Qiang Lü

Silica-/titania-coated Y2O3:Tm3+, Yb3+ nanoparticles with improvement in upconversion luminescence induced by different thickness shells

In order to improve the upconversion (UC) luminescence of lanthanide-doped nanoparticles (NPs), different sized Y2O3:Tm3+, Yb3+ NPs were synthesized using the Pechini type sol-gel method, and their surfaces were coated with different thickness of SiO2 or TiO2 shells using the Stober method. The results indicate that large-sized NPs have more intense UC luminescence intensities than small-sized NPs. The core-shell structures can enhance the UC luminescence intensities. Comparing with the UC luminescence intensity of noncoated NPs, the UC luminescence intensities of SiO2-coated NPs for the sintering time of 60 min and the coating time of 30, 60, 90, and 120 min are enhanced by 1.53, 1.54, 1.40, and 1.16 times, respectively. According to the relative variable ratios of the UC luminescence intensities, a competition process between two mechanisms was proposed to explain the effects of different thickness shells and different shell materials on the UC luminescence intensities. One mechanism is the role convers...

Enhanced 1.5μm emission and simultaneously suppressed green upconversion emission in Er:LiNbO3 crystals heavily codoped with MgO

Enhanced 1.5μm emission and simultaneously suppressed green upconversion emission was observed for Er(1.0mol%):LiNbO3 crystals codoped with 8mol% MgO. The heavy MgO codoping results in the lengthening of the 1.5μm emission by 11.7%. Time-resolved spectroscopy study shows that the decay trace of the green upconversion emission of the Er∕Mg codoped crystal is obviously nonexponential. These spectroscopic characteristics are associated with the lower concentration of the cluster sites of Er3+ ion, the higher nonradiative energy transfer upconversion probability, and the lower OH− content in the crystal.

Red upconversion emission in LiNbO 3 codoped with Er 3+ and Eu 3+

Red upconversion (UC) emission at 626 nm is obtained from a LiNbO(3) crystal codoped with Er(3+) and Eu(3+) under 800 nm femtosecond laser excitation. Energy transfer from ((2)H(11/2,),(4) S(3/2)) levels of Er(3+), which are excited by excited state absorption, to (5)D(1) of Eu(3+) followed by rapidly relaxing to (5)D(0) nonradiatively leads to this red UC emission. The energy transfer efficiency and Er-Eu transfer microparameter of approximately 30% is obtained in LiNbO(3):Er(3+)(1.0 mol%),Eu(3+)(0.1 mol%). These initial experimental results indicate that the red UC emission can be obtained from Er(3+)/Eu(3+) codoped system under diode laser excitation.

Yellow-green upconversion luminescence of Dy3+ ion in LiNbO3 crystal heavily codoped with ZnO

The observed Dy3+ ion upconversion luminescence in LiNbO3 crystal heavily codoped with ZnO was spectrally and temporally analyzed by microsecond time-resolved spectrum under 806nm intense femtosecond laser excitation at room temperature. Absorption spectrum and modified Judd-Ofelt approach were used to investigate its spectroscopic properties. The bright blue and intense yellow emissions are assigned to transitions F9∕24→H15∕26 and F9∕24→H13∕26, respectively. It is concluded that the main upconversion mechanism is excited state absorption by pump power dependence in combination with luminescence intensity temporal evolvements.

Enhanced green upconversion emission of Er(3+) through energy transfer by Dy(3+) under 800 nm femtosecond-laser excitation.

Er(3+) green upconversion (UC) emission corresponding to the transition of (4)S(3/2) ((2)H(11/2))-->(4)I(15/2) is enhanced in a Er/Dy-codoped LiNbO(3) crystal compared with Er-doped LiNbO(3) under 800 nm femtosecond-laser excitation at room temperature. The upconversion mechanisms are proposed based on spectral, kinetic, and pump-power dependence analyses. The energy-transfer efficiency from Dy(3+)((4)F(9/2)) to Er(3+)((4)F(7/2)) is 33%, which results in the enhancement of green UC emission. This energy transfer is advantageous for the Er(3+) UC emission sensitized by Dy(3+), especially in a low-phonon-energy host matrix.

