Phonon-assisted Energy Transfer Research Articles

Fabrication of upconversion emissive LaOCl phosphors doped with rare-earth ions for bioimaging probes

Lanthanum oxychloride nanoparticles containing trivalent erbium ions were prepared through a self-hydrolysis process of hydrated chlorides of lanthanum and erbium. The phosphors were characterized by X-ray diffraction to be found in a single oxychloride phase. Irradiation on the powder with a 980-nm laser diode resulted in frequency upconversion into intense green emissions around 525--550 nm and red emissions around 650--670 nm. Stokes emission and excitation power dependence of the upconversion emission were also measured, and the dominant upconversion mechanisms were deduced to be the excited state absorption (ESA) and phonon-assisted energy transfer (PET) for green and red emissions, respectively.

Optical spectroscopy and energy transfer of Er^3+/Ce^3+ in B_2O_3-doped bismuth-silicate glasses

Ce3+ and B2O3 are introduced into erbium-doped Bi2O3-SiO2 glass to enhance the luminescence emission and optic spectra characters of Er3+. The energy transfer from Er3+ to Ce3+ will obviously be improved with the phonon energy increasing by the addition of B2O3. Here, the nonradiative rate, the lifetime of the 4I11/2→4I13/2 transition, and the emission intensity and bandwidth of the 1.5 μm luminescence with the 4I13/2→4I15/2 transition of Er3+ are discussed in detail. The results show that the optical parameters of Er3+ in this bismuth-borate-silicate glass are nearly as good as that in tellurite glass, and the physical properties are similar to those in silicate glass. With the Judd-Ofelt and nonradiative theory analyses, the multiphonon decay and phonon-assisted energy-transfer (PAT) rates are calculated for the Er3+/Ce3+ codoped glasses. For the PAT process, an optimum value of the glass phonon energy is obtained after B2O3 is introduced into the Er3+/Ce3+ codoped bismuth-silicate glasses, and it much improves the energy-transfer rate between Er3+4I11/2→4I13/2 and Ce3+2F5/2→2F7/2, although there is an energy mismatch.

Effect of CsBr addition on the emission properties of Tm3+ ion in Ge-Ga-S glass

Absorption and emission properties of Tm 3+ in (1-x) (Ge 0.25 Ga 0.10 S 0.65 )-xCsBr glass (x = 0.00–0.12) were investigated. Upon 10 mol% CsBr addition, the absorption cross sections of Tm 3+ decreased accompanied by a large increase in the lifetime of the Tm 3+ : 3 H 4 level to 1.23 ms since Tm 3+ ions were surrounded by the Br ions of [GaS 3/2 Br] − units. As the concentration of Tm 3+ increased, the 3 H 4 level lifetime decreased due to cross relaxation (Tm 3+ : 3 H 4 , 3 H 6 →Tm 3+ : 3 F 4 , 3 F 4 ). Temperature dependence of the 3 H 4 level lifetime showed that cross relaxation in Tm 3+ is a phonon-assisted energy transfer process. When Tm 3+ were surrounded by Br ions, cross relaxation among Tm 3+ was also suppressed due to a decrease in the transition probability among Tm 3+ energy levels, a decrease in phonon energy of the host glasses, as well as an increase in the number of phonons participating in the cross relaxation process. The potential of Tm 3+ -doped Ge-Ga-S-CsBr glasses for S-band fiber amplifiers is also discussed.

Anti-Stokes fluorescent cooling by energy transfer

A new model on the mechanism of anti-Stokes fluorescent cooling is proposed, based on which incident laser induces a phonon-assisted energy transfer from one luminescent center to another in an optical medium and causes medium cooling when the energy transfer is an anti-Stokes process and absorbs phonons. Here, we develop the model in two cases: two-phonon-assisted energy transfer for a small energy mismatch and one-phonon-assisted energy transfer for a large energy mismatch. The dependence of relative cooling efficiency on photon energy and temperature is also discussed in evaluating the new mechanism.

Dynamical processes of Ln 3+ ions doped in LaF 3 nanocrystals embedded in transparent oxyfluoride glass

The transverse and longitudinal relaxations from the excited electronic levels of rare earth ions (Ln 3+) in nanocrystals are reviewed. Employing the frequency up-converted emission, we have studied the ion–ion interactions and the distribution of the ions within Pr 3+:LaF 3 nanocrystals embedded in oxyfluoride glass. The phonon-assisted energy transfer between Pr 3+ ions is discussed and the relation between the energy transfer rate and the buildup of the up-converted emission is established. The results suggest that the Pr 3+ concentration inside the nanocrystals is 5–10 times that of the average concentration. A jump in the energy transfer rate is observed as the concentration increases, and the possible processes behind it are discussed.

