Optical properties and energy transfer among Nd in Nd:Ca2Al2SiO7 crystals for diode pumped lasers

The energy levels of neodymium in the Nd³⁺:Ca₂Al₂SiO₇ (CAS) laser material with gehlenite structure are reported. As the Nd³⁺:Ca₂Al₂SiO₇ compound presents a broad absorption around 806 nm, it is a good candidate for diode pumped laser. The ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I11/2 emission have been recorded and the fluorescence branching ratios calculated from the Judd-Ofelt analysis are 0.41 and 0.47 respectively. The emission cross section at 1.06 μm (⁴F_3/2→⁴I11/2 transition) is 5 × 10^-20 cm². The decay profiles of the Nd³⁺ emission have been analyzed for several Nd³⁺ concentrations using the kinetic microparameters related to the cross relaxation ( and R₀≈6 Å) and the energy migration probabilities ( ). In the Nd:CAS laser material, the optimal concentration corresponding to the maximum of the fluorescence intensity is determined to be around 2.7 × 10²⁰ Nd³⁺ ions cm^-3. The Nd³⁺-Nd³⁺ interactions are not very strong in this material as the optical concentration value is two times higher than in the Nd:YAG laser material.     

Optical properties, fluorescence mechanisms and energy transfer in Tm, Ho and Tm -Ho doped near-infrared laser glasses, sensitized by Yb

The optical properties of the rare elements Tm³⁺, Ho³⁺ and Yb³⁺ were systematically investigated in various glasses. The Tm³⁺ doped aluminozircofluoride glass shows higher quantum efficiency, longer lifetime and stronger fluorescence intensity than Tm³⁺ doped YSGG crystal and other Tm³⁺ doped glasses for the ³H₄ → ³H₆ transition. Similar quantum efficiency, longer lifetime and stronger fluorescence intensity were also found in Ho³⁺ doped aluminozircofluoride glass for the ⁵I₇ → ⁵I₈ transition. The higher quantum efficiencies of Tm³⁺ and Ho³⁺ in aluminozircofluoride glass are due to the longer lifetime and the lower phonon energy. The fluorescence mechanisms and energy transfer in the Yb³⁺ -Tm³⁺ system, Yb³⁺ -Ho³⁺ system and Yb³⁺ - Tm³⁺ -Ho³⁺ system were studied. The very strong fluorescence intensities in the Yb³⁺ -Tm³⁺ system for Tm³⁺ and the Yb³⁺ -Tm³⁺ -Ho³⁺ system for Ho³⁺ which are 1.68 times that of Tm³⁺ doped YSGG crystal and 2.25 times that of Tm³⁺---Ho³⁺ codoped YSGG crystal are attributed to the efficient Yb³⁺ → Tm³⁺, Yb³⁺ → Ho³⁺ and Tm³⁺ → Ho³⁺ energy transfer processes. The fluorescence processes are described by cross relaxations of ²F_5/2 → ³H₅ → ³H₄ → ³H₆ → ²F_7/2 and²F_5/2 → ³H₅ (or ²F_5/2 → ⁵I₆ → ³H₅) → ³H₄ → ⁵I₇ → ⁵I₈ → ³H₆ → ²F_7/2.     

A method for determining the branching ratio for the superallowed decay: C(0, gs) → B(0, 1.74 MeV) + e + ν

We describe a new way of determining the strength of the superallowed branch of the β-decay of ¹⁰C, ¹⁰C(0⁺, gs) → ¹⁰B(0⁺, 1.74 MeV) + e⁺ + ν. Precise knowledge of the branching ratio is needed to compute the experimental ft-value and the weak vector coupling constant.     

