The effect of implanting nitrogen on the optical absorption and electron paramagnetic resonance spectra of silica

Silica samples (type III, Corning 7940) were implanted with N using multiple energies to produce a layer ∼600 nm thick in which the concentration of N was constant to within ±5%. The optical absorption spectra of the samples were measured from 1.8 to 6.5 eV. Electron paramagnetic resonance (EPR) measurements were made at ∼20.3 and 33 GHz for sample temperatures ranging from 77 K to 100 K for most measurements. The components identified in the EPR spectra, based on comparison with reported parameters, were due to E′ centers and peroxy radicals. By comparing the changes in the optical absorption at 5.85 eV with the changes in the concentrations of the various EPR components and with the reports in the literature, we conclude that there is an additional band at 5.7-5.9 eV other than the E′ center band. We conclude that the bands between 2 and 6.5 eV and the EPR spectral components produced by implantation of N are due to radiation damage processes; neither optical bands nor EPR components related to N are detected.     

Optical properties of Cr ion in lithium metasilicate Li2O · SiO2 transparent glass-ceramics

The optical properties of Cr³⁺ ions in lithium metasilicate (Li₂O · SiO₂) transparent glass-ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li₂O · SiO₂) crystal. The percent crystallinity and crystalline size were ranging 65-75% and 20-35 nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr³⁺ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr³⁺ ions substitute for three Li⁺ ions and occupy the distorted octahedral site between single [SiO₄]_n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq = 13 088 cm⁻¹, B = 453 cm⁻¹, Dq/B = 2.89 and C = 2036 cm⁻¹. The near-infrared luminescence centered at 1250 nm was not detected in the deep pink glass-ceramics unlike emerald green glass.     

Photosensitive defects in silica layers implanted with germanium ions

Ge-implanted silica layers have been investigated by high-power pulsed synchrotron-photoluminescence (PL), photoluminescence excitation spectroscopy (PLE), and optically stimulated electron emission (OSEE) with respect to association of excitation and absorption bands to respective emission bands and lifetimes of excited defect states. In this way singlet-singlet (4.35 eV) and triplet-singlet (3.18 eV) radiative transitions from excited states of oxygen-deficient centers (ODC) in Ge-doped silica glass are characterized by their absorption and emission bands as well as their lifetimes. The main channel for non-radiative relaxation of photoexcitation is electron emission by the OSEE effect. The OSEE shows non-radiative transitions of surface and bulk E′-centers found with concentrations of (2.7-3.4) × 10¹² cm⁻² and (2-4) × 10¹⁶ cm⁻³, respectively.     

Physical properties of ZnSe films prepared by two-source evaporation and a study of post doping effect

Two-source thermal evaporation method was utilized to prepare hard ZnSe thin films, the films were then immersed in silver nitrate solution for different time periods. The optical properties of the films were measured from the transmittance spectra. X-ray pattern of the films were also included. Final compositions of the resulting films were measured by EMPA method and comparisons between compositions by EMPA vs. optical absorbance were also reported. The dc electrical conductivity increased and a small shift in the optical band gap was also observed.     

The effect of Ti and O on the optical properties and microstructure of Ag nanocrystals formed in silica by sequential ion implantation

The implantation of Ti and O results in the modification of the silica with polarizable Ti ions. After annealing the Ti ions are incorporated with the O in the network to form a Ti–O–silicate layer. Subsequent implantation of Ag into this layer results in the formation of Ag metal nanocrystals with significantly modified optical properties compared to samples without the Ti ions. The incorporation of the Ti results in a shift in the wavelength of the surface plasmon resonance of the Ag nanocrystals and a change in the magnitude of the surface plasmon resonance absorption. The results are interpreted using effective medium theory.     

Optical properties of Ga3PO7 single crystals

Ga₃PO₇ crystal with size 18×15×12 mm³ and good optical quality has been grown by the top-seeded solution growth (TSSG) slow-cooling method from a Li₂O–3MoO₃ flux system. The Vickers microhardness and optical properties of the as-grown Ga₃PO₇ crystal have been carefully studied. The results show that the Ga₃PO₇ crystal belongs to the harder materials, and Meyer's index numbers for (1 1 0) and (0 0 1) planes were about 1.45 and 1.60, respectively. The Ga₃PO₇ crystal has a wide transmission range from 215 to 4300 nm. The smaller difference between the refractive indices values of n_o and n_e indicated that the Ga₃PO₇ crystal is an optically uniaxial negative crystal.     