Photon Statistics from Coupled Quantum Dots

We present an optical study of two closely stacked self-assembled InAs/GaAs quantum dots. The energy spectrum and correlations between photons subsequently emitted from a single pair provide not only clear evidence of coupling between the quantum dots but also insight into the coupling mechanism. Our results are in agreement with recent theories predicting that tunneling is largely suppressed between nonidentical quantum dots and that the interaction is instead dominated by dipole-dipole coupling and phonon-assisted energy transfer processes.

Cr 3 + → Nd 3 + energy transfer in the YAl3(BO3)4 nonlinear laser crystal

The energy transfer between Cr3+ and Nd3+ ions in Cr3+, Nd3+:YAl3(BO3)4 (YAB) codoped crystals has been investigated by means of steady-state and time-resolved laser spectroscopies. The mainly nonradiative character of the energy transfer has been evidenced from the comparison between the emission spectra and the donor fluorescence decays. The analysis of the donor decay profiles has been used to determine the main dipole–dipole character of the transfer, in excellent agreement with the predictions made from the Dexter [J. Chem Phys. 21, 836 (1953)] model, as well as with the dependence of the energy-transfer rates on the acceptor concentration. The temperature behavior of the transfer rate has been analyzed considering the thermal-induced population redistribution between the E2 and T24 states of Cr3+ ions and the existence of the phonon-assisted energy transfer. We have obtained that 80% of the Cr3+→Nd3+ energy transfer in YAB is taking place without the assistance of lattice phonons whereas almost 20% is taking place through multiphonon emission.

Quantum cutting by cooperative energy transfer inYbxY1−xPO4:Tb3+

We present experimental evidence for cooperative energy transfer from ${\mathrm{Tb}}^{3+}$ to two ${\mathrm{Yb}}^{3+}$ ions and a determination of the energy-transfer rate. Energy transfer from ${\mathrm{Tb}}^{3+}$ to ${\mathrm{Yb}}^{3+}$ was investigated by luminescence measurements on $({\mathrm{Yb}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x})\mathrm{P}{\mathrm{O}}_{4}$ doped with 1% ${\mathrm{Tb}}^{3+}$. Time-resolved luminescence experiments were analyzed using Monte Carlo simulations based on theories for phonon-assisted, cooperative, and accretive energy transfer. The luminescence decay curves of the $^{5}D_{4}$ emission from ${\mathrm{Tb}}^{3+}$ show an excellent agreement with simulations based on cooperative energy transfer via dipole-dipole interaction, while a phonon-assisted or an accretive energy-transfer mechanism cannot explain the experimental results. The energy-transfer rate to two nearest-neighbor ${\mathrm{Yb}}^{3+}$ ions is $0.26\phantom{\rule{0.3em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$. This corresponds to an upper limit of the energy-transfer efficiency of 88% in $\mathrm{Yb}\mathrm{P}{\mathrm{O}}_{4}$. Application of cooperative energy transfer has prospects for increasing the energy efficiency of crystalline Si solar cells by photon doubling of the high energy part of the solar spectrum.

Spectroscopic properties of Tm3+ ions in PbO–PbF2–Bi2O3–Ga2O3 glasses for S-band optical amplifications

The spectroscopic properties of Tm3+ ions in (57−x)PbO–xPbF2–25Bi2O3–18Ga2O3(mol%) glasses were investigated through measurements of absorption, emission, and lifetime measurements. The PbF2 substitution in this glass system raised the lifetime of the 1.48μm emission. The lifetime for the H43 level was changed from 272to416μs when the PbF2 content increased from x=0to15. In a 47PbO–15PbF2–25Bi2O3–18Ga2O3 glass, the phonon-assisted energy transfer involving two phonons with the energy of 550cm−1 dominated the cross relaxation of the Tm3+: H43, H63→F43, F43 route in spite of the presence of the overlap between the H43→F43 emission and H63→F43 absorption spectra. As a result, PbO–PbF2–Bi2O3–Ga2O3 glasses were thought to be useful for the S-band optical fiber amplifiers.

Optical Characterization of Eu3+Ions in CdSe Nanocrystal Containing Silica Glass

Silica glass samples containing CdSe/Eu3+ ions were prepared by sol-gel route. Size distribution and optical band gap of the nanoparticles were calculated from absorption spectrum. It is observed that the presence of CdSe nanocrystallites enhances the fluorescence of europium in silica glass. The phonon sideband spectrum associated with the excitation transition 7F0-5D2 is used to analyze the electron-phonon coupling and nonradiative deexcitiation of the rare earth ions in the glass host, The observed fluorescence enhancement is discussed on the basis of phonon assisted energy transfer from electron-hole recombination of the CdSe nanocrystallites to the rare earth ion and multiphonon relaxation.

