Nd3+ ions in KYF4 can occupy different sites. The energy transfer processes that take place between them are responsible for the unusual behavior of Nd3+. From the spectroscopic measurements at different temperatures we found an anomalous spectroscopic behavior in the low temperature range (20K - 100K). Based on an energy transfer model, we proposed an explanation to this abnormal behavior. We derived the formula to describe the deexcitation process and make a data fitting on our dynamic decay measurements.

High-quality manganese-doped zinc sulfide (ZnS:Mn) thin films have been deposited on various substrates using pulsed laser deposition (PLD). Effects of back-filled Ar pressure and substrate temperature on the structural as well as optical properties of ZnS:Mn films were studied. Structural properties of these films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Photoluminescence (PL) and optical transmittance were used to characterize the optical properties of these films. Our results reveal that ZnS:Mn films were polycrystalline with a mixed phase structure consisting of both wurtzite and zinc-blende structure. The ratio of these two structures was strongly depended on the change of substrate temperature. Low substrate temperature facilitated the formation of zinc-blende structure while the wurtzite phase became dominant at high substrate temperature. ZnS:Mn films with preferred wurtzite structure were obtained at a substrate temperature as low as 450°C. An orange-yellow emission band was observed at ∼590 nm. As the substrate temperature increased, the peak of this PL band shifted to a shorter wavelength. Furthermore, shifts in the absorption edge and the energy gap due to the change in substrate temperature were also observed. The variation in these optical properties will be correlated to their structural change.

SrTiO3:Pr,Ga phosphor using Li2CO3 as a flux has been investigated as a red phosphor for the application to fluorescent displays operated at low voltage. In SrTiO3:Pr,Ga system, Pr3+can substitute for Sr2+ because the ionic radius of Pr3+almost coincides with that of Sr2+. Previous work, it was found by XRF analysis of SrTiO3:Pr,Ga single crystal that only a small fraction of Pr ions are incorporated in the SrTiO3 lattice. In the present study, the effect of Li addition into SrTiO3:Pr,Ga on the cathodoluminescence (CL) properties was examined at low acceleration voltage. Especially, thanks to the liquid phase of Li2CO3 during the sintering process, doped Li ions act as a lubricant for the efficient incorporation of Pr ions into SrTiO3:Pr,Ga lattice. Furthermore, it is found that the Li addition could enhance the generation of the characteristic emission of Pr-activated SrTiO3phosphors.

H+ ion implanted SOI structures formed by hydrogen ion slicing have been investigated by Raman spectroscopy and photoluminescence (PL). After implantation the wafers have been heat-treated by either furnace annealing (FA) or rapid thermal annealing (RTA). It has been found that implantation of 3 × 1017 H+/cm2 results in the formation of the amorphous Si layer (a-Si) inside silicon film on insulator. Structural transformations in a-Si depended on the annealing conditions. FA led to crystallization of a-Si and to the formation of monocrystalline silicon films. RTA results in the formation of the layers containing a high density of Si nanocrystals. A comparison of the Raman measurements with the PL data allows to conclude that PL bands obtained near 420 and 500 nm are not associated with the radiative recombination in Si nanocrystals.

ZnS quantum dots (QDs) chemically synthesized in PVP stabilizing medium have been coated with Zn(OH)2, SiO2and polystyrene (PS) shells as inorganic and organic passivation agents. to synthesize ZnS/Zn(OH)2, ZnS/SiO2and ZnS/PS QDs. PL properties of inorganically passivated ZnS/Zn(OH)2 and ZnS/SiO2 had reported band edge enhancement of 8-10 times, while organically passivated ZnS/PS QDs exhibit tremendous enhancement of band edge emission as much as 10-15 times,. Therefore inorganic and organic coating can passivate trap states of different energies on the surface of ZnS QDs.

Europium doped alumina Al2O3optical waveguides were prepared by pulsed laser deposition (PLD) using a KrF laser. The targets were obtained by sintering doped powders synthesized by a sol-gel method. Depending on the oxygen pressure used during the deposition, Eu3+ (for 0.1 mbar) or Eu2+ (for 10−5 mbar) are obtained in the films. Two kinds of Eu2+ ions are present, with a 4f-5d broad excitation band peaking at 330 nm and emission bands located at 490 nm or 585 nm respectively. For Eu3+ doped films, the usual 5D0 to the 7FJ multiplets emission spectra were observed. The emission lines are strongly inhomogeneously broadened. Low temperature site selective fluorescence measurements were achieved in order to correlate the different Eu3+ sites observed with the structure of the films (amorphous or γ crystallized).

