has similar optical damage resistance to Mg-doped LiNbO(3), but the doping threshold of Sn is only 2.5 mol.%, where its distribution coefficient is 0.98. Hence Sn(4+) ion turns out to be another good choice for increasing optical damage resistance of LiNbO(3).

Recent Advances in the Photorefraction of Doped Lithium Niobate Crystals

The recent advances in the photorefraction of doped lithium niobate crystals are reviewed. Materials have always been the main obstacle for commercial applications of photorefractive holographic storage. Though iron-doped LiNbO3 is the mainstay of holographic data storage efforts, several shortcomings, especially the low response speed, impede it from becoming a commercial recording medium. This paper reviews the photorefractive characteristics of different dopants, especially tetravalent ions, doped and co-doped LiNbO3 crystals, including Hf, Zr and Sn monodoped LiNbO3, Hf and Fe, Zr and Fe doubly doped LiNbO3, Zr, Fe and Mn, Zr, Cu and Ce triply doped LiNbO3, Ru doped LiNbO3, and V and Mo monodoped LiNbO3. Among them, Zr, Fe and Mn triply doped LiNbO3 shows excellent nonvolatile holographic storage properties, and V and Mo monodoped LiNbO3 has fast response and multi-wavelength storage characteristics.

absorption spectra of pure lithium niobate crystals

The absorption spectra of congruent pure and nearly stoichiometric lithium niobate crystals have been investigated. Experimental results show that each absorption band consists of three components. The absorption peaks were considered to relate to the stretching vibration of protons located at 336 pm O-O bonds in oxygen triangles nearest to the Li site. The absorption peak corresponds to protons directly substituting for ions; the 3481 and peaks are suggested to be associated with protons occupying intrinsic defects near and two different ion environments cause these two absorption peaks.