Photoconductivity and Thermoluminescence of SrAl_2O_4∶Eu,Dy-A Study of Persistent Phosphors with Delocalization Processes with Prof. YEN

1 IntroductionA wide range of luminescent materials havebeen studied by optical spectroscopy techniques.Prof.William M.Yen has made tremendous contri-bution to this research area.One of his interests inthe recent years was in delocalization processes thatstrongly affect the luminescence efficiency[     

Effect of nanocrystals on up-conversion luminescence of Er3+,Yb3+ co-doped glass-ceramics

Different up-conversion luminescent spectra of Er3+ ions were observed in the oxyfluoride glass-ceramics.The ratio of two fluorides in the original compositions was modified in order to form different nanocrystals.The intensity of up-conversion luminescence increased sharply when the ratio of PbF2 and CdF2 was 40:10.The data of differential thermal analysis and X-ray diffraction were used to explain the optimization fluoride ratio. The intensity of up-conversion luminescence is not only decided by the crystallizability but also mainly related with the stoichiometric proportion of fluoride nanocrystals in the glass-ceramics.     

The effect of intermolecular interactions on photoluminescence of a porphyrin side-chain polymer

Photoluminescence properties and exciton decay dynamics in a porphyrin side-chain polymer, poly[porphyrin acrylate- acrylonitrile (abbreviated p[(por)A-AN]), have been investigated by femtosecond time-resolved photoluminescence spectroscopy. All the luminescences of p[(por)A-AN] films are due to the emissive decay of the photoexcited singlet excitons in the porphyrins. The luminescence efficiencies and lifetimes are increased for samples from pure films to dilute blend films. However, they are increased as the intrachain concentration of the porphyrin sidechain groups is decreased. The intrachain rotation motions of porphyrin sidechain groups result in the initial ultrafast luminescence decays, which are much faster than those due to the interchain interactions. All the samples show no significant red-shift and broadening of the transient luminescence spectra. The interchain and intrachain nonradiative exciton relaxation processes may play an important role in the luminescence dynamics in the p[(por)A-AN] films. The possible origin of different intrachain and interchain dynamic behaviours in p[(por)A-AN] films is discussed.     

Ultraviolet and visible upconversion dynamics in Er^3＋：YAlO3 under ^2H11/2 excitation

The luminescence of Er^3＋：YAlO3 in ultraviolet visible and infrared ranges under the 518 nm excitation of the multiples ^2H11/2 have been investigated. Ultraviolet （275 nm and 318 nm）, violet （405 nm and 413 nm） and blue （474 nm） upconversion and infrared downconversion luminescence has been observed. By means of measuring the fluorescence decay curves and using the theory of rate equations, the luminescence kinetics was studied in detail and the processes of energy transfer upconversion （ETU） and excitation state absorption （ESA） were proposed to explain the upconversion phenomena.     

Photoluminescence Mechanism of ZnO：Zn Investigated by Microwave Dielectric Spectrometry

We investigate the decay process of photoelectrons from a luminescent material of ZnO:Zn using a microwave dielectric spectrometry. Electrons in the conduction band are found to decay exponentially and the lifetime is 781 ns, while the time interval of decay from the maximum to half of this value is 470ns. ZnO:Zn is a green luminescent material at its central wavelength of 510 nm. Compared to the decay of electrons in the conduction band, the decay process of the luminescence is faster, and the time interval of decay from the maximum to half of the maximum is about 100 ns. We believe that the mechanism of the ZnO:Zn visible luminescence is recombination luminescence, and find that our theoretical simulation is in agreement with the experimental results.     

