Z. G. Zhang

Bright white upconversion luminescence in rare-earth-ion-doped Y2O3 nanocrystals

Room temperature bright white upconversion (UC) luminescence in Yb3+–Tm3+–Er3+ ions doped Y2O3 nanocrystals was obtained under single-wavelength diode laser excitation of 976nm. The white light consists of the blue, green, and red UC radiations which correspond to the transitions G41→H63 of Tm3+, H11∕22∕S3∕24→I15∕24, and F9∕24→I15∕24 of Er3+ ions, respectively. The UC mechanisms were proposed based on spectral, kinetic, and pump power dependence analyses. The calculated color coordinates display that white light can be achieved in a wide range of pumping powers, which promises their potential applications in the field of displays, lasers, photonics, and biomedicine.

Two-color upconversion in rare-earth-ion-doped ZrO2 nanocrystals

To develop fluorescent labels for multicolor imaging, rare-earth-ion-doped ZrO2 nanocrystals were prepared by a complex precursor method. Laser excitation of 976nm induced single fluorescent bands of green and red upconversion (UC) in ZrO2:Er3+ and ZrO2:Er3++Yb3+ nanocrystals, respectively. A suppression ratio (SR) parameter was introduced, defined as the UC intensity ratio of the main band to all the other detected impurity bands, and SR values in the order of 10–100 were experimentally obtained, demonstrating the excellent monochromaticity of the UC labels. Thus, the two-color UC labels obtained are potentially ideal to be used for biological multicolor imaging.

Enhancement of the upconversion radiation in Y2O3:Er3+ nanocrystals by codoping with Li+ ions

We report on an innovative route to increase the upconversion (UC) green radiation by two orders of magnitude in Y2O3:Er3+ nanocrystals through tailoring Er3+ ions’ local environment with Li+ ions under diode laser excitation of 970nm. Theoretical investigations based on the steady-state rate equations indicate that such enhancement arises from a combining effect of the tailored lifetime of the intermediate I11∕24(Er) state, the suppressed cross relaxation H11∕22(Er)+I15∕24(Er)→I9∕24(Er)+I13∕24(Er) process, and the enlarged nanocrystal size induced by the Li+ ions. The proposed route here may constitute a promising step to solve the low efficiency problem in UC materials.

Ultraviolet upconversion fluorescence in rare-earth-ion-doped Y2O3 induced by infrared diode laser excitation.

Room-temperature ultraviolet emission of Tm(3+) ions at 298 ((1)I(6)-->(3)H(6)), 364 ((1)D(2)-->(3)H(6)), and 391 nm ((1)I(6)-->(3)H(5)) was obtained in Y(2)O(3):Yb(3+)-Tm(3+) by continuous-wave diode laser excitation of 980 nm. Power dependence analysis demonstrates that five- and six-photon upconversion processes populate the (1)D(2) and (1)I(6) states, respectively. We believe that the (1)D(2) population originates from the cross relaxation (1)G(4)+(3)F(4)-->(3)H(4)+(1)D(2) of the Tm(3+) ions, while subsequent energy transfer from Yb(3+) to Tm(3+) excites the (1)D(2) state to the upper (1)I(6) state. High multiphoton-induced ultraviolet emission is also expected for other trivalent rare-earth ions similar to Tm(3+).

Simultaneous detection of multiple-gas species by correlation spectroscopy using a multimode diode laser

Simultaneous detection of multiple-gas species has for the first time (to our knowledge) been demonstrated by using a multimode diode-laser-based correlation spectroscopy (MDL-COSPEC) scheme. Concentration measurements of a mixture of CO(2) and CO gas were performed by probing overlapping line spectra around 1.57microm using an MDL. Species identification and corresponding quantitive analysis were implemented by correlating the recorded absorption signals of the sample gas mixtures with those of the reference gases of particular interest, attaining accuracies of 2% and 1%, respectively. MDL-COSPEC is a generic technique with potential application for simultaneous detection of multiple gases having resolvable narrow lines.

Concentration evaluation method using broadband absorption spectroscopy for sulfur dioxide monitoring

We report on an approach for sulfur dioxide monitoring using broadband absorption spectroscopy in the ultraviolet spectral range. The method was applied in real-time measurements and has the advantage of straightforward data evaluation, limited susceptibility for interference from other gases, and low degree of complexity compared with other real-time optical detection techniques having the same precision. Concentration measurement is demonstrated at atmospheric pressure, which is of interest for pollution emission monitoring, yielding a detection limit of about 1ppm with 3s integration time. Comparison is made with low pressure measurements for validation of the accuracy of the method.

Ultraviolet-blue upconversion emissions of Ho 3+ ions

Ultraviolet-blue upconversion (UC) radiations of Ho3+ ions at 240, 290, 360, 385, 418, 445, 485 nm were observed in hexagonal NaYF4:Yb3+∕Ho3+ powders under diode laser excitation of 970 nm. UC mechanism analyses illustrate that successive energy transfers from Yb3+ to Ho3+ generate emissions at 240, 360, 385, and 418 nm, while cross relaxations between Ho3+ ions evoke UC emissions at 290 and 445 nm. Power dependence analyses indicate that these UC emissions already have intense saturation effects even at low-power density range of 0.3–38 W∕cm2. Theoretical calculations based on steady-state equations demonstrate the proposed UC mechanisms and explain well the observed saturation effects.

Anomalous power dependence of upconversion emissions in Gd2O3:Er3+ nanocrystals under diode laser excitation of 970 nm

Visible green and red upconversion (UC) emissions with anomalous power dependence were observed in Gd2O3:Er3+ nanocrystals at room temperature under diode laser excitation of 970 nm. The green and red UC radiations both yield an “s”-shape power dependence, in marked contrast to the quadratic ones of the bulk counterparts. A closed positive looping UC mechanism that differs from conventional PA mechanism is proposed to explain the observed five- or six-photon processes in the “s”-shape power dependence. Power dependence analysis of the 1.55 μm emissions from the I413/2 state experimentally demonstrates our proposed model.

Experimental and theoretical energy levels, transition probabilities and radiative lifetimes in Yb III

The analysis of the spectrum of Yb III has been extended allowing us to establish 11 new energy level values. In addition, radiative lifetimes of two excited states of Yb III have been measured for the first time using time-resolved laser-induced fluorescence following two-photon excitation. The good agreement between experimental results and semi-empirical calculations performed with the relativistic Hartree-Fock method including core-polarization effects allows the determination of transition probabilities for 15 lines.

Doubly Ionized Thorium: Laser Lifetime Measurements and Transition Probability Determination of Interest in Cosmochronology

The first lifetime measurements have been performed for six levels of doubly ionized thorium, an important cosmochronometer in astrophysics for estimating the age of the Galaxy. The levels, belonging to the 5f2, 5f7p, 7s7p, and 6d7p configurations of Th III, have been measured by the time-resolved laser-induced fluorescence method and compared with relativistic Hartree-Fock calculations including configuration interaction and core-polarization effects. Taking advantage of the excellent agreement between theory and experiment, a first set of transition probabilities of astrophysical interest has been deduced for this ion from a combination of the experimental lifetimes and of the theoretical branching fractions.