Confinement on energy transfer between luminescent centers in nanocrystals

The luminescence dynamics of optical centers in nanocrystals depends critically on the phonon density of states (PDOS), which is quite distinct from that of bulk materials. It is shown that energy transfer (ET) in nanocrystals is confined by discrete PDOS as well as direct size restriction. Temperature-, concentration-, and size-dependence of the fluorescence decay from the S3/24 state of Er3+ in Y2O2S nanocrystals have been investigated using laser spectroscopic experiments and computational simulations. A set of microscopic rate equations that govern the evolution of the excitation probability Pi(t) are solved iteratively using a Monte Carlo method. The simulations of ET based on a theoretical model with five parameters are in good agreement with the experimental results. It is shown that phonon-assisted ET processes in Er3+:Y2O2S nanocrystals contribute partly to the fluorescence decay at 295 K, and is negligible at 5 K. For applications, the nanoconfinement effects on ET may significantly reduce the efficiency of sensitized or upconversion luminescence due to the lack of low-frequency phonon modes and restricted excitation migration in nanophosphors.

Energy transfer processes of Er3+ in Y AlO3

In this study the upconversion of the4I13/2 and4I11/2 multiplets, and the cross-relaxation (CR) processes of4S3/2 and4I9/2 ofEr3+ inY AlO3, which arerelated to its 3 µm laser operation, have been investigated. The dominant energy transfer (ET) pathways arediscussed and the ET rates of the contributing multipolar interaction mechanismshave been estimated, based upon the Holstein–Lyo–Orbach or the Miyakawa andDexter formalisms for phonon-assisted ET. The Kushida formalism has beenemployed for the investigation of migration processes, and the matrix elementsof Kaminskii for the relevant electronic transitions have been used in ourcalculations. The major results are that at room temperature, the contributions of2H11/2 (particularly from electric quadrupole–electric quadrupole interactions) to the CR and migration of4S3/2, and to theupconversion of 4I11/2,are important, and are even the dominant ones in some cases.

Analysis of cross relaxation between Tm3+ ions in PbO−Bi2O3−Ga2O3−GeO2 glass

Mechanisms of cross relaxation (3H4, 3H6→3F4, 3F4) in Tm3+ -doped PbO−Bi2O3−Ga2O3−GeO2 glass were analyzed. The energy gap between the absorption (3H6→3F4) and emission (3H4→3F4) spectra was approximately 600 cm−1. A large decrease in cross relaxation rates at low temperatures indicated that cross relaxation is controlled by a phonon-assisted energy transfer. The bending vibration of the Ga–O–Ga bonds between GaO4 tetrahedra (∼550 cm−1) was directly responsible for this energy transfer. Although GeO2 was added to enhance the thermal stability of PbO−Bi2O3−Ga2O3 glass, the phonon mode associated with vibration of GeO2 (∼770 cm−1) was not involved in the cross relaxation process.

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

We report large thermal enhancement of the frequency upconversion (UC) process due to cooperative energy transfer (CET) in a Yb3+/Tb3+ codoped fluoroindate glass obtained by heating the sample from 308 to 530 K. To study the influence of multiphonon transitions in the UC process we chose anti-Stokes quasiresonant excitation of Yb3+ ions which were used as sensitizers. UC of radiation at 1064 nm into blue and green light was obtained. Various emission lines of Tb3+ were observed between 400 and 700 nm due to CET from Yb3+ to Tb3+ ions. 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.

1.48 μm emission properties and the cross-relaxation mechanism in chalcohalide glass doped with Tm 3+

1.48 μm emission properties and the cross-relaxation mechanism of Tm 3+ ions in 0.7(Ge 0.25As 0.10S 0.65) + 0.15GaS 3/2 + 0.15CsBr glass were investigated. Both the relative intensity ratio of the 1.48 μm emission to 1.82 μm and the measured lifetime of the 3H 4 level decreased with increasing Tm 3+ concentration. When temperature decreased from room temperature to 20 K, lifetimes of the 3H 4 level increased from 670 to 970 μs. At the same time, the critical distance for cross-relaxation decreased from 1.11 to 0.93 nm. These results indicate that cross-relaxation ( 3H 4, 3H 6 → 3F 4, 3F 4) became less effective as temperature decreased. Analysis of the temperature dependence of cross-relaxation rates showed that cross-relaxation in Tm 3+ is a phonon-assisted energy transfer process. The major phonon contributing to the process is from the Ga–Br vibration in [GaS 3/2Br] − units.

