Local Phonon Energy Research Articles

Crystallization of bronze-like perovskite in potassium tantalum germanate glasses: Glass ceramic preparation and its optical properties

Structural, optical and photoluminescent properties of Europium-doped potassium germanate glassy materials were investigated in terms of tantalum content and nanoscale crystallization of potassium tantalate K2Ta8O21. For such purpose, an Eu3+-doped germanate glass, Eu3+-doped tantalum germanate glass and Eu3+-doped tantalum germanate glass-ceramics were prepared and characterized by X-ray diffraction, Raman, UV–Vis–NIR and photoluminescence spectroscopies. Eu3+ emission properties were used as a sensitive probe to investigate its symmetry site into the host structure. The chemical microenvironment around Eu3+ ions have changed with tantalum concentration as well as bronze-like tantalum perovskite K2Ta8O21 phase crystallization into the glasses. The ions preferably occupy sites of higher symmetry and lower energy phonons, which attest the ions migration to the tantalum-rich crystalline environment. The correlation between the phonon side band (PSB) spectra and Raman vibrational modes, 5D0 → 7F2/5D0 → 7F1 ratio and experimental Eu3+5D0 lifetime support this assumption. The highest photoluminescence intrinsic quantum yield (QEuEu) is calculated for glass-ceramics heat-treated for 20 h by using Judd-Ofelt theory, indicating reduction of nonradiative decay processes due to the lower local phonon energy. The luminescence quenching for longer crystallization times demonstrates the importance of optimizing thermal treatment parameters to ensure proper distance between optically active ions. Accordingly, tantalum germanate glass-ceramics are promising for photonic devices as red laser, solid-state lighting and energy conversion applications, as well as for nonlinear optical applications considering the perovskite bronze-like crystallization.

Microwave-Assisted Spectroscopy of Vacancy-Related Spin Centers in Hexagonal SiC

Optically active spin centers associated with atomic scale defects in $\mathrm{Si}\mathrm{C}$ are promising candidates for quantum technology, owing to their outstanding optical and spin properties. Photoluminescence as a mature optical investigating tool is widely used for the identification of spin defects and exploration of their properties. However, in the case of spectrally overlapping contributions from different types of defects, traditional photoluminescence measurements cannot be used to separately obtain their optical and vibrational properties, such as the local phonon energy and the Debye-Waller factor. Here, we apply spin-resonant microwave-assisted spectroscopy to investigate the optical and vibrational properties of silicon vacancies in 6H-$\mathrm{Si}\mathrm{C}$ and divacancies in 4H- and 6H-$\mathrm{Si}\mathrm{C}$. We isolate contributions from each type of defect, investigate their local vibrational modes, and obtain the Debye-Waller factor. This work proves that microwave-assisted spectroscopy is a suitable tool for the investigation of optical and vibrational properties of a large variety of spin defects.

High tantalum oxide content in Eu3+-doped phosphate glass and glass-ceramics for photonic applications

Abstract Transparent Eu3+-doped tantalum phosphate glass-ceramics containing Na2Ta8O21 nanocrystals were successfully obtained by a two-step heat treatment. Structural analyses were performed using X-ray diffraction, Transmission Electron Microscopy, Nuclear Magnetic Resonance and Raman spectroscopy. Due to the high Ta/P ratio (= 2) the glass network is built from a mixed 3D PO4–TaO6 chain in which TaO6-rich regions are inserted and responsible for crystallization of an alkali tantalum oxide phase Na2Ta8O21. Eu3+ and their optical properties was used as a sensitive probe to study it site structure in the host matrix. The radiative properties of Eu3+ emission levels were predicted using the Judd-Ofelt theory. The chemical environment and local phonon energy around the rare-earth ions have been proposed based on phonon side band associated with the 7F0 → 5D2 transition, 5D0 → 7F2/7F1 relative intensity and narrowing of the emission bands. High tantalum content precursor glass exhibits high photoluminescence quantum efficiency due to low phonon energy and high refractive index (1.9461 at 633 nm). A perovskite-like bronze crystalline structure is formed in the glass-ceramic (nucleated at 925 °C for 0.5 h and grown at 940 °C for different times) and is considered promising for nonlinear optical applications and studies.