Compositional dependence of the optical properties of amorphous Sb2Se3−xSx thin films

Thin films of Sb₂Se_3−xS_x solid solutions (x = 0, 1, 2, and 3) were deposited by thermal evaporation of presynthesized materials on glass substrates held at room temperature. The films compositions were confirmed by using energy dispersive analysis of X-rays (EDAX). X-ray diffraction studies revealed that all the as-deposited films as well as those annealed at T_a < 423 K have amorphous phase. The optical constants (n, k) and the thickness (t) of the films were determined from optical transmittance data, in the spectral range 500-2500 nm, using the Swanepoel method. The dispersion parameters were determined from the analysis of the refractive index. An analysis of the optical absorption spectra revealed an Urbach’s tail in the low absorption region, while in the high absorption region an indirect band gap characterizes the films with different compositions. It was found that the optical band gap energy increases quadratically as the S content increases.     

Spectroscopic properties of Er-Yb co-doped glass ceramics containing BaF2 nanocrystals

A Er³⁺ and Yb³⁺ co-doped transparent oxyfluoride glass ceramic containing BaF₂ nanocrystals has been prepared. The formation of BaF₂ nanocrystals in the glass ceramic was confirmed by X-ray diffraction. Intense upconversion luminescence in the Er³⁺ and Yb³⁺ co-doped glass ceramic could be observed. Stark splitting of the Er³⁺ upconversion luminescence peaks in the glass ceramic indicated that Er³⁺ and Yb³⁺ had been incorporated into the BaF₂ nanocrystals. Near infrared luminescence decay curves showed that the Er³⁺ and Yb³⁺ co-doped glass ceramic had higher luminescence efficiency than the precursor glass.     

Sol–gel synthesis and optical properties of size controlled Indium Arsenide nanocrystals embedded in silica glasses

III–V semiconductor Indium Arsenide (InAs) nanocrystals embedded in silica glasses was synthesized by combining the sol–gel process and heat treatment in H₂ gas. The size of InAs nanocrystals can be easily controlled via changing the In and As content in the starting materials and the heating temperature in a H₂ gas atmosphere. Absorption measurements indicate a blue shift in energy with a reduction on the In and As content in the SiO₂ gel glasses as a result of quantum confinement effects. A near-infrared photoluminescence with peak at 3.40 μm was observed at 6 K under 514.5 nm Ar⁺ laser excitation from InAs nanocrystals embedded in the silica gel glasses.     

Phase separation and crystallization in the system SiO2-Al2O3-P2O5-B2O3-Na2O glasses

The phase separation and crystallization behavior in the system (80 − X)SiO₂ · X(Al₂O₃ + P₂O₅) · 5B₂O₃ · 15Na₂O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al₂O₃-P₂O₅-SiO₂ and forms a continuous phase. Glasses containing a larger amount of Al₂O₃-P₂O₅ (X = 40 and 50) readily crystallize and precipitates tridymite type AlPO₄ crystals. It is estimated that the phase separation occurs forming continuous Al₂O₃-P₂O₅-SiO₂ phase at first, and then tridymite type AlPO₄ crystals precipitate and grow in this phase. Highly transparent glass-ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass-ceramics are 20-30 nm and about 50%, respectively.     

Phase-field simulations of Te-precipitate morphology and evolution kinetics in Te-rich CdTe crystals

Te precipitates are one of principal defects that form during cooling of melt-grown CdTe or CZT crystals when grown Te-rich. Many factors such as the kinetic properties of intrinsic point defects (vacancy, interstitial, and antisite defects); stresses associated with the lattice mismatch between precipitate and matrix; temperature gradients and extended defects (dislocations, twin and grain boundaries); non-stoichiometric composition; thermal treatment history all affect the formation and growth/dissolution of Te precipitates in CdTe. A good understanding of these effects on Te precipitate evolution kinetics is technically important in order to optimize material processing and obtain high-quality crystals. This research develops a phase-field model capable of investigating the evolution of coherent Te precipitates in a Te-rich CdTe crystal undergoing cooling from the melt. Cd vacancies and Te interstitials are assumed to be the dominant diffusing species in the system, which is in two-phase equilibrium (matrix CdTe and liquid Te inclusion) at high temperatures and three-phase equilibrium (matrix CdTe, Te precipitate, and void) at low temperatures. Using available thermodynamic and kinetic data from experimental phase diagrams and thermodynamic calculations, the effects of Te interstitial and Cd vacancy mobility, cooling rates and stresses on Te precipitate, and void evolution kinetics are investigated.     

