Optimization of passively <Emphasis Type="BoldItalic">Q</Emphasis> -switched Er:Yb:Cr:phosphate glass laser: theoretical analysis and experimental results

This article presents the results of theoretical analysis and experimental studies on passively Q-switched flash-lamp pumped Er: Glass laser employing Co²⁺:MgAl₂O₄ saturable absorber. Using two methodologies derived from basic rate equations, laser performance was optimized in terms of the output energy, pulse width and efficiency. The efficacy of these approaches is compared with each other and these results matched well when evaluated for a given system. Theoretical results have been compared with the actual experimental results. For optimum combination of output mirror reflectivity (R=70%) and initial transmission of saturable absorber (T₀=83%), single pulse energy of 10.77 mJ, 35.6 ns pulse width was obtained with a Q-switching efficiency of 12.7%. The experimental results match well with the theoretical analysis. To the best of our knowledge, this is the first attempt to analyse passively Q-switched Er:Glass laser by Co²⁺:MgAl₂O₄ from both these approaches. PACS 42.60.Gd, 42.55.Rz     

A comparative study on diode-pumped continuous wave Tm:Ho:YVO<Subscript>4</Subscript> and Tm:Ho:GdVO<Subscript>4</Subscript> lasers

In this paper, we presented experimental results concerning on the laser characteristics of two microchip lasers emitting in the 2 μm range, Tm:Ho:YVO₄ microchip laser and Tm:Ho:GdVO₄ microchip laser. At a heat sink temperature of 283 K, the maximum output power of Tm:Ho:YVO₄ laser and Tm:Ho:GdVO₄ laser is 47 and 34 mW under absorbed pump power of 912 mW, respectively. High efficiency can be achieved for both lasers at room temperature. Nevertheless, compared with Tm:Ho:GdVO₄ laser, Tm:Ho:YVO₄ laser can operate on single frequency with high power easily. At the heat sink temperature of 288 K, as much as 16.5 mW of 2052.3 nm single-longitudinal-mode (SLM) laser was achieved for Tm:Ho:YVO₄ laser. Under the same condition, only 8 mW of 2048.5 nm SLM laser was achieved for Tm:Ho:GdVO₄ laser.     

High-frequency tunable EPR spectroscopy of non-Kramers ions in AgGaSe<Subscript>2</Subscript>: Cr,AgGaS<Subscript>2</Subscript>: Cr,and CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript>: Cr crystals

Crystals of the chalcopyrite family, AgGaSe₂, AgGaS₂, and CdGa₂S₄, doped with chromium ions have been investigated using high-frequency broad-band EPR spectroscopy in the range 65–530 GHz at T = 4.2 K. It has been revealed that, in the AgGaSe₂ and AgGaS₂ crystals, the Cr²⁺ ions occupy positions with orthorhombic and tetragonal symmetry, whereas the previously investigated CdGaS₄ crystals contain only tetragonal centers. The observed spectra have been described in the framework of the spin-Hamiltonian formalism. Apart from the divalent chromium centers, the EPR lines attributed to non-Kramers ions are observed in the frequency range 300–450 GHz for all the crystals under investigation. The nature of these lines has been discussed.     

Performance of photorefractive crystals as derived from EPR-based defect studies: application to BaTiO<Subscript>3</Subscript>:Rh and Ba<Subscript>0.77</Subscript>Ca<Subscript>0.23</Subscript>TiO<Subscript>3</Subscript>:Rh

A method is introduced which allows us to predict the performance of a photorefractive material quantitatively using electron paramagnetic resonance (EPR)-based defect studies. This includes the determination of the defect densities and the parameters governing their light-induced charge changes. On this basis the effective trap densities and the photorefractive response times are calculated. All quantities can be determined without theoretical simplifications such as employed in previous approaches to the problem. The method is applied to BaTiO₃ and congruently melting Ba_0.77Ca_0.23TiO₃, both doped with rhodium. The iron defects, present as background contaminations, are fully taken into account. Their influence on the intensity saturating the space charges is calculated on this basis. The complete energy dependences of the absorption cross sections of all optically active Rh and Fe defects are given. PACS 42.70.Nq; 76.30.Mi; 78.40.-q     

