Wen-Zhi Wu

Upconversion luminescence of CdTe nanocrystals by use of near-infrared femtosecond laser excitation

We study the steady-state and time-resolved luminescent properties of CdTe nanocrystals by one- and two-photon excitation with a femtosecond laser. We observe that 1208 nm excitation causes a shift of the emission peak of about 20 nm to the infrared compared with 400 nm laser excitation. It is found that upconversion luminescence is composed of a photoinduced trapping and a band edge excitonic state and produces the observation of biexponential decay kinetics. We conclude that the redshift of the emission peak is caused by the relative change in luminescence intensity between excitonic and trapping states.

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.

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.

Effect of solvent on absorption spectra of all-trans-β-carotene under high pressure

The absorption spectra of all-trans-beta-carotene in n-hexane and carbon disulfide (CS(2)) solutions are measured under high pressure at ambient temperature. The common redshift and broadening in the spectra are observed. Simulation of the absorption spectra was performed by using the time-domain formula of the stochastic model. The pressure dependence of the 0-0 band wavenumber is in agreement with the Bayliss theory at pressure higher than 0.2 GPa. The deviation of the linearity at lower pressure is ascribed to the reorientation of the solvent molecules. Both the redshift and broadening are stronger in CS(2) than that in n-hexane because of the more sensitive pressure dependence of dispersive interactions in CS(2) solution. The effect of pressure on the transition moment is explained with the aid of a simple model involving the relative dimension, location, and orientation of the solute and solvent molecules. The implication of these results for light-harvesting functions of carotenoids in photosynthesis is also discussed.