Annihilation of E′ center defects in silica glass by hydrogen treatment

It was found, using electron paramagnetic resonance (EPR), that the signal of E′ centers in silica glass totally disappeared following a 1 h heat-treatment at 1000 °C under hydrogen atmosphere. However, by subsequent heat-treatment at the same temperature under a dry nitrogen atmosphere, some of the E′ centers re-appeared.     

Generation and annealing of defects in virgin fused silica (type III) upon ArF laser irradiation: Transmission measurements and kinetic model

The transmission of ArF laser pulses in virgin fused silica (type III) samples changes during N = 10⁶ pulses at an incoming fluence H_in = 5 mJ cm⁻² pulse⁻¹. The related absorption is determined by the pulse energy absorption coefficient α(N, H_in) using a modified Beer’s law, yielding initial values α_ini around 0.005 cm⁻¹, a maximum α_max ? 0.02 cm⁻¹ at N = 10³-10⁴ and stationary values 0.0045 cm⁻¹ ? α_end ? 0.0094 cm⁻¹ after N ≈ 6 × 10⁵ pulses. The development α(N, H_in = const.) is simulated by a rate equation model assuming a pulse number dependent E′ center density E′(N). E′(N) is determined by a dynamic equilibrium between E′ center generation and annealing. Generation occurs photolytically from the precursors ODC II and unstable SiH structures upon single photon absorption and from strained SiO bonds via two-photon excitation. Annealing in the dark periods between the laser pulses is dominated by the reaction of E′ with H₂ present in the SiO₂ network. The development α(N, H_in = const.) is observed for the very first sample irradiation only (virgin state). The values α_end are not accessible by simple spectrophotometer measurements.     

Optical,structural investigations and band-gap bowing parameter of GaInN alloys

We have performed a detailed investigation of the photoluminescence features taken at 2 K on a series of Ga_xIn_1−xN alloys grown by metal-organic vapour-phase epitaxy through the whole composition range. The evolution of the photoluminescence lineshape of GaInN alloys in the indium-rich region is dominated by doping effects rather than by band-gap tailing effects correlated to existence of random chemical crystal inhomogeneities. The lineshape of the photoluminescence indicates a residual electron concentration of about 10¹⁸–10¹⁹ cm⁻³ in the bulk part of the epilayers. The value we get for the bowing parameter is b=2.8 eV.     

Optical and structral properties of pure and Ce-doped nanocrystals of barium titanate

Nanocrystals barium titanate (nc-BT) and Ce-doped barium titanate were synthesized by an unusual hydrothermal process. X-ray diffraction and Raman spectroscopy were used to investigate their microstructures, as well as their lattice vibration and photoluminescence (PL) spectra. Particle sizes as small as 20 nm were measured by X-ray diffraction pattern via Scherer equation. The critical size relative to the cubic-tetragonal transition is about 48 nm in nc-BT below which ferroelectricity vanishes. The lattice parameter ratio c/a of is equal to 1.003, much smaller than 1.01 for that of the bulk crystal, leading to a weakening of ferroelectricity. Hydroxyl defects are observed in the Ce-doped nc-BT samples which show a vibrational band at 3300 em t. Good crystallization character and the lattice perfection were characterized by Raman spectra in the nanophase barium titanate. Strong PL spectra centered at 696 and 585 nm were observed in the pure and Ce-doped nc-BT, respectively. PL dependence on temperature and annealing time was examined. A molecule-like recombination and the charge transfer between Ce⁴⁺ and Ce ³⁺ are proposed to elucidate the luminescence process in the nc-BT and Cc-doped nc-BT system.     

Growth and characterization of glycine picrate—Remarkable second-harmonic generation in centrosymmetric crystal

Remarkable second-harmonic generation (SHG) efficiency has been observed in the glycine picrate (GP) though it crystallizes in centrosymmetric structure. Bulk single crystals of GP with a good size of ~20×10×3 mm³ have been successfully grown by the slow cooling method in aqueous medium. Powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) and FT-Raman studies have confirmed, respectively, the crystal structure and functional groups of the grown crystal. Crystalline perfection of single crystals has been evaluated by high-resolution X-ray diffractometry (HRXRD) using a multicrystal X-ray diffractometer and found that the grown crystals are nearly perfect. Nonlinear optical (NLO) behavior of glycine picrate crystals has been studied for the first time by Kurtz powder technique and its second-harmonic generation efficiency is found to be 2.34 times higher than that of KDP. Transparency of crystals in UV–vis–NIR region has also been studied. Dielectric measurements have been carried out using an impedance analyzer over a wide range of frequency (100 Hz–3 MHz) at room temperature. The slight decrease in dielectric constant has been observed as the frequency is increased and the dielectric loss is very low for the entire frequency range. The ac conductivity is almost constant up to 1 MHz and sudden increase has been observed above this frequency.     