Thermally-boosted pumping of Nd:LiYF<Subscript>4</Subscript> using Ti:Sapphire laser

We report for the first time (to our knowledge) a 1053 nm Nd:LiYF₄ (Nd:YLF) laser by directly pumping into the upper lasing level with a tunable Ti:sapphire laser. The results obtained for direct upper laser level pumping at 872 and 880 nm of Nd:YLF were compared with traditional 806 nm pump band excitation. Highly efficient 1053 nm continuous-wave (CW) laser emission under direct pumping at 880 nm in an 8 mm thick, 1.0 at % Nd:YLF crystal is obtained. The slope efficiency is improved from 39.1% for traditional pumping at 806 nm to 63.9% for direct pumping at 880 nm.     

Growth and Study of Scintillation Properties of Orthovanadate Crystals Ca:GdVO<Subscript>4</Subscript> and Ca:YVO<Subscript>4</Subscript>

New scintillation orthovanadate crystals Ca:GdVO₄ and Ca:YVO₄ featuring intrinsic luminescence are grown. The spectra of pulsed cathodoluminescence of new scintillators and their luminescence decay time are studied. Using γ-rays with an energy of 662 keV from the ¹³⁷Cs source, total absorption spectra (so-called photopeaks) for new orthovanadate crystals are measured. It is shown that the light yield of Ca:YVO₄ and Ca:GdVO₄ crystals is 28100 photon/MeV and 14000 photon/MeV, respectively.     

Accuracy in determination of the temperature and partial pressure of CO<Subscript>2</Subscript> in CO<Subscript>2</Subscript>:N<Subscript>2</Subscript>:H<Subscript>2</Subscript>O:NO<Subscript>2</Subscript> mixtures by multiple-frequency laser probing

We present the results of analysis of the errors introduced by hot-band transitions 11¹0-01¹1, 03¹0-01¹1, 12⁰0-12⁰1 of the CO₂ molecule and the absorption lines of the H₂O and NO₂ molecules in determination of the temperature and partial pressure of CO₂, included in the gas mixture CO₂: N₂:H₂O: NO₂ at atmospheric pressure, by multiple-frequency laser probing using a CO₂ laser tunable over the lines of the 00⁰1-[10⁰0,02⁰0]_I,II ground-state laser transitions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 810–815, November–December, 2007.     

Enhancement of nonvolatile blue photorefractive properties in LiNbO<Subscript>3</Subscript>:In:Fe:Cu crystals

The nonvolatile photorefractive characteristics of LiNbO₃:Fe:Cu and In-doped LiNbO₃:Fe:Cu crystals are investigated. The stronger nonvolatile blue photorefraction observed can be ascribed to its remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers, which is one or two orders of magnitude higher than those obtained in conventional two-color recordings under the same recording conditions. Furthermore, it is interesting that, compared with LiNbO₃:Fe:Cu, the recording properties, such as the saturation refractive index change, nonvolatile sensitivity and response time at 488 nm wavelength are enhanced in LiNbO₃:In:Fe:Cu crystals under the same recording conditions. The so-called damage-resistant dopants such as In³⁺ ions in red photorefraction are not damage resistant at 488 nm wavelength but they enhance the blue photorefraction. PACS  42.40.Ht; 42.40.Lx; 42.70.Ln     

A tuberculosis model: The case of ‘reasonable’ and ‘unreasonable’ infectives

Epidemics such as tuberculosis (TB), can be represented by a finite number of states and transition rules that govern the spread of the disease in each discrete time step. This paper uses a graph theoretic approach to investigate TB interactions in a community where infectives are categorized. A threshold value, , for ‘reasonable’ infectives is proposed. The results show that an epidemic will not ensue as long as the threshold is surpassed. Simulations presented show that unreasonable infectives can amplify the epidemic.     

