Two-step Excitation Mechanism Research Articles

Enhancement of visible emission in an Ho3+-doped fluorozirconate fiber using dual-wavelength pumping at 44X nm

Optical absorption and fluorescence spectra in the visible spectral range were investigated for an Ho3+-doped fluorozirconate fiber. From the absorption spectrum, the Judd–Ofelt (J–O) intensity parameters, Ω2, Ω4, and Ω6, were calculated along with the spectroscopic parameters, including radiative decay rates (AJJ′), radiative branching ratios (βJJ′), and radiative lifetimes (τrad) for the main emission levels, including 5I7, 5I6, 5F5, 5F4+5S2, 5F3, and 3K7+5G4 of Ho3+ ions. The excited-state absorption (ESA) mechanism of blue diode-pumped visible emission for an Ho3+-doped fluorozirconate fiber was investigated for the first time under blue pumping at 44X nm. Emission bands originating from 3K7+5G4 levels at peak wavelengths of 579 and 680 nm show a two-step excitation mechanism. These two emissions are enabled by ESA for both 5I6 and 5I7 levels, and the process to the 3K7+5G4 levels after nonradiative decay from the 5G2 level is ESA from the 5I6 level, whereas the process to the 3K7+5G4 levels after nonradiative decay from the 3H6 level is ESA from the 5I7 level. It has been revealed that enhancement of visible emission was enabled by introducing the ESA at 440 nm in an Ho3+-doped fluorozirconate fiber under dual-wavelength pumping at 449 + 440 nm.

Large-Scale High-Yield Synthesis of PdCu@Au Tripods and the Quantification of their Luminescence Properties for Cancer Cell Imaging

The synthesis of anisotropic branched gold nanoparticles remines to be challenging as their arm number and arm length could hardly be controlled, greatly limited their biomedical application. We report the large-scale high-yield synthesis of PdCu@Au tripods, and, for the first time, their two-photon luminescence properties with quantitative characterization of the two-photon action cross section as well as quantum yield. By introducing nitrogen protection to the synthesis of the PdCu bimetallic cores, this approach eliminates the oxidative etching caused by oxygen in the air, providing a 2.5 times higher synthetic yield of 70.4 %, which enables the large-scale preparation of PdCu@Au at ca. 380 mg per batch. By the conformal coating of PdCu bimetallic cores, the PdCu@Au tripods are prepared with a purity of >90 % with average arm length 45.3 ± 5.6 nm that is ideal for biomedical applications. The PdCu@Au tripods demonstrate a much brighter two-photon luminescence than that from Au nanorods, with a 3.6 ± 0.9 times larger two-photon action cross section and comparable quantum yield. Our result also shows the two-photon luminescence property of PdCu@Au tripods could be tuned by their distinct localized surface plasmon resonance property and, in turn, the different amount of Au coating. This tunability could be explained by the recently-proposed two-step excitation mechanism of two-photon luminescence in Au nanoparticle. The folate-targeted in vitro two-photon luminescence imaging of MDA-MB-435 breast cancer cells were also demonstrated to show the great potential using PdCu@Au tripods as novel multi-functional platforms for cancer theranostics.

Thresholds for nonlinear recording of fluorescent centers in chromone-doped polymer films

Nonlinear recording of stable fluorescent microstructures in PMMA films doped with photosensitive compound 2-(furan-2-yl)-3-(thiophen-2-carbonyl)-chromen-4-one is investigated. Energy and power density thresholds for recording stable fluorescent marks and for film destruction and photobleaching of the fluorescent marks by laser pulses were measured in wide spectral (from 530 to 620 nm) and temporal (from 80 to 7 ns) ranges. The evidences of the sequential two-step excitation mechanism for the studied dye are presented. It was shown that picosecond pulse duration range is preferable for two-photon data recording. Quasi-Gaussian and donut shaped fluorescent marks with almost the same cross-section were recorded by femtosecond pulses with rather small difference in energies.

YAl 3(BO 3) 4:Yb&Tm a nonlinear crystal: Up- and down-conversion phenomena and excited state relaxations

Excitation and relaxation phenomena within the Yb 3+–Tm 3+ system in the YAl 3(BO 3) 4 (YAB) crystal were investigated upon continuous wave and short pulse excitation at 975 and 660 nm. With the 975 nm excitation, an unusually intense luminescence from 3H 4, and a considerably weaker one from 1G 4 of Tm 3+ have been observed at 800 (IR) and 480 nm (VIS), respectively. The intensity ratio of the IR to VIS up-converted emissions is strongly affected by dopant concentrations. The dependence of the anti-Stokes emission intensities on the excitation power implies a two-step excitation mechanism of the 3H 4 luminescence and a three-step excitation of the 1G 4 one. Down-converted emission from the 2F 5/2 ytterbium level has been observed following the 660 nm excitation of the 3F 2 thulium state. The optical and kinetics results provided that energy transfer processes are responsibly for the up- and down-conversion phenomena. Even though the Yb 3+–Tm 3+ energetic system is far from resonance, the energy transfer processes are surprisingly efficient in the YAB host. A high intensity of the up-converted emission at 800 nm may offer a laser potential.