Several independent groups have observed optical cooling by means of anti-Stokes luminescence in condensed media. The most promising materials are grouped into two categories: ion-doped glasses and organic dye solutions. It is this latter group that we focus our efforts on. Recent studies by our group show that irradiating a solution of rhodamine 101 in the long wavelength wing of the absorption spectrum results in the observation of optical cooling. To improve upon the initial observation of a few degree drop in temperature requires a better understanding of the conditions and phenomena leading to anti-Stokes luminescence in dye solutions. We develop a thermal lensing experiment to obtain fluorescence quantum yields of various dye solutions. The importance of concentration, choice of solvent, deuteration, and acidification are discussed.

We study optical processes relevant to optical amplification and lasing in CdSe nanocrystal quantum dots (NQD). NQDs are freestanding nanoparticles prepared using solution-based organometallic reactions originally developed for the Cd chalcogenides, CdS, CdSe and CdTe [J. Am. Chem. Soc. 115, 8706 (1993)]. We investigate NQDs with diameters ranging from 2 to 8 nm. Due to strong quantum confinement, they exhibit size-dependent spectral tunability over an energy range as wide as several hundred meV. We observe a strong effect of the matrix/solvent on optical gain properties of CdSe NQDs. In most of the commonly used solvents (such as hexane and toluene), gain is suppressed due to strong photoinduced absorption associated with carriers trapped at solvent-related interface states. In contrast, matrix-free close packed NQD films (NQD solids) exhibit large optical gain with a magnitude that is sufficiently high for the optical gain to successfully compete with multiparticle Auger recombination [Science 287, 10117 (2000)]. These films exhibit narrowband stimulated emission at both cryogenic and room temperature, and the emission color is tunable with dot size [Science 290, 314 (2000)]. Moreover, the NQD films can be incorporated into microcavities of different geometries (micro-spheres, wires, tubes) that produce lasing in whispering gallery modes. The facile preparation, chemical flexibility and wide-range spectral tunability due to strong quantum confinement are the key advantages that should motivate research into NQD applications in optical amplifiers and lasers.

We have used Near-field Scanning Optical Microscopy (NSOM) and Electron Probe Microanalysis (EPMA) to study the topographic and microscopic optical properties of indium gallium phosphide (In1−xGaxP) samples grown by Liquid Phase Epitaxy (LPE) on gallium arsenide substrates. Photoluminescence (PL) intensity images gathered using NSOM exhibit strong, highly localized variations in the optical properties of these samples that are seen to occur roughly in registry with the surface topography. Shifts in the PL peak position (by 27 meV) occur across highly mismatched samples with high In content, whereas no shifts were seen for In1−xGaxP films with a nearly lattice matched composition. Compositional fluctuations lead to these PL peak energy shifts, measured by NSOM with a resolution of 250 nm. These composition fluctuations arise from the known solid-solid miscibility gap in the In1−xGaxP system at temperatures used for the growth of these samples.

Quantum splitting phosphors (QSPs) are phosphors that could convert VUV radiation into more than one visible photon. These phosphors have the potential to improve the efficacy of current Hg fluorescent lamps and/or Xe lamps by reducing the Stokes shift energy loss after VUV excitation (λexc=185 nm for Hg lamps or 147 nm and 172 nm for Xe lamps provided the emission color of the phosphor matches the eye sensitivity. The current technology in QSPs and their potential limitations will be discussed in this paper. At GE-CRD, we have discovered and developed QSPs that meet the requirements for use in current Hg based fluorescent lamps. The steady state and time resolved optical properties of one of these phosphors, SrAl12O19:Pr3+,Mg2+, has been measured to estimate the maximum quantum efficiency and onset of concentration quenching in this phosphor. The maximum quantum efficiency for SrAl12O19:Pr3+,Mg2+ has been calculated to be ∼125-135% for 185 nm excitation with an upper bound on the Pr3+ doping level of ∼1%.

It is an interesting exercise in materials science to explore simple rules relating the electronic properties of ternary systems to those of their binary constituents. In the present work, we have investigated the electronic structures of the large band gap gallates MGa2O4(M=Mg, Ca, Ba and Zn) and the corresponding binary oxides MO and Ga2O3. Using first-principles band structure methods, we find that the metal atoms in MO control the width of the O 2p-like valence band and the size of the optical band gap. Covalent metal-oxygen bonding is much more pronounced in Ga2O3 and leads to characteristic structure in the valence band density of states. These basic features are retained in the ternary compounds where the covalent admixture to the chemical bond is largest between Ga and O, and the transitions across the band gap involve the Ga2O3 sublattice.

Aqueous colloidal solutions of well dispersed YVO4:Ln (Ln = Eu, Nd) nanoparticles are synthesized through precipitation reactions at room temperature. In the case of YVO4:Eu, a luminescence quantum yield of 15% is found, which is not as high as in the bulk due to the existence of residual crystalline defects and nonradiative relaxations from the hydroxylated surface. Appropriate hydrothermal annealing and deuteration of the surface allow to rise the yield up to 38%. Incorporation of the nanocrystals into a transparent silica matrix is achieved through preliminary coating of the particles with a functionnalized silicon alkoxide and further dispersion into a sol-gel precursor solution. Such sol-gel materials doped with YVO4:Nd nanocrystals are transparent and exhibit the typical emission at 1.06 μm of the Nd3+ ion.