A spectroscopic study of the vaterite form YBO 3:Eu 3+processed by sol–gel technique

In the vaterite form YBO 3, Y 3+ ions lie in three different polyhedra. The use of Eu 3+ as fluorescent structural probe confirms such description by exhibiting three characteristic optical spectra which can be easily isolated using laser-induced site selective excitation. The 5D 0-relaxation rate has been evaluated in the temperature range 300–15 K for YBO 3:Eu (50 mol%) and EuBO 3 samples. The recorded evolutions are explained by phonon-assisted energy transfer mechanism leading to cross-over from fast migration to limited diffusion process below 90 K.

Energy exchange between optically excited Silicon Nanocrystals and Molecular Oxygen

ABSTRACTWe report on the photosensitizing properties of optically excited Silicon (Si) nanocrystal assemblies that are employed for an efficient generation of singlet oxygen. Spin triplet state excitons confined in Si nanocrystals transfer their energy to molecular oxygen (MO) adsorbed on the nanocrystal surface. This process results in a strong suppression of the photoluminescence (PL) from the Si nanocrystal assembly and in the excitation of MO from the triplet ground state to singlet excited states. The high efficiency of the energy transfer if favored by a broad energy spectrum of photoexcited excitons, a long triplet exciton lifetime and a highly developed surface area of the nanocrystal assembly. Due to the specifics of the coupled system Si nanocrystal – oxygen molecule all relevant physical parameters describing the photosensitization process are accessible experimentally. This includes the role of resonant and phonon-assisted energy transfer, the dynamics of energy transfer, and its mechanism.

Energy transfer in rare earth doped Y 3Al 5O 12 at very high temperatures

Phonon assisted energy transfer from the 2F 5/2 state of Yb 3+ to the 7F 0 state of Tb 3+ in Y 3Al 5O 12 has been investigated up to 1850 K. Together with prior data from the literature, the results suggest that the temperature dependence of the energy transfer probability over the entire range is well described by the electron coupling to phonon modes at the two ends of the spectrum only. Additionally, it is found that the electron–phonon coupling parameter for the lowest energy phonons is an order of magnitude larger than that for those with the highest energy.

Thulium cross-relaxation in a low phonon energy crystalline host

We report on the cross-relaxation of ${\mathrm{Tm}}^{3+}$ in a low phonon energy chloride host. The temperature and concentration dependence for ${\mathrm{Tm}}^{3+}$ cross-relaxation in the host ${\mathrm{YCl}}_{3}$ was studied between 300 and 500 K, in crystals of ${\mathrm{YCl}}_{3}$ with concentrations ranging from 0.7 to $7\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}.$ Crystals were grown from melts of anhydrous ${\mathrm{YCl}}_{3}$ and ${\mathrm{TmCl}}_{3}$ powders under ${\mathrm{Cl}}_{2}$ atmosphere using a self-seeded vertical Bridgeman technique. Fluorescence spectra and lifetimes resulting from quasi-cw pumping with a 0.5-W, 811-nm diode were collected and analyzed for the spectral region 1100--2000 nm versus temperature for three crystals with varying ${\mathrm{Tm}}^{3+}$ densities. Each of the fluorescence spectra contain three broad features centered at 1200, 1490, and 1800 nm that reflect the populations of the first three excited levels of ${\mathrm{Tm}}^{3+}.$ Rates for multiphonon relaxation and cross-relaxation were determined from the fluorescence lifetime data. An increase in 1200-nm fluorescence as the temperature rises is evidence of an endothermic cross-relaxation process. This suggests a fundamentally new mechanism for optical cooling. A macroscopic rate equation model with its temperature and concentration dependence given by theories for phonon-assisted energy transfer is fit to the spectral data.

Characterization of broadband amplified spontaneous emission from an Er 3+–Tm 3+ co-doped silica fiber

We report spectral characteristics of amplified spontaneous emission (ASE) from an Er–Tm co-doped silica fiber. When pumped at 980 nm, the ASE yielded a 3 dB bandwidth over 90, 1460–1550 nm. The changes in ASE with regard to temperature, pumping scheme, and fiber lengths were measured experimentally, which strongly suggest that phonon-assisted energy transfer between the sensitizing ion, Er 3+, and the acceptor ion, Tm 3+, plays an important role in the bandwidth increment.