Enhancement of Er I13/2 population in Y2O3 by energy transfer to Ce

We report measurements of the energy transfer between Er³⁺ and Ce³⁺ in Y₂O₃. The transition between the Er^{3+ 4}I_11/2 and ⁴I_13/2 excited states can be stimulated by energy transfer to Ce³⁺, augmenting the population in the ⁴I_13/2 state at the expense of that in the ⁴I_11/2 state. Experiments were performed on Y₂O₃ planar waveguides doped with 0.2 at.% erbium and 0–0.42 at.% cerium by ion implantation. From measurements of Er³⁺ decay rates as a function of cerium concentration we derive an energy transfer rate constant of 1.3×10⁻¹⁸ cm³/s. The efficiency of the energy transfer amounts to 0.47 at 0.42 at.% cerium. The energy transfer rate constant measured in Y₂O₃ is two times smaller for Er³⁺→Ce³⁺ than that for Er³⁺→Eu³⁺ in the same material.     

Growth and spectroscopy of Dy doped in ZnWO4 crystal

Single-crystal ZnWO₄:Dy³⁺ was grown by Czochralski technique. The XRD, absorption spectra as well as fluorescence spectrum are investigated and the Judd–Ofelt intensity parameters Ω₂, Ω₄, Ω₆ are obtained to be 7.76 × 10⁻²⁰ cm², 0.57 × 10⁻²⁰ cm², 0.31 × 10⁻²⁰ cm², respectively. Calculated radiative transition rate, branching ratios and radiative lifetime for different transition levels of ZnWO₄:Dy³⁺ crystals are presented. Fluorescence lifetime of ⁴F_9/2 level is 158 μs and quantum efficiency is 66%.The most intense fluorescence line at 575 nm correlative with transition ⁴F_9/2 → ⁶H_13/2 is potentially for application of yellow lasers.     

Low-temperature, direct synthesis of β-SiC by metal vapor vacuum arc ion source implantation

Crystalline β-SiC surface layers with strong (111) preferred orientation were synthesized by direct ion implantation into Si(111) substrates at a low temperature of 400°C using a metal vapor vacuum arc ion source. Both X-ray diffraction and Fourier transform infrared spectroscopy reveal an augment in the amount of β-SiC with increasing implantation doses at 400°C. Scanning electron microscopy shows the formation of an almost continuous SiC surface layer after implantation at 400°C with a dose of 7×10¹⁷/cm². The full width at half maximum of the X-ray rocking curve of β-SiC(111) was measured to be 1.4° for the sample implanted at a dose of 2×10¹⁷/cm² at 700°C, revealing a good alignment of β-SiC with the Si matrix.     

Energy transfer luminescence in (Eu,Nd) : tellurite glass

Here, we bring out an infrared transmitting new optical glass based on TeO₂ added with AlF₃ and LiF, containing dual rare earth ions (Eu³⁺,Nd³⁺) as the dopants with a purpose to examine their luminescence and also the decay times pertaining to a prominent transition of Eu³⁺ (⁵D₀ → ⁷F₂ at 615 nm) as a function of temperature both in the presence and absence of Nd³⁺ ions. The energy transfer rates (W_tr), critical distances (R₀) and transfer efficiencies (η_tr) have been evaluated based on the measured lifetime data of this glass.     

Structural properties of swift heavy ion beam irradiated Fe/Si bilayers

The Fe/Si multilayers were prepared by electron beam evaporation in a cryo-pumped vacuum deposition system. Ag⁺ and Au⁺ ions of 100 MeV at two different fluencies such as 1 × 10¹² ions/cm² and 1 × 10¹³ ions/cm² at a pressure of 10^− 7 torr were used to irradiate the Fe/Si samples. The irradiated samples were analyzed by High-Resolution XRD and it reveals that the irradiated films are having polycrystalline nature and it confirms the formation of the β-FeSi₂. The structural parameters such as crystallite size (D), strain (ε) and dislocation density (δ) have been evaluated from the XRD spectrum. The role of the substrates and the influence of swift heavy ions on the formation of β-FeSi₂ have been discussed in this paper.     