Long-term processes of cathodoluminescence degradation of thin film phosphors Zn2SiO4:Ti and Zn2GeO4:Mn were investigated in a wide range of e-beam energies, current and power densities. The time dependencies describing the decreasing of emission intensity have been found. At higher current densities of e-beam irradiation, the specific behavior of long-term degradation processes was observed, which is characterized by rapid initial degradation and a slower long term decrease. The most probable mechanisms are proposed for long-term processes of degradation in the investigated phosphors.

Photoluminescence (PL) studies were carried out on a-Se and a few Ge20Se80−xBix and Ge20Se70−xBixTe10 bulk glassy semiconductors at 4.2 K with Ar+ laser as excitation source. While a-Se and samples with lesser at% of Bi show fine structured PL with a large Stokes shift, samples with higher at% of Bi did not show any detectable PL. The investigations show at least three radiative recombination transitions. Features extracted by deconvoluting the experimental spectra show that the discrete gap levels associated with the inherent coordination defects are involved in the PL transitions. Absence of PL in samples with higher Bi at% are explained on the basis of nonradiative transition mechanisms. Overall PL mechanism involving gap levels in chalcogenide glasses is illustrated with the help of a configurational coordinate diagram.

Photo luminescence (PL) signal from the aluminum nitride (AlN) films, excited by near UV (363.8 nm) laser has been measured at the room temperature. The AlN films are deposited by radio frequency (RF) sputtering of aluminum in argon-nitrogen-hydrogen gas mixture. Positions of the PL peaks maximums are influenced by the AlN preparation regimes. The analysis of the PL data is based on the results of the structural studies and electron spectrum investigations.

Nanocrystalline ZnS:Pb2+ is synthesized via a precipitation method. The luminescence is studied and the influence of the size of the nanocrystals on the luminescence properties is investigated. Nanocrystalline ZnS:Pb2+ shows a white emission under UV excitation. At least two luminescence centers are involved. One center is identified as a Pb2+ ion located on a regular Zn2+ site and gives a red emission under 480 nm excitation. The luminescence properties of this emission are characteristic for transitions on Pb2+ ions. The other centers are not as well defined and give a broad green emission band under 380 nm excitation and also show luminescence properties typically observed for Pb2+. The green emission probably originates from a charge-transfer like D-band emission of Pb2+ in ZnS close to a defect (e.g. an S2− vacancy or an O2− ion on an S2− site). A relation between the temperature quenching of the emissions and the band gap is observed and indicates that photoionization occurs.

Luminescence of nanocrystalline ZnSe:Mn2+ and ZnSe:Cu2+ prepared via an organic chemical synthesis method are described. The spectra show distinct ZnSe, Mn2+ and Cu2+ related emissions, all of which are excited via the host lattice. The Mn2+ emission wavelength depends on the concentration of Mn2+incorporated into the ZnSe lattice, which is attributed to the presence of Mn2+ pair-states at higher concentrations. The ZnSe:Cu2+ luminescence was studied as a function of the crystal-size. Temperature-dependent photoluminescence spectra and photoluminescence lifetime measurements are also presented and the results are compared to those of Mn2+ and Cu2+ in bulk ZnSe.

Temperature and power dependent photoluminescence measurements were carried out on the multi-layer structure of GeSi dots grown on Si(001) substrate by gas-source molecular beam epitaxy. The transfer of photon-induced carriers from wetting layers into the dots and the region near the dots was evidenced. Different power dependent behaviors of the photoluminescence peak position were observed for the dots and the wetting layer. Accordingly, type-II and type-I band alignments were proposed for the dots and the wetting layers, respectively. After annealing, the photoluminescence peaks from the dots and the wetting layers showed blueshift due to the atomic intermixing. For the samples annealed at temperature above 850°C for 5min, the band alignment of the dots changes from type-II to type-I.

Glasses containing nominally 1, 2, 5, and 10 wt% Sm2O3 in Na2O-Al2O3-SiO2 and Al2O3-SiO2 were prepared from metal alkoxide solution using the sol-gel process. After low temperature heat treatment in air, the glasses were heated up to 800 °C under a flowing H2 atmosphere to reduce Sm3+ into Sm2+. Samarium ions in the divalent and trivalent states were identified by fluorescence measurements. The fluorescence properties of Sm2+ions are discussed in relation to concentration of Sm2O3 and the glass matrix composition. Preliminary results of pressure studies on the luminescence spectra and lifetime of Sm2+ in the glasses are presented as well.