Surface morphology,structural and optical properties of polar and non-polar ZnO thin films: A comparative study

The polar and non-polar ZnO thin films were fabricated on cubic MgO (1 1 1) and (0 0 1) substrates by plasma-assisted molecular beam epitaxy. Based on X-ray diffraction analysis, the ZnO thin films grown on MgO (1 1 1) and (1 0 0) substrates exhibit the polar c-plane and non-polar m-plane orientation, respectively. Comparing with the c-plane ZnO film, the non-polar m-plane ZnO film shows cross-hatched stripes-like morphology, lower surface roughness and slower growth rate. However, low-temperature photoluminescence measurement indicates the m-plane ZnO film has a stronger 3.31 eV emission, which is considered to be related to stacking faults. Meanwhile, stronger band tails absorbance of the m-plane ZnO film is observed in optical absorption spectrum.     

Cathodoluminescence of epitaxial GaN and ZnO thin films for scintillator applications

A comparative study of cathodoluminescence ultraviolet photon yields and decay times of large area GaN and zinc oxide (ZnO) layers grown for scintillator applications by metalorganic vapor phase epitaxy is presented. Silicon-doped GaN and non-intentionally-doped ZnO yield up to 1.4±0.2 photons/kVe⁻ and 1.3±0.2 photons/kVe^– at room-temperature, respectively. For GaN the decay times scatter between 0.4 and 0.9 ns, and for ZnO between 2.5 and 3.0 ns. The GaN and the ZnO absorption coefficients, α, internal efficiencies, η_i, and radiative constants, B, are determined. The characteristics of thin-film scintillators based on these materials are compared with commercially available granular scintillators.     

Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

The morphology and microstructure induced in high quality fused silica by UV (355 nm) laser pulses at high fluence (10-45 J/cm²) have been investigated using a suite of microscopic and spectroscopic tools. The laser beam has a near-Gaussian profile with a 1/e² diameter of ∼0.98 mm at the sample plane and a pulse length FWHM (full width at half maximum) of 7.5 ns. The damage craters consist of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser-generated shock waves, which also compact the crater wall, ∼10 μm thick and ∼20% higher in density. The size of the damage crater varies with laser fluence, number of pulses, and laser irradiation history. In the compaction layer, there is no detectable change in the Si/O stoichiometry to within ±1.6% and no crystalline nano-particles of Si were observed. Micro- (1-10 μm) and nano- (20-200 nm) cracks are found, however. A lower valence Si³⁺ species on the top 2-3 nm of the compaction layer is evident from the Si 2p XPS. The results are used to construct a physical model of the damage crater and to gain critical insight into laser damage process.     

Optical properties of and concentration quenching in Bi2O3-B2O3-Ga2O3 glasses

Er₂O₃-doped Bi₂O₃-B₂O₃-Ga₂O₃ glasses were prepared by the conventional melt-quenching method, and the Er³⁺:⁴I_13/2 → ⁴I_15/2 fluorescence properties are studied for different Er³⁺ concentrations. when the Er₂O₃ concentration increases from 0.03 to 3.0 mol%, the measured lifetime of Er³⁺:⁴I_13/2 level decrease from 2.24 to 0.9 m s, and from 0.25 to 0.20 m s for the Er³⁺:⁴I_11/2 level. The fast energy migration among Er³⁺ ions cause the reduction of lifetime of the ⁴I_13/2 level, whereas the change in the ⁴I_11/2 level is mainly due to a cooperative upconversion process (⁴I_11/2, ⁴I_11/2) → (⁴F_7/2, ⁴I_15/2). Based on the dipole-dipole interaction theory, the interaction parameter, C_Er,Er, for the migration rate of Er³⁺:⁴I_13/2 ↔ ⁴I_13/2 was calculated to be 32 × 10⁻⁴⁰ cm⁶ s⁻¹.