Charge carrier transport in Cu(COOH)2 organic molecular single crystal

The temperature dependences of the d.c. electrical conductivity (σ) and thermoelectric power (TEP) of low-dimensional Cu(COOH)₂ single crystal have been studied. The electrical conductivity is highly anisotropic (σ_‡/σ = 3.0 × 10²) and the crystal undergoes a Peierls transition at 208 K and two polymorphous phase transitions, namely → β and β → at 327 and 377 K respectively. The conductivity in this crystal is found to be mainly due to a non-adiabatic hopping mechanism. TEP measurements carried out at various temperatures show that the conductivity is due to the mobility of holes.     

Synthesis and the luminescent properties of europium-activated Ca2SnO4 phosphors

The synthesis and photoluminescent (PL) properties of calcium stannate crystals doped with europium grown by mechanically activated in a high energy vibro-mill have been investigated. The characteristics of Ca₂SnO₄:Eu³⁺ phosphors were found to depend on the amounts of europium ions. The XRD profiles revealed that the system, (Ca_1−xEu_x)₂SnO₄, could form stable solid solutions in the composition range of x = 0–7% after being calcined at 1200 °C. The calcined powders emit bright red luminescence centered at 618 nm due to ⁵D₀ → ⁷F₂ electric dipole transition. Both XRD data and the emission ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) reveal that the site symmetry of Eu³⁺ ions decreases with increasing doping concentration. The maximum PL intensity has been obtained for 7 mol% concentration of Eu³⁺ in Ca₂SnO₄.     

Temperature dependent luminescence in (Eu, Dy) doped tellurite glass

This short paper reports both the photoluminescence and the lifetime measurements of a prominent emission transition (⁵D₀→⁷F₂) of Eu³⁺ both in the presence and absence of the codopant rare earth ion (Dy³⁺) in an optical glass of the composition (79−x)TeO₂+6AlF₃+15LiF+xLn₂O₃ as a function of temperature down to 10 K.     

Investigation of stress behaviors and mechanism of void formation in sputtered TiSix films

We have investigated the stress behaviors and a mechanism of void formation in TiSi_x films during annealing. TiSi_x thin films were prepared by DC magnetron sputtering using a TiSi_2.1 target in the substrate temperature range of 200–500 °C. The as-deposited TiSi_x films at low substrate temperature (<300 °C) have an amorphous structure with low stress of 1×10⁸ dynes/cm². When the substrate temperature increases to 500 °C, the as-deposited TiSi_x film has a mixture of C49 and C54 TiSi₂ phase with stress of 8×10⁹ dynes/cm². No void was observed in the as-deposited TiSi_x film. Amorphous TiSi_x film transforms to C54 TiSi₂ phase with a random orientation of (311) and (040) after annealing at 750 °C. The C49 and C54 TiSi₂ mixture phase transforms to (040) preferred C54 TiSi₂ phase after annealing over 650 °C. By increasing substrate temperature, the transformation temperature for C54 TiSi₂ can be reduced, resulting in relieved stress of TiSi₂ film. The easy nucleation of the C54 phase was attributed to an avoidance of amorphous TiSi_x phase. We found that amorphous TiSi_x→C54 TiSi₂ transformation caused higher tensile stress of 2×10¹⁰ dynes/cm², resulting in more voids in the films, than C49→C54 transformation. It was observed that void formation was increased with thermal treatment. The high tensile stress caused by volume decreases in the silicide must be relieved to retard voids and cracks during C54 TiSi₂ formation.     

Infrared optical properties of Bi2Ti2O7 thin films by spectroscopic ellipsometry

Bi₂Ti₂O₇ thin films have been grown directly on n-type GaAs (1 0 0) by the chemical solution decomposition technique. X-ray diffraction analysis shows that the Bi₂Ti₂O₇ thin films are polycrystalline. The optical properties of the thin films are investigated using infrared spectroscopic ellipsometry (3.0–12.5 μm). By fitting the measured ellipsometric parameter (Ψ and Δ) data with a three-phase model (air/Bi₂Ti₂O₇/GaAs), and Lorentz–Drude dispersion relation, the optical constants and thickness of the thin films have been obtained simultaneously. The refractive index and extinction coefficient increase with increasing wavelength. The fitted plasma frequency ω_p is 1.64×10¹⁴ Hz, and the electron collision frequency γ is 1.05×10¹⁴ Hz, and it states that the electron average scattering time is 0.95×10⁻¹⁴ s. The absorption coefficient variation with respect to increasing wavelength has been obtained.     

