Ai-Hua Li

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.

Effect of beta-Ring Rotation on the Structures and Vibrational Spectra of beta-Carotene: Density Functional Theory Analysis

The effect of beta-ring rotation on the structures and vibrational spectroscopic characteristics of beta-carotene, including infrared (IR) intensities and Raman activities, is analyzed using density functional theory. Two stable isomers having Ci symmetry are obtained. The reversion of bond lengths is ascribed to the hyperconjugation effect. The natural bond orbital (NBO) charge analysis suggests that the NBO charges of C5 can be used to estimate the degree of pi-electron delocalization. These structural variations are used to analyze and assign the vibrational spectra. It is concluded that (a) the similar rotational angle dependencies of nu1 and nu2 frequencies justify the contribution of C=C stretch vibrations to the nu2 mode and explain the same conjugation length dependencies of nu1 and nu2 frequencies in polyenes, (b) the nu1 mode can be assigned to the C=C stretching in the central part of polyene chain, whereas beta-rings play an important role in nu2 and IR1 bands, especially for the all-trans isomer, and (c) the transition dipole moment of the calculated IR1 absorption band is relevant to the conjugation degree and the crossing angle between the eigenvectors of the polyene chain and the C5=C6 stretching vibration. This theoretical analysis, together with our previous Raman spectral experiments, suggests that the C6-C7 bond is easier to be twisted than other parts of beta-carotene molecule and so provides an insight into the structures of carotenoids and the properties of binding sites in carotenoproteins.

In/Er-codoped LiNbO3 crystals with enhanced 1.5 μm emission and suppressed upconversion emission

1.5 μm emission and green upconversion emission properties in In/Er-codoped LiNbO3 crystals were investigated by using both steady-state and time-resolved emission spectra. The 1.5 mol % In2O3 codoping results in the lengthening of 1.5 μm emission lifetime by 13.9% and simultaneously the shortening of 550 nm green upconversion emission lifetime by 52.5%. The decay behavior of the green upconversion emission of the In/Er-codoped crystal shows obviously nonexponential. These spectroscopic characteristics are related with the lower concentration of the cluster sites of Er3+ ion and possible cross-relaxation energy transfer processes. The above emission results are further supported by the theoretical emission results based on Judd–Ofelt theory.

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.

Effect of End Groups on the Raman Spectra of Lycopene and β-Carotene under High Pressure

The Raman spectra of all-trans-lycopene in n-hexane were measured under high pressure, and the results compared with those of β-carotene. The different pressure effects on Raman spectra are analyzed taking into account the different structures of lycopene and β-carotene molecules. It is concluded that: (a) the vibronic coupling between the S1 and S0 states of β-carotene is stronger than that of lycopene, (b) the diabatic frequency increment of the ν1 mode is more susceptible to pressure than that of the ν2 mode for lycopene, and (c) β-rings rotation can relieve the pressure effect on the C=C bond length in β-carotene. This work provides some insights for elucidating the structural and environmental effects on Raman spectra of carotenoids.

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.