Optical reflectance of blue bronze crystals near the Peierls transition

We report on the reflectance of blue bronze crystals in the visible frequency range as a function of temperature. The observed large oscillations in the reflectance at all wavelengths at temperatures near the Peierls transition are interpreted as interference effects stemming from a surface layer. It is found that the surface layer, which appears to be tied to fluctuations, can be as much as 1.6 microns thick.     

Absorption and emission characteristics of Er3NbO7 phosphor: A comparison with ErNbO4 phosphor and Er:LiNbO3 single crystal

Er₃NbO₇ phosphor was synthesized by sintering a mixture of Er₂O₃ and Nb₂O₅ powder in a molar ratio of 3:1 at 1600 °C over 55 h. Optical absorption and emission characteristics of Er³⁺ ions in the calcined Er₃NbO₇ powder were investigated and discussed compared with ErNbO₄ phosphor and a Z-cut congruent Er (2 mol%):LiNbO₃ single crystal. The absorption and emission studies show that, due to different crystal structures, the spectroscopic properties of these niobates have some differences in spectral shape, peak position, and relative intensity, especially at 1.5 μm. The most obvious spectral feature of the Er₃NbO₇ is that the spectral structure of band instead of peak is observed in its absorption or emission spectrum due to the existence of local structural disorder and multiple Er³⁺ sites. The Er₃NbO₄ shows stronger upconversion emission than the single crystal but weaker than the ErNbO₄. Experimental results show that energy transfer upconversion and/or excited state absorption play a dominant role in the upconversion emissions, and, at higher pump level (>200 mW), the thermal effect becomes significant and results in drop of the upconversion intensity. The 1.5 μm lifetimes of Er³⁺ ion in the Er₃NbO₇, ErNbO₄ phosphor, and in the Er:LiNbO₃ crystal are measured to be ∼5.3, 2.0, and 2.4 ms, respectively. In combination with the measured Raman spectra, the quantum efficiency, multiphonon nonradiative decay rate, and theoretical radiative lifetime of the 1.5 μm emission of the two powder materials are expected. The differences in upconversion intensity and measured 1.5 μm lifetime between the three materials are explained qualitatively.     

Energy transfer and photoextinction from Ln to Tb and Eu in aqueous chloride solutions

Energy transfer and photoextinction from Ln³⁺ to Tb³⁺ and Eu³⁺ in aqueous chloride solutions were studied by absorption, emission and excitation spectra. The energy gaps below the luminescent terms of Gd³⁺, Tb³⁺ and Eu³⁺ are spanned by 10, 5 and 4 quanta, respectively, of the highest energy vibration in aqueous solution, and luminescence is observed in each case. This is not so for other Ln³⁺ where nonradiative deactivation dominates over luminescence. It was verified that only Gd³⁺ could transfer energy to Tb³⁺ and Eu³⁺ ions in aqueous or acid solutions. The ions Pr³⁺, Nd³⁺, Sm³⁺, Dy³⁺, Ho³⁺ and Er³⁺ exhibited strong competitive absorption at certain wavelengths, resulting in the photoextinction of Tb³⁺ and Eu³⁺ emission.     

Application of the chemical vapor-etching in polycrystalline silicon solar cells

This paper reports a study of the application of chemical vapor-etching (CVE) for the rear surface and in the emitter of polycrystalline silicon (pc-Si) solar cells. The CVE technique consists of exposing pc-Si wafers to a mixture of HF/HNO₃. This technique is used to groove the rear surface of the pc-Si wafers for acid vapors rich in HNO₃ (HNO₃/HF > 1/4), in order to realize rear-buried metallic contacts (RBMC) and the formation of a porous silicon (PS) layer on the frontal surface of the cell for volume ratio of HNO₃/HF = 1/7. A significant increase of the spectral response in the long wavelength range was observed when a RBMC is formed. This increase was attributed to the reduction of the effective thickness of the base of the cells and grain boundary Al gettering. The achievement of a PS layer on the emitter of the pc-Si cells passivates the surface and reduces the reflectivity. The dark I-V characteristics of pc-Si cells with emitter-based PS show an important reduction of the reverse current together with an improvement of the rectifying behaviour. The I-V characteristic under AM1.5 illumination shows an enhancement of both short circuit current density and fill factor. The internal quantum efficiency is improved, particularly in the short wavelengths region.     