Blue up-conversion with excitation into Tm ions at 808 nm in YVO4 crystals co-doped with thulium and ytterbium

The up-conversion of infrared radiation at 808 nm, emitted by a diode laser, into blue emission centered at 480 nm in 1 at.% Tm, 5 at.% Yb: YVO4; 1 at.% Tm, 8 at.% Yb: YVO4 and 2 at.% Tm, 5 at.% Yb: YVO4 has been studied. The highest intensity of blue emission is found for the 1 at.% Tm, 8 at.%Yb: YVO4 system. The power dependence of up-converted emission upon continuous-wave excitation as well as the time evolution of its intensity upon short-pulse excitation were found to be consistent with a two-step excitation mechanism in which the forward Tm3+–Yb3+ energy transfer is followed by the back Yb3+–Tm3+ energy transfer. The effect of dopant concentrations on the up-conversion process is interpreted taking into account dynamics of the excited states involved.

Infrared-to-visible conversion of radiation in YVO 4 crystals doped with Yb 3+ and Tm 3+ ions

Up-conversion processes in Yb 3+ and Tm 3+ doped YVO 4 crystals have been investigated upon continuous wave and short pulse excitation around 975 nm. The energy transfer efficiency from Yb 3+ to Tm 3+ has been experimentally assessed, using results of pulsed laser excitation. Infrared emission originating in the 3H 4 level and a considerably weaker blue emission originating in the 1G 4 level of Tm 3+ has been observed. The ratio of the IR to blue up-converted emission intensities is about 25:1 at room temperature. The dependence of the intensity of both emissions on the excitation power implies a two-step excitation mechanism of the 3H 4 emission and a three-step excitation one of the 1G 4 emission. Temperature dependencies of IR and blue up-converted emission have been discussed based on optical absorption and emission spectra and lifetimes recorded at several temperatures between 4.2 and 300 K. Up-converted blue emission in YVO 4:Yb+Tm is qualitatively similar to that observed in other matrices containing ytterbium and thulium but high intensity of up-converted IR emission associated with the 3H 4– 3H 6 transition of Tm 3+ may offer a laser potential.

Infrared-to-visible conversion of radiation in YVO4 crystals doped with Yb3+ and Tm3+ ions

Up-conversion processes in Yb 3+ and Tm 3+ doped YVO 4 crystals have been investigated upon continuous wave and short pulse excitation around 975 nm. The energy transfer efficiency from Yb 3+ to Tm 3+ has been experimentally assessed, using results of pulsed laser excitation. Infrared emission originating in the 3 H 4 level and a considerably weaker blue emission originating in the 1 G 4 level of Tm 3+ has been observed. The ratio of the IR to blue up-converted emission intensities is about 25:1 at room temperature. The dependence of the intensity of both emissions on the excitation power implies a two-step excitation mechanism of the 3 H 4 emission and a three-step excitation one of the 1 G 4 emission. Temperature dependencies of IR and blue up-converted emission have been discussed based on optical absorption and emission spectra and lifetimes recorded at several temperatures between 4.2 and 300 K. Up-converted blue emission in YVO 4 :Yb+Tm is qualitatively similar to that observed in other matrices containing ytterbium and thulium but high intensity of up-converted IR emission associated with the 3 H 4 – 3 H 6 transition of Tm 3+ may offer a laser potential.

Theoretical resolution of theH−resonance spectrum up to then=4threshold. I. States of1Po,1Do,and1Fosymmetries