Sensitized holmium upconversion emission in LiNbO3 triply doped with Ho3+, Yb3+, and Nd3+

Observed is 15 times enhancement of Ho3+ green upconversion (UC) emission in Ho3+(0.75 mol %)/Nd3+(0.75 mol %) doped lithium niobate crystal after incorporating with 4 mol % of Yb3+ under 800 nm femtosecond laser excitation. Yb3+ acting as bridging ion increases the energy transfer efficiency from Nd3+ F43/2 level to Ho3+ in these triply doped systems. The nonradiative energy transfer efficiency from Nd3+ F43/2 to Yd3+ F25/2 level can reach to 56%. The UC emission mechanisms are proposed based on spectral, kinetic, and pump power dependence analyses. The green UC emission is accomplished through a multi-ion interaction involving ground state absorption by the Nd3+ (I49/2→H29/2,F45/2) followed by three successive energy transfer processes involving one Nd3+-Yb3+ (F43/2+F27/2→I49/2+F25/2) and two Yb3+-Ho3+ pairs (F25/2+I58→F27/2+I56 and F25/2+I56→F27/2+S52,F54); whereas the red is dominated by excited state absorption of the Ho3+ (I57→F55) following two successive energy transfer processes involving the Nd3...

MgO-codoping effects on the spectroscopic properties of Er3+-doped LiNbO3

Optical absorption, spectrally resolved polarized upconversion fluorescence, and time-resolved luminescence spectra are used to investigate the Er3+ spectroscopic properties in a series of congruent LiNbO3 crystals codoped with Er3+ (1 mol %) and MgO (X mol %, X=0, 2, 4, 6, and 8). The absorption spectra indicate that the transition cross section of Er3+ ions decreases with increasing MgO concentration; it is explained as the improvement of the Er3+-site symmetries. A new Er3+ energy site with low energy, which appears in the long wavelength side of the polarized fluorescence spectra for the samples heavily codoped with MgO, is demonstrated in the time-resolved luminescence spectra. An important finding is that mild codoping with MgO clearly does not facilitate the formation of clustered sites, whereas heavy codoping clearly does facilitate it in Er3+-doped LiNbO3 crystals. These results are important to those who consider the material as a 1.5 μm laser or fiber optical cable.

Visible and ultraviolet upconversion emission in LiNbO3 triply doped with Tm3+, Yb3+, and Nd3+

Visible and ultraviolet upconversion (UC) emission is observed under 800 nm femtosecond laser excitation in LiNbO3 crystals triply doped with Tm3+, Yb3+, and Nd3+ at room temperature. Energy transfer (ET) from Nd3+ to Yb3+ then to Tm3+ is very important in this UC emission process. The overlapping between the emissions of D12→F34 and G14→H36, which makes up of blue emission band, is confirmed by transient investigation. From the pump energy dependence investigation, it is known that the dominant populating mechanism for the G14 state is the two-photon process, and that for D12 is the three-photon process. In our UC emission model, the G14 state is populated by the ET of F25/2(Yb3+)+H34(Tm3+)→F27/2(Yb3+)+G14(Tm3+), D12 state is populated by the ET of F32+H34→D12+H36 among Tm3+ ions. For LiNbO3 crystals doped with Tm3+ to the concentration of 0.9 mol %, the measured lifetimes of G14 and D12 are ∼80 and 4 μs.

Measurement of infrared level lifetime by upconversion luminescence

Based on repetition frequency-dependent excited state absorption (ESA) upconversion (UC) luminescence, a method to measure the lifetime of an IR intermediate level is proposed so long as ESA UC luminescence can occur in the rare earth ions. The feasibility of this idea is demonstrated via a theoretical simulation. A Er(3+):LiNbO₃ crystal ESA UC luminescence under femtosecond laser excitation is used to illustrate this measurement method, and the obtained 1.5 μm lifetime of 2.31 ms is shorter than previous reported values. This method can obviate the influence of radiation trapping effect on lifetime measurement, which is crucial in the traditional pulse sampling technique.

Two-photon-excited luminescence in a Tb3+-doped lithium niobate crystal pumped by a near-infrared femtosecond laser

Blue (487.6 nm), green (544.1 nm), yellow (582.1 nm), and red (623.6 nm) upconversion (UC) luminescences are achieved in a Tb3+-doped lithium niobate crystal when an 800 nm femtosecond laser is loosely focused onto the sample at room temperature. The relationship between UC luminescence intensity and the pump energy indicates that a two-photon excitation process is dominant in this UC luminescence phenomenon. The Tb3+ sensitive temperature dependence of the luminescence intensity is demonstrated via an obvious reduction of luminescence intensity with durative laser irradiation.