Light scattering in single crystals of nonlinear-optical photorefractive LiNbO<Subscript>3</Subscript>:Cu and LiNbO<Subscript>3</Subscript>:Zn

Comparative studies are made of the photorefractive scattering of light in nonlinear single crystals of lithium niobate with congruent compositions (LiNbO₃) doped with "photorefractive" Cu [0.015 mass %] and "nonphotorefractive" Zn [0.5 mass %] cations. For the first time it is found that single crystals doped with "photorefractive" and "nonphotorefractive" cations have different indicatrices for photorefractive light scattering. The aperture angle for photorefractive scattering reaches its steady state value more rapidly with high laser powers than with low. However, at high powers laser induced heating of the crystal is greater, and this leads to a narrowing of the scattering indicatrix. It is also found that photorefractive scattering in these single crystals depends on the region of the boule from which a sample has been cut. This indicates that there is a nonuniform distribution over the boule of the imperfections with localized electrons which determine the magnitude of the photorefractive effect.     

Actively mode-locked Tm-Ho:LiYF<Subscript>4</Subscript> and Tm-Ho:BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> lasers

We report on the generation of mode-locking pulse trains with high average output powers from diode-pumped Tm-Ho:LiYF₄ and Tm-Ho:BaY₂F₈ lasers emitting at around 2 μm. The highest output power of 365 mW was obtained with the Tm-Ho:YLF₄ laser, whereas the shortest pulse duration of 120 ps and the widest tunability range of 59 nm was achieved with the Tm-Ho:BaY₂F₈ laser. PACS 42.55.Xi; 42.60.Fc; 42.72.Ai; 42.55.Rz; 42.70.Hj     

Luminescence of CaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>:Eu crystals

We have studied photoluminescence and thermoluminescence (PL and TL) in CaGa₂Se₄:Eu crystals in the temperature range 77–400 K. We have established that broadband photoluminescence with maximum at 571 nm is due to intracenter transitions 4f⁶ 5d–4f⁷ (⁸S_7/2) of the Eu²⁺ ions. From the temperature dependence of the intensity (log I–10³/T), we determined the activation energy (E _a = 0.04 eV) for thermal quenching of photoluminescence. From the thermoluminescence spectra, we determined the trap depths: 0.31, 0.44, 0.53, 0.59 eV. The lifetime of the excited state 4f6 5d of the Eu²⁺ ions in the CaGa₂Se₄ crystal found from the luminescence decay kinetics is 3.8 μsec. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 112–116, January–February, 2009.     

Modulated magnetic structure of weak rhombohedral ferromagnets α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga and FeBO<Subscript>3</Subscript>:Mg

The effect of diamagnetic impurities on the stability of the homogeneous magnetic state of rhombohedral antiferromagnets with weak ferromagnetism (α-Fe₂O₃:Ga and FeBO₃:Mg) is studied experimentally. It is shown that the application of an external magnetic field in the basal plane in the crystals under study in a certain temperature range induces a magnetic superstructure along the hard magnetization axis, which can be presented in the form of a ripplon phase with the azimuth of the local ferromagnetism vector oscillating about the direction of the field. The preferred orientation of the discovered modulated structures relative to crystallographic directions in the basal plane of α-Fe₂O₃:Ga and FeBO₃:Mg is studied, and the dependence of the spatial period of the superstructure on the applied magnetic field and temperature is analyzed. The magnetic-field-induced transition of the studied crystals from a homogeneous to an inhomogeneous magnetic state is described phenomenologically on the basis of the thermodynamic potential with gradient terms. In the discussion of physical reasons for magnetic order parameter modulation in weak ferromagnetic doped with diamagnetic ions, preference is given to the mechanism associated with the emergence of uniaxial magnetic centers with a random distribution of azimuths of easy axes in the basal plane of the crystal in the vicinity of impurities. A model describing the formation of a modulated magnetic state in α-Fe₂O₃:Ga and FeBO₃:Mg is proposed, according to which the competition between magnetoanisotropic and Zeeman interactions in the inhomogeneous magnetic phase of these crystals leads to periodic deviations in the direction of the local ferromagnetism vector from the direction of magnetization.     