We report on a theoretical approach to the calculation of wave functions, energies $E,$ and widths $\ensuremath{\Gamma}$ of high-lying resonances of ${\mathrm{H}}^{\ensuremath{-}},$ with application to the identification of 76 states of ${}^{1}{P}^{o},$ ${}^{1}{D}^{o},$ and ${}^{1}{F}^{o}$ symmetries up to the $n=4$ threshold, with widths down to about $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$--$1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\mathrm{a}.\mathrm{u}.,$ depending on symmetry and threshold. The overwhelming majority of these resonances have not been detected experimentally. Previous calculations by different methods allowed the identification of 35 of these states, with only very few cases having a level of accuracy comparable to the one of the present work. We suggest that the measurement of these resonances might become possible via two-step excitation mechanisms using ultrasensitive techniques capable of dealing with the problems of very small widths and preparation cross-sections. In this work, the ${}^{1}D$ state at $10.872\mathrm{eV}$ above the ${\mathrm{H}}^{\ensuremath{-}}{1s}^{2}{}^{1}S$ ground state, already prepared and measured by electron scattering as well as by two-photon absorption, is considered as the stepping stone for the possible probing of resonances of ${}^{1}{P}^{o},$ ${}^{1}{D}^{o},$ and ${}^{1}{F}^{o}$ symmetries via absorption of tunable radiation of high resolution. By classifying the results according to the Gailitis-Damburg model of dipole resonances (a product of a ${1/r}^{2}$-like potential) we find that there are unperturbed as well as perturbed series, in analogy with the Rydberg spectra of neutrals and positive ions (a product of a $1/r$-like potential). For the former, the agreement with the Gailitis-Damburg predictions as to the relationship of the extent of the outer orbital and of the energies and widths of states is excellent. The perturbed series result from interchannel coupling and the remaining electron correlation. One of the effects is the existence of overlapping resonances. For example, for two ${}^{1}{P}^{o}$ states below the $n=3$ threshold there is degeneracy on the energy axis ${(E}_{1}=\ensuremath{-}0.0555763612\mathrm{a}.\mathrm{u}.$ and ${E}_{2}=\ensuremath{-}0.0555763099\mathrm{a}.\mathrm{u}.)$ but the widths differ $({\ensuremath{\Gamma}}_{1}=1.14\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\mathrm{eV}$ and ${\ensuremath{\Gamma}}_{2}=5.45\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{eV}).$ We also comment on whether consideration of the relativistic Lamb shift splitting of the hydrogen thresholds is sufficient for deciding the truncation of the resonance series. Our calculations were carried out by implementing previously published theories, whereby the resonance $E\mathrm{'}\mathrm{s}$ and \ensuremath{\Gamma}'s are determined from properly selected complex eigenvalues of non-Hermitian Hamiltonian matrices constructed in terms of physically relevant square integrable real and complex function spaces representing the localized and asymptotic parts of the resonance eigenfunctions. For the ${\mathrm{H}}^{\ensuremath{-}}$ series of resonances, the physical relevance of the real functions implies the systematic construction of basis sets with average $〈r〉$ extending to thousands of atomic units, in order to account for the extreme diffuseness of the outer orbital as each threshold is approached. The complex one-electron basis sets are Slater-type orbitals of a complex coordinate ${\mathrm{re}}^{\ensuremath{-}i\ensuremath{\theta}}.$ Their inclusion into the overall calculation and their optimization via the variation of nonlinear parameters (including \ensuremath{\theta}) accounts for the contribution of the asymptotic part of the resonance, and for the energy width and shift beyond the real energy ${E}_{o}$ of the localized part.

Excited-state absorption mechanisms in red-laser-pumped uv and blue upconversions in Tm3+-doped fluoroaluminate glass.

The mechanism of red-pumped frequency-upconversion fluorescence observed in the uv and blue ranges of frequencies for a Tm 3+ -doped fluoroaluminate glass was investigated by single-wavelength pumping of a tunable 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-amino-styryl)-4H-pyran dye laser. It was found that the dependences of fluorescence intensity on the incident excitation power were quadratic both for the 0.36- and 0.45-μm emissions, which originate from 1 D 2 , and for the 0.48-μm emission from 1 G 4 , indicating two-step excitation mechanisms

Rocket measurements of O2 atmospheric (0-0) and OH meinel bands in the night airglow.

O2(0-0) atmospheric and OH Meinel bands in the night airglow have been simultaneously measured using multi-color rocket-borne photometers in midlatitude. Atomic oxygen densities have been deduced from O2 airglow intensity profiles and the direct and two-step excitation mechanisms for the O2 airglow have been examined. The obtained atomic oxygen densities agree with in-situ measurements. The altitude profiles of the observed OH(8-3), (7-3) and (6-2) bands are reproduced by calculation using calculated atomic oxygen density profiles. It is clearly demonstrated that time-to-time variations of the altitude profile of atomic oxygen is basically important for the change of the OH altitude profile. The quenching rate of OH by O has been estimated to be small, with a best-fitted value of 5.1×10-12cm3sec-1.

Coupled channels analysis of 12C

Existing data for the ${0}_{2}^{+}$ state in $^{12}\mathrm{C}$ at 50 and 67.5 MeV are compared with coupled channels calculations. Calculations are done with a realistic potential which includes the Lorentz-Lorenz-Ericson-Ericson p-wave correction and terms second order in the nuclear density. These are compared to results obtained using a phenomenological Kisslinger potential to investigate the sensitivity to the details of the \ensuremath{\pi}A interaction. The suppression of the forward angle cross sections by the Lorentz-Lorenz-Ericson-Ericson effect is insufficient to bring the direct transition into agreement at both energies. The interference between the one- and two-step excitation mechanisms is required to reproduce the forward angle data. The dependence of the cross section on the choice of monopole transition density is examined.

Use of metastable ions for a soft X-ray laser

A two-step excitation mechanism involving metastable ions of the Li + like series of ions (Li +, Be 2+, B 3+, C 4+, etc.) is proposed with lasing frequencies in the range of 200-40 Å. A particular design feature allows repetitive excitation leading to true cavity lasing with a maximum output of 10 -6 J per pulse in a pulse train.