Red-green-blue upconversion emission and energy-transfer between Tm3+ and Er3+ ions in tellurite glasses excited at 1.064 mu m

Red, green, and blue emission through frequency upconversion and energy-transfer processes in tellurite glasses doped with Tm3+ and Er3+ excited at 1.064 mum is investigated. The Tm3+/Er3+-codoped samples produced intense upconversion emission signals at around 480, 530, 550 and 660 nm. The 480 nm blue emission was originated from the (1)G(4)-->H-3(6) transition of the Tm3+ ions excited by a multiphoton stepwise phonon-assisted excited-state absorption process. The 5 30, 5 50 nm green and 660 mn red upconversion luminescences were identified as originating from the H-2(11/2), S-4(3/2) --> I-4(15/2) and F-4(9/2) --> I-4(15/2) transitions of the Er3+ ions, respectively, populated via efficient cross-relaxation processes and excited-state absorption. White light generation employing a single infrared excitation source is also examined. (C) 2003 Elsevier B.V. (USA). All rights reserved.

Energy transfer between Tm3+ and Er3+ ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in TM3+/Er3+ doped telluride glass pumped at the commercial diode laser pump wavelength similar to 800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified, A 6-fold enhancement of the Er3+ visible frequency upconversion fluorescence at similar to 660 nm is observed due to the inclusion of Tm3+ ions. This is evidence of the relevant contribution of the route Er-1(I-4(11/2)) + Er-2(I-4(13/2)) -> Er-1(I-4(15/2)) + Er-2(F-4(9/2)) to the process. Energy migration among pumped I-4(9/2) level reducing the efficiency of the upconversion emission rate (H-3(11/2), S-4(3/2), and F-4(9/2)) is observed for Er3+ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported. (c) 2006 Elsevier B.V. All rights reserved.

Thermally enhanced cooperative energy-transfer frequency upconversion in terbium and ytterbium doped tellurite glass

Infrared-to-visible frequency upconversion through cooperative energy-transfer and thermal effects in Tb3+/Yb3+-codoped tellurite glasses excited at 1.064 mum is investigated. Bright luminescence emission around 485, 550, 590, 625 and 65 nm, identified as due to the D-5(4) --> F-7(J) (J= 6, 5, 4, 3, and 2) transitions of the terbium ions, respectively, was recorded. The excitation of the D-5(4) emitting level of the Tb3+ ions is assigned to cooperative energy-transfer from pairs of ytterbium ions.. The effect of temperature on the upconversion process was examined and the results revealed a fourfold upconversion enhancement in the 300-500 K interval. The enhancement of the upconversion process is due to the temperature dependence of the Yb3+-sensitizer absorption cross-section under anti-Stokes excitation. A rate-equation. model using multiphonon-assisted absorption for the ytterbium excitation combined with the energy migration effect between Yb-Yb pair, and Tb3+ ground-state depopulation via multiphonon excitation of the F-7(J) excited states describes quite well the experimental results. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Energy transfer assisted frequency upconversion in Ho 3+ doped fluoroindate glass

Experimental results are reported which show a strong evidence of energy transfer between Ho 3+ ions in a fluoroindate glass excited by a pulsed laser operating at 640 nm. We identified the origin of the blue and green upconverted fluorescence observed as being due to a Ho 3+-Ho 3+ pair interaction process. The dynamics of the fluorescence revealed the pathways involved in the energy transfer assisted upconversion process. © 2002 American Institute of Physics.

Phonon-assisted cooperative energy transfer and frequency upconversion in a Yb3+/Tb3+ codoped fluoroindate glass

Phonon-assisted cooperative energy transfer and frequency upconversion (UC) in Yb3+/Tb3+ codoped fluoroindate glass were investigated. Anti-Stokes quasiresonant excitation of Yb3+ ions was used to study the influence of multiphonon transitions in the UC process. A rate equation model was used to describe the temperature dependence of the UC emission intensities and the theoretical results are in good agreement with the experimental data.

Dynamics of energy transfer and frequency upconversion in Tm3+ doped fluoroindate glass

The spectroscopic properties of Tm3+-doped fluoroindate glasses (FIG) were described by single wavelength pumping in the red region. The Judd-Ofelt (J-O) theory was used to obtain the quantum efficiency of the 4f-4f transitions and other spectroscopic parameters. The dynamics of the fluorescence was investigated and energy transfer (ET) processes among Tm3+ ions were studied. The results indicate that a two-step one-photon absorption process is responsible for the ultraviolet upconversion (UC) emissions, and dipole-dipole interaction provides the main contribution for ET rate is equal to the decay rate of noninteracting among active ions.