Excitation cross-section and lifetime of the excited state of erbium ions in avalanching light-emitting Si : Er : O diodes

Electroluminescence (EL) characteristics of avalanching silicon diodes fabricated by Er and O co-implantation and subsequent annealing have been studied. Saturation of the Er-related EL intensity is achieved under the avalanche regime at current density an order of magnitude lower than that under the tunnel regime. Under avalanche regime at 300 K, the effective cross-section for excitation of Er³⁺ ions is equal to 2.3×10⁻¹⁶ cm² and the lifetime of the excited state is equal to 380 μs being four times higher than these values in tunneling diodes.     

Photoluminescence of CaGa<Subscript>2</Subscript>S<Subscript>4</Subscript>:Pr Polycrystals

The photoluminescence (PL) and PL excitation spectra of CaGa₂S₄ polycrystals doped with praseodymium are studied in the regions of the activator absorption and the fundamental absorption of the host. It is found that the PL excitation spectrum consists of two regions: broadband absorption in the range of 200-380 nm corresponding to the fundamental absorption of the host and the narrow-band absorption of the dopant in the range of 430–515 nm. The luminescence spectra are different for different excitation wave-lengths, which occurs because Pr³⁺ ions substitute divalent cations occupying different crystallographic positions in the host crystal lattice.     

Luminescent characteristics of ZrO<Subscript>2</Subscript>:Pb nanopowders

ZrO₂ nanopowders doped with Pb ions are prepared by the coprecipitation method. The average size of the powders, measured by X-ray diffraction, is 4–6 nm. The emission spectra show a narrow band originating from the transition from the ¹S₀ ground state to the ³P₁ excited state of the Pb ions. A quenching of the photoluminescence is observed when the concentration of Pb²⁺ is higher than 5%. The influence of residual chlorine ions on the luminescent properties of the nanopowders is also discussed. PACS 78.55.-m; 81.05.Je; 61.72.Ww     

Republication of: Einsteinian <Emphasis Type="Italic">ds</Emphasis><Superscript>2</Superscript> in Newtonian fields. IX: The analog of the logarithmic potential

This is an English translation of the second of two papers by Tullio Levi-Civita, first published in 1917 and 1919. The papers are remarkable as being among the earliest in which exact solutions of Einstein’s equations were derived. Of the two solutions presented, the first (republished as an accompanying Golden Oldie) is better known today as the Bertotti-Robinson solution, and the second one (republished as this Golden Oldie) is the gravitational field of an infinite cylinder. The papers have been selected by the Editors of General Relativity and Gravitation for republication in the Golden Oldies series of the journal. This republication is accompanied by an editorial note written by Malcolm MacCallum, and by a brief biography of the author, compiled from internet sources by Andrzej Krasiński.     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

Multiphonon relaxation of optical excitations in CsCdBr<Subscript>3</Subscript>:Pr<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Nd<Superscript>3+</Superscript> crystals

The technique of calculation of the n-phonon transition rates between electronic sublevels of impurity rare earth ions in dielectric crystals is developed in the case when n>2. The n-phonon transition probabilities are calculated according to the 1st and 2nd orders of perturbation theory. The Hamiltonian of the electron-phonon interaction is constructed in the framework of the exchange charge model and developed as series in relative displacements of the rare earth ion and ligands. The contribution of the lattice anharmonicity on the probabilities of n-phonon transitions is taken into account. On the basis of the developed technique, the nonradiative relaxation rates of ⁴ G _7/2 multiplet of Nd³⁺ ions in LiYF₄:Nd³⁺crystal and ³P₁ multiplet of Pr³⁺ ions in CsCdBr₃:Pr³⁺ crystal were computed. The results of our calculations show that the 2nd order terms in the expressions for the probabilities studied here are comparable with, and in some cases prevail over the 1st order terms. An account of lattice anharmonicity in case of LiYF₄:Nd³⁺ crystal substantially modifies the corresponding multiphonon relaxation rates. The calculated nonradiative relaxation rates for both crystals agree well with the experimental data.