Yb to Er energy transfer and rate-equations formalism in the eye safe laser material Yb:Er:Ca2Al2SiO7

Rate equations formalism is used to predict the population ratio of the Er^{3+ 4}I_13/2 levels involved in the 1.55 μm laser transition in the Yb:Er:CAS laser materials. An effective Yb → Er energy transfer, favourable to the Er³⁺ 1.55 μm laser emission, is demonstrated in this laser host. Indeed, the Yb → Er transfer and the Er → Yb back transfer rates are calculated to be 6 x 10⁻¹⁶ and 0.45 x 10⁻¹⁶ cm³ s⁻¹, respectively. Attempts of codoping the system with Nd³⁺, Eu³⁺ and Ce³⁺ have been realised in order to increase the population of the Er^{3+ 4}I_13/2 laser emitting level. Best results are obtained with Ce³⁺ ion since in the sample containing 6 x 10²⁰ Ce³⁺/cm³, the Er^{3+ 4}I_11/2 level lifetime is divided by a factor of 3 while the Er^{3+ 4}I_13/2 fluorescence lifetime remains unaffected. On the contrary, codoping with Nd³⁺ or Eu³⁺ ions simultaneously decreases the Er^{3+ 4}I_11/2 and ⁴I_13/2 kinetics parameters. The role of the other parameters such as Yb/Er concentrations ratios is also discussed.     

Characteristics of the radiation damage seen in the silicon microstrip detector of Fermilab experiment E771

The central region of the silicon microstrip detector used in Fermilab experiment E771 was subjected to a peak fluence of 9.5 × 10¹³ p/cm² induced by 800 GeV protons over a two-month period. Fourteen 300 μm thick planes manufactured by Micron Semiconductor were operated at bias voltages ranging from 84 to 109 V. Analysis of data from low intensity beam triggers taken near the end of the run shows that the mean pulse height from our amplifiers began to decline at a fluence of approximately 2 × 10¹³ p/cm² and fell to near zero by 6 × 10¹³ p/cm². We show that the use of fast amplifiers contributed to this early loss of signal.     

Optically active Er–Yb codoped Y2O3 films produced by pulsed laser deposition

Optically active Er³⁺:Yb³⁺ codoped Y₂O₃ films have been produced on c-cut sapphire substrates by pulsed laser deposition from ceramic Er:Yb:Y₂O₃ targets having different rare-earth concentrations. Stoichiometic films with very high rare-earth concentrations (up to 5.5 × 10²¹ at cm^− 3) have been achieved by using a low oxygen pressure (1 Pa) during deposition whereas higher pressures lead to films having excess of oxygen. The crystalline structure of such stoichiometric films was found to worsen the thicker the films are. Their luminescence at 1.53 μm and up-conversion effects have been studied by pumping the Yb³⁺ at 0.974 μm. The highest lifetime value (up to 4.6 ms) is achieved in films having Er concentrations of ≈ 3.5 × 10²⁰ at cm^− 3 and total rare-earth concentration ≈ 1.8 × 10²¹ at cm^− 3. All the stoichiometric films irrespective of their rare-earth concentration or crystalline quality have shown no significant up-conversion.     