Phosphate glasses with high tantalum oxide contents: Thermal, structural and optical properties

Glasses in the binary system KPO3-Ta2O5 were prepared by the melt-quenching technique. Highly modified phosphate glasses with up to 40 mol% Ta2O5 were obtained. The samples were characterized by Differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) analysis, Ultraviolet–Visible (UV–Vis) and infrared (IR) transmittance, Raman and luminescence spectroscopies. Thermal, structural and optical properties are strongly dependent of the tantalum oxide content. Thermal behavior and Raman data are in agreement with a progressive insertion of tantalum units in the phosphate network through P-O-Ta bonds. These bonds enhance the network connectivity due to TaO6 units cross-linking the linear phosphate chains. In addition, higher Ta2O5 contents in the glass network results in tantalum oxide-rich regions detected as Ta-O-Ta bridging bonds, apparently acting as crystallization nuclei. Emission spectra of Eu3+-doped glasses pointed out a decrease in the symmetry of the Eu3+ sites by increasing Ta2O5 content. A decrease in the excited state lifetimes was also identified due to the progressive Ta-rich environment around Eu3+ ion. Finally, photoluminescence quantum efficiency increases with tantalum insertion due to the reduction of local phonon energy around rare-earth ions. In this sense, high tantalum phosphate glasses are built up from TaOx polyhedra within the phosphate chains. The presence of TaOx clusters allows the improvement of optical and thermal properties and corroborates with the potential application of these materials photonic devices.

Influence of local phonon energy on quantum efficiency of Tb3+-Yb3+ co-doped glass ceramics containing fluoride nanocrystals

The Tb3+ single-doped and Tb3+-Yb3+ co-doped glass ceramics with the precipitation of CaF2, CaF2-SrF2 solid state solution and SrF2 nanocrystals were designed and prepared by taking different amounts of CaF2 and SrF2 as the starting fluorides to investigate the influence of the crystalline phase on the total quantum efficiency. The formation of the fluoride nanocrystals and the incorporation of the doped rare earth ions into the fluoride nanocrystals were proved by the XRD measurement. The energy transfer from Tb3+ to Yb3+ was studied by the steady and time resolved spectra. The total internal quantum efficiencies were calculated based on the measured Tb3+ lifetime, which was about 10.5% improved in the SrF2 nanocrystals precipitated glass ceramics compared with that in the CaF2 nanocrystals precipitated glass ceramics mainly due to the lower phonon energy environment. Meanwhile, the total external quantum efficiencies were evaluated with the integrating sphere measurement system, which were 18.6%, 19.3% and 24.4%, respectively, for the CaF2, CaF2-SrF2 and SrF2 nanocrystals precipitated glass ceramics. Additionally, obvious difference between the calculated total internal quantum efficiency and the measured total external quantum efficiency was also discussed.

Visible spectroscopic properties of SiO2–Na2O–Al2O3–LaF3 glass ceramics doped with Dy3 + and Ho3 + under blue LED excitation

SiO2–Na2O–Al2O3–LaF3 glasses were doped with various rare earth ions to find the proper active ions for blue LED (λ=455nm) color conversion. Based on photo-luminescence (PL) spectra, a combination of Dy3+ and Ho3+ was selected and their visible spectroscopic properties were investigated for possible candidate of white color conversion of a blue LED. Concentrations of Dy3+ and Ho3+ were varied to find the proper combination, and 2mol% DyF3 and 1mol% HoF3 co-doped glass showed reasonable visible emissions. When the glasses were heat treated, emission intensities from both ions were clearly increased by the formation of LaF3 nano-crystals, which were identified by X-ray diffraction and TEM analyses. Emission peak at ~525nm newly appeared due to nano-crystal formation and the related local phonon energy change nearby Dy3+ ion. Energy transfer between Dy3+ and Ho3+ was also observed and discussed. Photo-luminescence excitation (PLE) spectra were measured and also discussed.

An investigation into the lattice thermal conductivity of random nanowire composites

The lattice thermal conductivity of compact random silicon and germanium nanowire composites was investigated by using a Monte-Carlo (MC) simulator, which was developed based on the gray medium approximation and unstructured grids. By defining the local equilibrium temperature as the one which preserves the local phonon energy in the interested heterogeneous subregion, we are able to obtain the effective thermal conductivity of random nanowire composites and explore the effects of the wire shape and the composition concentration on it. The results show that among the three kinds of wire shapes investigated – triangle, quadrangle, and voronoi, the random quadrilateral nanowire composites are the best thermal conductor under a fixed interface density or a fixed characteristic wire size and that the dependence of the effective thermal conductivity on the silicon volume concentration is approximately parabolic. The influences of these two factors are equally strong. Moreover, the parabolic dependence can be well explained by the effective medium approximation model for three bond percolation systems.

Luminescent Ions in Silica-Based Optical Fibers

We present some of our research activities dedicated to doped silica-based optical fibers, aiming at understanding the spectral properties of luminescent ions, such as rare-earth and transition metal elements. The influence of the local environment on dopants is extensively studied: energy transfer mechanisms between rare-earth ions, control of the valence state of chromium ions, effect of the local phonon energy on thulium ions emission efficiency, and broadening of erbium ions emission induced by oxide nanoparticles. Knowledge of these effects is essential for photonics applications.