Enhancement of upconversion luminescence due to the formation of nanocrystals in Er~(3+)-doped tellurite glasses

Optically transparent Er3 -doped tellurite-based nanocrystallized glasses with the composition of 70TeO2·15Li2O·15Nb2O5·0.5Er2O3 (mol) have been prepared by a conventional melting quenching and the subsequent heat treatment processes. The sizes of grown nanocrystals in glass matrix appear to be 35-50 nm from the X-ray diffraction (XRD) measurement. The microhardness measurement shows that the Vickers hardness values of the nanocrystallized tellurite glasses are larger (33%-62%) than those in the base glass. The Raman spectra imply that the maximum phonon energy of the based glass decreases and shifts from 668 to 638 cm-1 after heat-treatment. Visible upconversion luminescence and infrared luminescence of the base glass and heat-treated glasses under 980-nm laser diode (LD) excitation are investigated. The 524-, 546- and 656-nm upconversion intensities by 980-nm pumping increase significantly.     

UV and visible upconversion luminescence in Er^3＋：YAG under red laser excitation

This paper reports that the ultraviolet and visible upconversion luminescence from the ^4S3/2, ^2G9/2 and ^2P3/2 levels have been observed in Er^3＋：YAG following 647.2 nm excitation of the ^4F9/2 multiple. Upconversion luminescence intensity dependence on pump power was recorded. The measured decay profiles were theoretically fitted by kinetics theory and the basically good agreements were achieved. The results indicate that some energy transfer processes proposed to explain the observed upconversion phenomena are reasonable.     

Improved anti-Stokes energy transfer between rare earth ions in Er（0.5）Yb（9.5）：FOV oxyfluoride vitroceramics explains the strong color reversal

The widely used energy transfer theory is a foundation of luminescence,in which the rates of Stokes and antiStokes processes have the same calculation formula.An improvement on the anti-Stokes energy transfer to explain the fluorescence intensity reversal between the red and green fluorescence of Er(0.5)Yb(9.5):FOV is reported in the present article.The range of the intensity reversal Σ was measured to be 877.Dynamic processes for 16 levels were simulated.A coefficient,the improvement factor of the intensity ratio of Stokes to anti-Stokes processes in quantum Raman theory compared to classical Raman theory,is introduced to successfully describe the anti-Stokes energy transfer.A new method to calculate the distance between the rare earth ions,which is critical for the energy transfer calculation,is proposed.The validity of these important improvements is also proved by experiment.     

Characteristics of Solid State Cathodoluminescence of PPV

Based on our previous discovery [Chem.Phys.Lett.325(2000)420] of the solid-state cathodoluminescence from organic luminescent materials in inorganic/organic heterojunction,we study characteristics of this new kind of electric-field-induced luminesecence by means of examining its oscillogram.We prepared three devices with differect structures in which PPV was used as luminescent layer,and SiO2 was used as accelerating layer.The experimental results might be understood only by means of the existence of solid-state cathodoluminescence.This new kind of luminescence makes it possible to produce new type of flat panel display.     

In situ luminescence measurement of 6H-SiC at low temperature

To understand the evolution of defects in SiC during irradiation and the influence of temperature,in situ luminescence measurements of 6H-SiC crystal samples were carried out by ion beam induced luminescence(IBIL)measurement under2 MeV H^+ at 100 K,150 K,200 K,250 K,and 300 K.A wide band(400-1000 nm)was found in the spectra at all temperatures,and the intensity of the IBIL spectra was highest at 150 K among the five temperatures.A small peak from 400 nm to 500 nm was only observed at 100 K,related with the D1 defect as a donor-acceptor pair(D-A)recombination.For further understanding the luminescent centers and their evolution,the orange band(1.79 eV)and the green band(2.14 eV)in the energy spectrum were analyzed by Gaussian decomposition,maybe due to the donor-deep defect/conduction band-deep defect transitions and Ti related bound excition,respectively.Finally,a single exponential fit showed that when the temperature exceeded 150 K,the two luminescence centers’resistance to radiation was reduced.     