Optical properties and energy-transfer frequency upconversion of Yb 3+-sensitized Ho3+- And Tb3+-doped lead-cadmium-germanate glass and glass ceramic

In this report we investigate the optical properties and energy-transfer upconversion luminescence of Ho3+- and Tb3+/Yb 3+-codoped PbGeO3-PbF2-CdF2 glass-ceramic under infrared excitation. In Ho3+/Yb 3+-codoped sample, green(545 nm), red(652 nm), and near-infrared(754 nm) upconversion luminescence corresponding to the 4S 2(5F4) → 5I8, 5F5 → 5I8, and 4S2(5F4) → 5I 7, respectively, was readly observed. Blue(490 nm) signals assigned to the 5F2,3 → 5I8 transition was also detected. In the Tb3+/Yb3+ system, bright UV-visible emission around 384, 415, 438, 473-490, 545, 587, and 623 nm, identified as due to the 5D3(5G6) → 7FJ(J=6,5,4) and 5D4→ 7FJ(J=6,5,4,3) transitions, was measured. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the former samples present much higher upconversion efficiencies. The dependence of the upconversion emission upon pump power, and doping contents was also examined. The results indicate that successive energy-transfer between ytterbium and holmium ions and cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The viability of using the samples for three-dimensional solid-state color displays is also discussed.

Energy transfer and frequency upconversion involving triads of Pr3+ ions in (Pr3+, Gd3+) doped fluoroindate glass

Blue and ultraviolet luminescence in (Pr3+, Gd3+) doped fluoroindate glass is studied for excitation in the red region (≈590 nm). Frequency upconversion (UC) is observed due to energy transfer (ET) among three Pr3+ ions initially excited to the D21 state corresponding to the ET process D21 + D21 + D21 → S01 + H53 + H53. Additionally, UC luminescence from states P 72 6 and I 72 6 of Gd3+ is observed for an excitation wavelength resonant with transitions of the Pr3+ ions. The characterization of the luminescence signals allowed to determine ET rate among the Pr3+ ions and provides evidence of interconfigurational ET between Gd3+ and Pr3+ ions. © 2006 American Institute of Physics.

Chaotic Dynamics in a Low-Energy Transfer Strategy to the Equilateral Equilibrium Points in the Earth-Moon System

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electronic energy transfer processes in pi-conjugated polymers

Currently pi-conjugated polymers are considered as technologically interesting materials to be used as functional building elements for the development of the new generation of optoelectronic devices. More specifically during the last few years, poly-p-phenylene materials have attracted considerable attention for their blue photoluminescence properties. This Thesis deals with the optical properties of the most representative blue light poly-p-phenylene emitters such as poly(fluorene), oligo(fluorene), poly(indenofluorene) and ladder-type penta(phenylene) derivatives. In the present work, laser induced photoluminescence spectroscopy is used as a major tool for the study of the interdependence between the dynamics of the probed photoluminescence, the molecular structures of the prepared polymeric films and the presence of chemical defects. Complementary results obtained by two-dimensional wide-angle X-ray diffraction are reported. These findings show that the different optical properties observed are influenced by the intermolecular solid-state interactions that in turn are controlled by the pendant groups of the polymer backbone. A significant feedback is delivered regarding the positive impact of a new synthetic route for the preparation of a poly(indenofluorene) derivative on the spectral purity of the compound. The energy transfer mechanisms that operate in the studied systems are addressed by doping experiments. After the evaluation of the structure/property interdependence, a new optical excitation pathway is presented. An efficient photon low-energy up-conversion that sensitises the blue emission of poly(fluorene) is demonstrated. The observed phenomenon takes place in poly(fluorene) derivatives hosts doped with metallated octaethyl porphyrins, after quasi-CW photoexcitation of intensities in the order of kW/cm2. The up-conversion process is parameterised in terms of temperature, wavelength excitation and central metal cation in the porphyrin ring. Additionally the observation of the up-conversion is extended in a broad range of poly-p-phenylene blue light emitting hosts. The dependence of the detected up-conversion intensity on the excitation intensity and doping concentration is reported. Furthermore the dynamics of the up-conversion intensity are monitored as a function of the doping concentration. These experimental results strongly suggest the existence of triplet-triplet annihilation events into the porphyrin molecules that are subsequently followed by energy transfer to the host. After confirming the occurrence of the up-conversion in solutions, cyclic voltammetry is used in order to show that the up-conversion efficiency is partially determined from the energetic alignment between the HOMO levels of the host and the dopant.

Vibrational spectroscopic monitoring of CO2-O energy transfer: cooling processes in atmospheres of Venus & Mars

The vibrational excitation of CO2 by a fast-moving O atom followed by infrared emission from the vibrationally excited CO2 has been shown to be an important cooling mechanism in the upper atmospheresof Venus, Earth and Mars. We are trying to determine more precisely the efficiency (rate coefficient) of the CO2-O vibrational energy transfer. For experimental ease the reverse reaction is used, i.e. collision of a vibrationally excited CO2 with atomic O, where we are able to convert to the atmospherically relevant reaction via a known equilibrium constant. The goal of this experiment was to measure the magnitudes of rate coefficients for vibrational energy states above the first excited state, a bending mode in CO2. An isotope of CO2, 13CO2, was used for experimental ease. The rate coefficients for given vibrational energy transfers in 13CO2 are not significantly different from 12CO2 at this level of precision. A slow-flowing gas mixture was flowed through a reaction cell: 13CO2 (vibrational specie of interest), O3(atomic O source), and Ar (bath gas). Transient diode laser absorption spectroscopy was used to monitor thechanging absorption of certain vibrational modes of 13CO2 after a UV pulse from a Nd:YAG laser was fired. Ozone absorbed the UV pulse in a process which vibrationally excited 13CO2 and liberated atomic O.Transient absorption signals were obtained by tuning the diode laser frequency to an appropriate ν3 transition and monitoring the population as a function of time following the Nd:YAG pulse. Transient absorption curves were obtained for various O atom concentrations to determine the rate coefficient of interest. Therotational states of the transitions used for detection were difficult to identify, though their short reequilibration timescale made the identification irrelevant for vibrational energy transfer measurements. The rate coefficient for quenching of the (1000) state was found to be (4 ± 8) x 10-12 cm3 s-1 which is the same order of magnitude as the lowest-energy bend-excited mode: (1.8 ± 0.3) x 10-12 cm3 s-1. More data is necessary before it can be certain that the numerical difference between the two is real.