Raman spectra of undoped and uranium doped CaF2 single crystals

Raman scattering experiments for nominally pure and uranium doped CaF2 single crystals were presented.In all crystals, the Raman active T2g vibration mode of CaF2 was observed, whose frequency shift and fullwidth at half-maximum (FWHM) broadening correspond well with defects and impurities in CaF2 lattice.Additional Raman peaks develop in nominally pure CaF2 with high etch pits density and U6+:CaF2crystals. Part of additional Raman peaks in the experimental results, which are assumed due to vibration modes from F- interstitials and vacancies, are in well agreement with the theoretical predications by employing the Green-function formulation.     

Application of point-group bases to f−fd transitions of lanthanide and actinide ions doped in crystals

The application of point-group coupling coefficients to the modeling of f^N↔f^N−1d transitions is discussed. There are several possible coupling schemes for the states of the f^N−1d configuration. Formulae for matrix elements of the Hamiltonian for the f^N−1d configuration and the relative line strengths for f^N↔f^N−1d transitions are derived. As an example, the f-d absorption spectrum of the crystal Yb²⁺: SrCl₂ is calculated using coupling coefficients based on the software developed by Butler and co-workers, which makes use of the Racah-Wigner calculus. The advantages and disadvantages of various coupling schemes are demonstrated. These coupling schemes are related to the simple model for f-d transitions.     

The structure and optical properties of evaporated Samarium fluoride films

Samarium fluoride (SmF₃) films have been deposited on quartz, silicon and germanium substrates by vacuum evaporation method. The crystal structure of the films deposited on silicon substrate is examined by X-ray diffraction (XRD). The films deposited at 100 °C, 150 °C and 250 °C have the (1 1 1) preferred growth orientation, but the film deposited at 200 °C has (3 6 0) growth orientation. The surface morphology evolution of the films with different thickness is investigated with optical microscopy. It is shown that the microcrack density and orientation of thin film is different from that of thick film. The transmission spectrum of SmF₃ films is measured from 200 nm to 20 μm. It is found that this material has good transparency from deep violet to far infrared. The optical constants of SmF₃ films from 200 nm to 12 μm are calculated by fitting the transmission spectrum of the films using Lorentz oscillator model.     

Optical study of anthracene molecules doped in fluorene single crystals

Properties of two luminescence centers, which are observed in anthracene-doped fluorene crystal as concentration increases, have been investigated to clarify their origin. Excitation spectra and concentration dependence of absorption spectra were measured and two types of luminescence are attributed to anthracene molecule pair centers with weak and strong interaction. Dipole-dipole interactions between two anthracence molecules of various configurations are estimated and possible configuration for two types of luminescence is proposed.     

Investigation on optical transmission spectra of (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystals

The optical transmission spectra from 0.3 to 11 μm of relaxor ferroelectric single crystals (1−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT) were systematically studied at room temperature in this paper. The crystal is transparent between 0.45 and 5.5 μm and becomes completely absorbing around 0.4 μm in near UV region and 10 μm in infrared region. But the wavelength cutoff in near UV is much sharper than the long wavelength cutoff. As compared with other configurations, tetragonal single crystals possess the optimal transmission properties. The optical transmittance in the wavelength region from 0.45 to 5.5 μm is about 70%. The results show that tetragonal PMN-xPT single crystals are promising for a wide range of optical applications. Some discussions about the oxygen-octahedra structure that determines the basic energy level of the crystals are also presented on the optical properties of PMN-xPT single crystals.     

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Micro-lenses with well-defined optical parameters are generated on polymethylmethacrylate (PMMA) substrates doped with diphenyltriazene (DPT) by controlled use of a swelling effect generated under conditions of subablative excimer laser illumination. The surface profiles depend on the laser spot size and energy density. A sensitively balanced combination of matrix softening, substrate volume expansion due to photochemical nitrogen release, and surface tension is responsible for the final shape of the lenses. Complete arrays of identical lenses with 15 μm diameters and a focal length of 30 μm are produced by irradiation of (0.25 wt. %) DPT-PMMA with a single laser pulse at a wavelength of 308 nm and a fluence of 3 J/cm². It is shown experimentally and theoretically that appropriate volume expansion is possible without introducing internal light scattering due to the formation of small bubbles. Received: 7 April 1999 / Accepted: 8 April 1999 / Published online: 5 May 1999     