Electrical properties of reactively sputtered carbon nitride films

Carbon nitride films with β-C₃N₄ crystals of 200 nm grain size were grown on Si (1 0 0) substrates using magnetron sputtering. These films were characterized by transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Carbon nitride films had low surface roughness. Maximum N/C ratio of 0.5 was achieved in the films. Chemical bonds of sp, sp² and sp³ coexisted in the films. The fraction of each bond was related to the deposition conditions. The resistivities of the films were measured, which ranged from 1×10³ to 1×10⁷ Ω cm. The measured resistivity results indicated that the carbon nitride films had semiconductive properties. The resistivity largely depended on the sp³/sp² ratio. Effects of N₂ fraction, target current and substrate bias were investigated. All these deposition parameters had influence on the chemical bonds of the films, and on the resistivities too. High sp³/sp² ratio resulted in high resistivity.     

Dielectric properties of AlN films prepared by laser-induced chemical vapour deposition

The dielectric properties and electrical conductivity of AlN films deposited by laser-induced chemical vapour deposition (LCVD) are studied for a range of growth conditions. The static dielectric constant is 8.0 ± 0.2 over the frequency range 10²−10⁷ Hz and breakdown electric fields better than 10⁶ V cm⁻¹ are found for all films grown at temperatures above 130°C. The resistivity of the films grown under optimum conditions (substrate temperature above 170°C, NH₃/TMA flow rate ratio greater than 300 and a deposition pressure of 1–2 Torr) is about 10¹⁴ Ω cm and two conduction mechanisms can be identified. At low fields, F < 5 × 10⁵ V cm⁻¹ and conductivity is ohmic with a temperature dependence showing a thermal activation energy of 50–100 meV, compatible with the presumed shallow donor-like states. At high fields, F > 1 × 10⁶ V cm⁻¹, a Poole-Frenkel (field-induced emission) process dominates, with electrons activated from traps at about 0.7–1.2 eV below the conduction band edge. A trap in this depth region is well-known in AlN. At fields between 4 and 7 × 10⁵ V cm⁻¹ both conduction paths contribute significantly. The degradation of properties under non-ideal growth conditions of low temperature or low precursor V/III ratio is described.     

The CPLEAR electromagnetic calorimeter

A large-acceptance lead/gas sampling electromagnetic calorimeter (ECAL) was constructed for the CPLEAR experiment to detect photons from decays of π⁰s with momentum p_π⁰ ≤ 800 MeV/c. The main purpose of the ECAL is to determine the decay vertex of neutral-kaon decays K⁰ → π⁰π⁰ → 4γ and K⁰ → π⁰π⁰π⁰ → 6γ. This requires a position-sensitive photon detector with high spatial granularity in r−, −, and z−coordinates. The ECAL - a barrel without end-caps located inside a magnetic field of 0.44 T - consists of 18 identical concentric layers. Each layer of 1/3 radiation length (X₀) contains a converter plate followed by small cross-section high-gain tubes of 2640 mm active length which are sandwiched by passive pick-up strip plates. The ECAL, with a total of 6X₀ has an energy resolution of and a position resolution of 4.5 mm for the shower foot. The shower topology allows separation of electrons from pions. The design, construction, read-out electronics, and performance of the detector are described.     

Phase formation at rapid thermal annealing of Al/Ti/Ni ohmic contacts on 4H-SiC

Ohmic contacts to the top p-type layers of 4H-SiC p⁺–n–n⁺ epitaxial structures having an acceptor concentration lower than 1×10¹⁹ cm⁻³ were fabricated by the rapid thermal anneal of multilayer Al/Ti/Pt/Ni metal composition. The rapid thermal anneal of multilayer A1/Ti/Pt/Ni metal composition led to the formation of duplex cermet composition containing Ni₂Si and TiC phases. The decomposition of the SiC under the contact was found to be down to a depth of about 100 nm. The contacts exhibited a contact resistivity R_c of 9×10⁻⁵ Ω cm⁻² at 21°C, decreasing to 3.1×10⁻⁵ Ω cm⁻² at 186°C. It was found that thermionic emission through the barrier having a height of 0.097 eV is the predominant current transport mechanism in the fabricated contacts.