Long-Distance Electronic Energy Transfer in Light-Harvesting Supramolecular Polymers.

The efficient collection of solar energy relies on the design and construction of well-organized light-harvesting systems. Herein we report that supramolecular phenanthrene polymers doped with pyrene are effective collectors of light energy. The linear polymers are formed through the assembly of short amphiphilic oligomers in water. Absorption of light by phenanthrene residues is followed by electronic energy transfer along the polymer over long distances (>100 nm) to the accepting pyrene molecules. The high efficiency of the energy transfer, which is documented by large fluorescence quantum yields, suggests a quantum coherent process.

Creating a dynamic picture of the sliding clamp during T4 DNA polymerase holoenzyme assembly by using fluorescence resonance energy transfer

The coordinated assembly of the DNA polymerase (gp43), the sliding clamp (gp45), and the clamp loader (gp44/62) to form the bacteriophage T4 DNA polymerase holoenzyme is a multistep process. A partially opened toroid-shaped gp45 is loaded around DNA by gp44/62 in an ATP-dependent manner. Gp43 binds to this complex to generate the holoenzyme in which gp45 acts to topologically link gp43 to DNA, effectively increasing the processivity of DNA replication. Stopped-flow fluorescence resonance energy transfer was used to investigate the opening and closing of the gp45 ring during holoenzyme assembly. By using two site-specific mutants of gp45 along with a previously characterized gp45 mutant, we tracked changes in distances across the gp45 subunit interface through seven conformational changes associated with holoenzyme assembly. Initially, gp45 is partially open within the plane of the ring at one of the three subunit interfaces. On addition of gp44/62 and ATP, this interface of gp45 opens further in-plane through the hydrolysis of ATP. Addition of DNA and hydrolysis of ATP close gp45 in an out-of-plane conformation. The final holoenzyme is formed by the addition of gp43, which causes gp45 to close further in plane, leaving the subunit interface open slightly. This open interface of gp45 in the final holoenzyme state is proposed to interact with the C-terminal tail of gp43, providing a point of contact between gp45 and gp43. This study further defines the dynamic process of bacteriophage T4 polymerase holoenzyme assembly.

Temporally and spectrally resolved subpicosecond energy transfer within the peripheral antenna complex (LH2) and from LH2 to the core antenna complex in photosynthetic purple bacteria.

We report studies of energy transfer from the 800-nm absorbing pigment (B800) to the 850-nm absorbing pigment (B850) of the LH2 peripheral antenna complex and from LH2 to the core antenna complex (LH1) in Rhodobacter (Rb.) sphaeroides. The B800 to B850 process was studied in membranes from a LH2-reaction center (no LH1) mutant of Rb. sphaeroides and the LH2 to LH1 transfer was studied in both the wild-type species and in LH2 mutants with blue-shifted B850. The measurements were performed by using approximately 100-fs pulses to probe the formation of acceptor excitations in a two-color pump-probe measurement. Our experiments reveal a B800 to B850 transfer time of approximately 0.7 ps at 296 K and energy transfer from LH2 to LH1 is characterized by a time constant of approximately 3 ps at 296 K and approximately 5 ps at 77 K. In the blue-shifted B850 mutants, the transfer time from B850 to LH1 becomes gradually longer with increasing blue-shift of the B850 band as a result of the decreasing spectral overlap between the antennae. The results have been used to produce a model for the association between the ring-like structures that are characteristic of both the LH2 and LH1 antennae.

Energy transfer dynamics of nanocrystal-polymer composites

Steady-state and time-resolved photoluminescence spectroscopy are used to examine the photoluminescent properties of nanocrystal-polymer composites consisting of colloidal PbS nanocrystals blended with poly(2-methoxy-5(2-ethylhexyloxy)-p-phenylene vinylene). Quenching of the emission from the conjugated polymer due to the PbS nanocrystals is observed along with band edge emission from the ligand capped PbS nanocrystals. A decrease in the photoluminescence lifetime of MEH-PPV is also observed in the thin film nanocrystal-polymer composite materials. Photoluminescence excitation spectroscopy of the PbS nanocrystal emission from the composite shows features attributed to MEH-PPV providing evidence of a Forster transfer process.