Radiation and thermally induced effects in YAlO3:Mn crystals

We have studied effects induced by γ-radiation and temperature in Mn-doped YAlO₃ crystals. The studies have been performed by means of optical spectroscopy that include measuring of optical absorption changes induced by γ-radiation and elevated temperature as well as thermally stimulated luminescence (TSL). It has been shown that under γ-irradiation of YAlO₃:Mn crystals, along with the ionization of Mn_Al⁴⁺ ions (Mn_Al⁴⁺→Mn_Al⁵⁺+e⁻), some additional coloration processes take place. This additional coloration is characterized by a wide intense band centered at 26,000- that is ascribed to color centers intrinsic to YAlO₃ lattice. This coloration is removed by the way of crystal warming at , while the coloration caused by Mn_Al⁵⁺ ions is removed at higher temperature . The observed TSL glow of irradiated crystals reveals three peaks near 360, 400 and that correspond to three types of traps. Parameters of the traps have been determined. The TSL emission corresponds to intra-center luminescence of Mn_Al⁴⁺ and Mn_Y²⁺ ions. The possible ionization and trapping mechanisms in YAlO₃:Mn crystals are discussed.     

Raman study on the pressure-induced disordered state of polystyrene

The pressure-induced disordered state was first reported for iPS, the polymer showing thermal shrinkage. The low-frequency Raman study showed clearly that pressure introduces irregularity into the polymer backbone of iPS. It has also been revealed that there exist two types of vibrational modes, which show different sensitivities on the regularity of the main chain. These modes indicate two distinctive features of dispersion in the ordered form. In the disordered state, the dispersive modes smear out and the dispersionless one survives to be observed. We conclude that pressure causes the ordered-disordered conversion like thermal shrinkage. This conversion will be a common feature of the polymers showing thermal shrinkage and would be able to take place at relatively low pressure.     

Effect of cation doping on low-temperature specific heat of LaMnO3 manganite

We have investigated the thermodynamic properties of perovskite manganite LaMnO₃, the parent compound of colossal magnetoresistive manganites, with the Ca²⁺ doping at the A-site. As strong electron-phonon interactions are present in these compounds, the lattice part of the specific heat deserves proper attention. We have described the temperature dependence of the lattice contribution to the specific heat at constant volume (C_v(lattice)) of La_1−xCa_xMnO₃ (x=0.125, 0.175, 0.25, 0.35, 0.50, 0.67, 0.75) as a function of temperature (1 K–20 K) by means of a rigid ion model (RIM).The trends of specific heat variations with temperature are almost similar at all the composition. The Debye temperatures obtained from the lattice contributions are found to be in somewhat closer agreement with the experimental data. The specific heat values revealed by using RIM are in closer agreement with the available experimental data, particularly at low temperatures for some concentrations (x) of La_1−xCa_xMnO₃. The theoretical results at higher temperatures can be improved by including the effects of the charge ordering, van der Waals attraction and anharmonicity in the framework of RIM.     

Determination of the Li/Nb ratio in lithium niobate by means of birefringence and Raman measurements

Two optical methods for the determination of the Li/Nb ratio in lithium niobate (LiNbO₃) are discussed. Data for the optical birefringence method are presented, they cover the entire composition range in which LiNbO₃ can be fabricated and a wide spectral region. The line-widths of lattice modes have been measured by means of Raman scattering as a function of the Li/Nb ratio for single crystals with polished and rough surfaces.     

Influence of Ba-doping on structural and luminescence properties of Sr2SiO4:Eu phosphors

Ba²⁺-doped Sr₂SiO₄:Eu²⁺ phosphors were synthesized with the high-temperature solid-state reaction technique. The experimental results, summarized in the successful production of a single-phase powder with fine microstructure of spherical particles with smooth surface, suggest that Ba²⁺-doping favors the stabilization of α′-Sr₂SiO₄. Rietveld refinement of X-ray diffractograms suggests that Ba²⁺ and Eu²⁺ ions occupy the sites of Sr²⁺ in the lattice of α′-Sr₂SiO₄. The produced phosphors show two intense emission bands at green and yellow regions of spectrum, originated from Eu²⁺ ions accommodated at two different sites in the host crystal, whose peaks depend on the concentrations of Ba²⁺ and Eu²⁺. Intense and broad excitation spectra extend from ultraviolet to the blue region.     