Estimation of luminescence lifetime in frequency domain

Absorption is the origin of luminescence. But it must be noticed that the lifetime of luminescence might reversely influence the rate of absorption. In this paper, it is reported that the luminescence intensity of copper and manganese changes with the driving frequency at constant voltage.The variation of luminescent intensity depends only on the lifetime of luminescence but not on the type of quenching or other factors. Generally the rate of absorption is dominantly determined by the material property and the lifetime of luminescence centres, the absorption of shorter lifetime centre will be larger than that of the longer lifetime centre at the same excited condition.     

Intrinsic luminescence centers in γ- and θ-alumina nanoparticles

In this study, we investigate the photoluminescence(PL) properties of γ and θ-alumina nanoparticles synthesized by the chemical wet method followed by annealing. The obtained experimental results indicate the presence of some favorable near ultraviolet(NUV)-orange luminescent centers for usage in various luminescence applications, such as oxygen vacancies(F, F~+_2, F~(2+)_2, and F_2centers), OH related defects, cation interstitial centers, and some new luminescence bands attributed to trapped-hole centers or donor–acceptor centers. The energy states of each defect are discussed in detail. The defects mentioned could alter the electronic structure by producing some energy states in the band gap that result in the optical absorption in the middle ultraviolet(MUV) region. Spectra show that photoionazation of F and F_2 centers plays a crucial role in providing either free electrons for the conduction band, or the photoconversions of aggregated oxygen vacancies into each other, or mobile electrons for electrons-holes recombination process by the Shockley–Read–Hall(SRH)mechanism.     

Synthesis and characterization of Ca2Sn1-xCexO4 with blue luminescence originating from Ce^4＋ charge transfer transition

Ce^4＋-doped Ca2SnO4 with a one-dimensional structure, which emits bright blue light, is prepared by using a solid-state reaction method. The x-ray diffraction results show that the Ce^4＋ ions doped in Ca2SnO4 occupy the Sn^4＋ sites. The excitation and emission spectra of Ca2Sn1-xCexO4 appear to have broad bands with peaks at - 268nm and -442nm, respectively. A long excited-state lifetime （-83μs） for the emission from Ca2Sn1-xCexO4 suggests that the luminescence originates from a ligand-to-metal Ce^4＋ charge transfer （CT）. The luminescent properties of Ca2Snl_xCexO4 have been compared with those of Sr2CeO4, which is the only material reported so far to show Ce^4＋ CT luminescence. More interestingly, it is observed that the emission intensity of Ca2Sn1-xCexO4 with a small doping concentration （x - 0.03） is comparable to that of Sr2CeO4 in which the concentration of active centre is 100%.     

Coexisting Raman- and Rayleigh-Enhanced Four-Wave Mixing in Femtosecond Polarization Beats

Based on the polarization interference of Raman- and Rayleigh-enhanced four-wave mixing processes, heterodyne detection of the Raman, Rayleigh and coexisting Raman and Rayleigh femtosecond difference-frequency polarization beats is investigated in the cw and the three Markovian stochastic models, respectively. These two processes exhibit asymmetric and symmetric spectra, respectively, and the thermal effect in them can be suppressed by field-correlation method. Such studies of coexisting Raman- and Rayleigh-enhanced four-wave mixing processes can have important applications in coherence quantum control, and quantum information processing.     

Femtosecond Laser Induced Optical Waveguides and Micro—mirrors Inside Glasses

Optical Waveguides and micro-mirrors have been successfully induced inside fused silica glass and k9 glass,respectively,by focusing a 800nm femtosecond(fs)pulsed laser with a repetition rate of 1kHz,The change of refractive index was determined to be 0.001-0.008 in the fused silica glass and 0.006 in the k9 glass.The refractive index change is dependent on both the dose of irradiation and the power density of the fs pulsed laser,Photoluminescence was observed in the irradiated region,and was attributed to the defects induced by fs laser irradiation.We discuss the relationship between the optical property and the luminescent propert property of the irradiated region.     