Improvement of the Tm 3+: 3H 4 level lifetime in silica optical fibers by lowering the local phonon energy

The role of some glass network modifiers on the quantum efficiency of the near-infrared fluorescence from the 3H 4 level of Tm 3+ ion in silica-based doped fibers is studied. Modifications of the core composition affect the spectroscopic properties of Tm 3+ ion. Adding 17.4 mol% of AlO 3/2 to the core glass caused an increase of the 3H 4 level lifetime up to 50 μs, 3.6 times higher than in pure silica glass. The quantum efficiency was increased from 2% to approximately 8%. On the opposite, 8 mol% of PO 5/2 in the core glass made the lifetime decrease down to 9 μs. These changes of Tm 3+ optical properties are assigned to the change of the local phonon energy to which they are submitted by modifiers located in the vicinity of the doping sites. Some qualitative predictions of the maximum achievable quantum efficiency are possible using a simple microscopic model to calculate the non-radiative de-excitation rates.

Optical properties and local structure of rare-earth-doped amplifier for broadband telecommunication

Optical properties and local structure of rare-earth ions in glasses for broadband optical fiber amplifiers are reviewed. In a series of Er-doped glasses with broad 1.5 μm emission, heavy-metal oxide glasses are attracting great interest. Anomalous compositional dependences of optical properties of Er-doped antimony silicate glasses are shown. Structural implication of small compositional dependence observed for the Judd–Ofelt Ω 6 parameter and local phonon energy are discussed.

Electron-phonon interaction in the deep centre photoluminescence of Cds:Ag:Cl

The temperature dependence of the half-width of the red band in CdS:Ag:Cl upon chlorine concentration is studied. It is found that an increase in chlorine concentration ( N cl > 10 18cm −3) leads to an increase in the local phonon energy ( h ̷ ω e ∼N 1 2 cl) and a decrease in the number of phonons emitted. This change in the local lattice dynamics is interpreted as the result of screening by a complex neutral defect which ionizes between 80 and 150 K.

Deep centres related to defect complexes in wide-gap semiconductors

A model of binary complexes is proposed, taking into account Coulomb and short-range potentials of defects and their elastic interaction with the crystal lattice. The energy levels of such nearest-neighbour associates, the lineshape and polarization of luminescence and the photoionization cross section are calculated. The local phonon energy, distance between components and its charge state can be determined from experimental data on the basis of this theory. A number of complexes are described by the model in AIIBVI and AIIB2IIIC4VI compounds.

High-resolution optical spectroscopy of nickel ions in II-VI semiconductors: Isotope shifts at the 3T1(F) 3T1(P) and 3T1(F) 3A2(F) Ni2+ transitions in CdS and ZnS crystals.

Extremely narrow fine structures in the absorption, emission, and excitation spectra of the ${\mathrm{Ni}}^{2+}$ transitions $^{3}\mathrm{T}_{1}$(F)\ensuremath{\leftrightarrows}${\mathrm{}}^{3}$${\mathrm{T}}_{1}$(P) and $^{3}\mathrm{T}_{1}$(F)\ensuremath{\leftrightarrows}${\mathrm{}}^{3}$${\mathrm{A}}_{2}$(F) have been achieved by doping CdS and ZnS crystals with very small concentrations of nickel impurities. The incorporation of up to five different nickel isotopes leads to the appearance of clearly resolved isotope splittings of the lines. For the first time the dependence of such splittings on different transitions, different second neighbors, and on external parameters---such as the magnetic field, uniaxial stress, and temperature---has been measured. An adequate description of the assumed level structure and the isotope effect can be obtained by considering the influence of the local phonon energy and of the Jahn-Teller effect for both the initial and the final states of the transitions considered.

Photoionization cross sections in vanadium-doped silicon

Vanadium-doped silicon has been investigated using a photocapacitance method on n+-p and p+-n junctions. The photoionization cross-section spectra showed two levels at 0.47 eV below the conduction band and 0.485 eV above the valence band, and the complementary optical transition threshold energies were 0.665 and 0.77 eV, respectively. For the Ec−0.47-eV level, the electron photoionization cross section at the threshold energy depends significantly on temperature. This would be due to the photothermal ionization process via the excited states. For the Ev+0.485-eV level, the sum of the threshold energies in electron and hole emission processes was greater than the band gap. This indicates that phonon broadening due to lattice relaxation occurs in the photoionization spectrum. According to the configuration coordinate model, the local phonon energy and the lattice relaxation energy are approximately 350 K and 0.13 eV, respectively.

Charge carrier capture on semiconductor Coulomb centers with excitation of local center oscillations

Carrier lifetime in a semiconductor is calculated for capture in shallow Coulomb impurities with energy transfer to local center oscillations. The situation is considered in which the trap depth ɛI=Ry*/2 is less than the local phonon energy and the temperature is low: T ≪ ɛI. The electron-phonon interaction is accomplished by change in the dipole component of the impurity potential or the short distance component of its potential. Under certain conditions this mechanism can be more effective than the cascade mechanism.