Infrared absorption and reflectivity of double perovskite Sr2FeWO6

In this work we present temperature dependent infrared reflectivity and absorption of Sr₂FeWO₆ between 700 and 17 K measured from 40 to 10000 cm⁻¹. The reflectivity spectra show well defined phonon bands peaking at 143, 227, 377 and 625 cm⁻¹ assigned to overlapping vibrational modes split from those active in cubic perovskite. We have also verified that this compound is structurally stable in the whole temperature range and that its optical gap at ∼750 cm⁻¹ (95 meV) undergoes only a minor high temperature decrease ascribed to new thermally accessible levels.     

Influence of the plasma parameters and nitrogen addition on the electrical characteristics of DLC films deposited by inductively coupled plasma deposition

This work has been based on studies of the plasma parameters influence and nitrogen addition over on the electrical characteristics of diamond-like carbon (DLC) films deposited by inductively coupled plasma deposition (ICP) system. For these studies, it was used a mixture of methane with different flows of nitrogen, two different pressure processes and three different coil powers. The nitrogenated DLC films, had presented a great variation in their electric and structural properties with the nitrogen variation in the plasma. With the nitrogen addition, an increase in its dielectric constant of 1.7-7.4 to concentration of the 40% of the nitrogen has occurred. For high nitrogen concentrations (80% of nitrogen), the dielectric constant decreases (of 7.4 for 5.0). The resistivity of the films decreases with the nitrogen concentration increase (1.2 × 10⁹ Ω cm). Attributing semiconductors characteristics to DLC films. With the increase of nitrogen concentration, the sp³ hybridization increases, too. These characteristics were excellent for innumerable applications in electronic devices.     

Growth and characterization of a pure and doped nonlinear optical l-histidine acetate single crystals

Single crystals of pure, Cu²⁺and Mg²⁺ doped l-histidine acetate (LHA) were grown successfully by slow evaporation technique. The X-ray diffraction (XRD) studies were carried out for the pure and doped grown crystals. Absorption of these grown crystals was analyzed using UV-vis-NIR studies, and it was found that these crystals possess minimum absorption from 200 nm to 1500 nm. The pure and doped crystals are characterized by Fourier transform Raman (FT-Raman), thermal and photoconductivity studies. Vickers microhardness tests were carried out for the pure and doped crystals and the mechanical strengths were found. The dielectric constant and the dielectric loss with frequency were also studied.     

Luminescence spectra of doped and undoped lead tungstate crystals

TL spectra of undoped lead tungstate crystals exhibit glow peaks at 30 v K and 85 v K centred at 440 v nm, plus a peak at 50 v K at 530 v nm in an annealed sample. Annealing adds a 170 v K peak. Trivalent dopants of La 3+ and Y 3+ reduce the green luminescence, and Nb 5+ introduces a peak near 100 v K centred at 530 v nm; Sb introduces features between 40 v K and 90 v K and 150 and 180 v K. The luminescence emissions around 50 v K may be attributed to complex intrinsic defect centres, including (WO 4 ) m 3 . Of the four dopants studied in the present research, Sb +5 has the highest luminescence intensity. CL spectra show interesting anomalies near 170 v K which are linked to a phase change of water/ice nanoparticles trapped at dislocations.     

Luminescence of undoped β-Ga2O3 single crystals excited by picosecond X-ray and sub-picosecond UV pulses

Undoped β-Ga₂O₃ single crystals were grown using the floating zone technique under a pressure of 2 atm oxygen. Luminescence spectra of the crystals were measured with steady-state X-ray (<15 keV) and UV (258 nm, 4.8 eV) sources. The X-ray excitation produced a spectrum with a peak at 390 nm (3.2 eV) whereas the UV excited spectrum had a peak at 430 nm (2.9 eV). The luminescence rise and decay were also examined by using picosecond X-ray and sub-picosecond UV pulses. It was found that the X-ray pulse excitation gave a slower rise and a faster decay of the luminescence compared with the UV pulse excitation. These results suggest that X-ray excitation generates high energy electrons, building up luminescent states until those electrons lose their kinetic energies, giving rise to the formation of local hot spots in the gallium oxide crystals.