Tunneling current of the Coulomb-coupled quantum dots embedded in n-n junction

Taking account of the electron--electron (hole) and electron--hole interactions, the tunneling processes of the main quantum dot (QD) Coulomb-coupled with a second quantum dot embedded in n--n junction have been investigated. The eighteen resonance mechanisms involved in the tunneling processes of the system have been identified. It is found that the tunneling current depends sensitively on the electron occupation number in the second quantum dot. When the electron occupation number in the second dot is tiny, both the tunneling current peaks and the occupation number plateaus in the main QD are determined by the intra-resonance mechanism. The increase of the electron occupation number in the second dot makes the inter-resonance mechanism participate in the transport processes. The competition between the inter and intra resonance mechanisms persists until the electron occupation number in the second dot reaches around unity, leading to the consequence that the inter-resonance mechanisms completely dominate the tunneling processes.     

Effects of clouds, sea surface temperature, and its diurnal variation on precipitation efficiency

The effects of clouds, sea surface temperature, and its diurnal variation on precipitation efficiency are investigated using grid-scale data from nine equilibrium sensitivity cloud-resolving model experiments driven without large-scale vertical velocity. The precipitation efficiencies are respectively defined in surface rainfall, cloud, and rain microphysical budgets. We mathematically and physically demonstrate the relationship between these precipitation efficiencies. The 2℃ increases in spatiotemporal invariant sea surface temperature (SST) from 27℃ to 29℃ and from 29℃ to 31℃, and the inclusion of diurnal SST difference 1℃ and the 1℃ increase in diurnal SST difference generate opposite changes in the precipitation efficiency by changing ice cloud-radiation interactions. The radiative and microphysical processes of ice clouds have opposite effects on the precipitation efficiency because of the rainfall increase associated with the reduction in the saturation mixing ratio caused by the exclusion of radiative effects and the decrease in rainfall related to the reduction in net condensation caused by the exclusion of deposition processes. The radiative effects of water clouds on the precipitation efficiency are statistically insensitive to the radiative effects of ice clouds.     

Blue Luminescent Properties of Silicon Nanowires Grown by a Solid-Liquid-Solid Method

Silicon nanowires （SiNWs） were grown directly from n-（111） single-crystal silicon （c-Si） substrate based on a solid- liquid-solid mechanism, and Au film was used as a metallic catalyst. The room temperature photoluminescence properties of SiNWs were observed by an Xe lamp with an exciting wavelength of 350 nm. The results show that the SiNWs exhibit a strongly blue luminescent band in the wavelength range 40-480 nm at an emission peak position of 420 nm. The luminescent mechanism of SiNWs indicates that the blue luminescence is attributed to the oxygen-related defects, which are in SiOx amorphous oxide shells around the crystalline core of SiNWs.     

Virtual Photon Effects on Chaos in Generalized Lorenz-Haken Equation

The dynamics of an ensemble of two-level atoms injected into a single-mode cavity is studied in the exact atom-field interaction situation, in which the counter-rotating terms describing the so-called virtual photon processes neglected in the rotating-wave approximation, are considered. The cavity mode is driven by the injected classical field,and the atom is prepared in a coherent superposition of the two levels. We first derive the generalized Lorenz-Haken equation by using the technique of quantum Langevin equation, and then numerically study the dynamics of this equation.We find that the virtual photon processes have strong effects on the dynamics, which can cause the trajectory in phase space of strange attractor spiral around four focus points, and the trajectory is modulated by virtual photon processes.The chaos region in parameter space is now enlarged. It should be stressed that the strange attractor can exist in optical bistability, and whether the atomic coherences and classical field can inhibit chaos depends on the laser frequency.     

Attosecond ionization time delays in strong-field physics

Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laserbased pump-probe interrogation techniques have made possible the temporal resolution of ultrafast electronic processes on the attosecond timescale, including photoionization and tunneling ionization. These interrogation techniques include the attosecond streak camera, the reconstruction of attosecond beating by interference of two-photon transitions, and the attoclock. While the fo...