Exciton and intracenter luminescence in Cd<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Te/Cd<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Te quantum-well structures

Exciton luminescence and intracenter luminescence (IL) of Mn²⁺ ions in Cd_0.6Mn_0.4Te/Cd_0.5Mg_0.5Te structures with quantum wells (QWs) 7, 13, and 26 monolayers thick were studied. It was established that in QWs the intensity of exciton luminescence with respect to that of IL is a few orders of magnitude higher than that in bulk crystals. The spectral position of manganese IL profile changes noticeably in going from a bulk crystal to a QW of the same composition. The nonexponential parts of the IL decay curves are determined by excitation migration and the cooperative upconversion process, whose contribution is high under strong excitation and efficient migration. At 77 K, the IL decay constant τ within the exponential region increases with decreasing QW thickness. The decay constant τ in a QW, unlike in a bulk Cd_0.5Mn_0.5Te crystal, decreases substantially under cooling from 77 to 4 K.     

A dynamic phase-conjugate mirror based on CdF<Subscript>2</Subscript> crystals with bistable centers

A four-wave phase-conjugate mirror on reflection gratings recorded in a CdF₂ crystal with bistable centers has been studied experimentally. The mirror reflectivity and speed have been measured, and the coefficient of third-order nonlinearity of this medium has been estimated. The quality of the wave reflected from the phase-conjugate mirror has been investigated in model experiments.     

Phosphorescence of CaF<Subscript>2</Subscript>-Dy crystals

The decay kinetics of phosphorescence excited by x-rays in a CaF₂-Dy crystal is investigated. It is found that localized charge carriers recombine through tunneling. The conclusion is drawn that, in the initial stages of x-ray irradiation, the accumulation of charge carriers occurs in pairs. This process is caused by the formation of excimer-like molecular states during excitation of the crystal.     

Spectroscopic characteristics of Pr<Superscript>3+</Superscript> in biaxial La<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> crystal

A Pr³⁺-doped La₂(WO₄)₃ crystal grown by the Czochralski method has been investigated as a promising laser material. The principal axes of the optical indicatrix and Pr³⁺ concentration of the crystal were determined. The polarized absorption, fluorescence spectra and fluorescence decay curves of the main emission multiplets of the crystal were measured at room temperature. The spectroscopic parameters were obtained by the modified Judd–Ofelt theory combined with the normalized method. The peak stimulated emission cross-sections of the major emission lines were estimated. The good spectroscopic properties imply that the Pr³⁺:La₂(WO₄)₃ crystal is a potential laser gain medium for solid-state laser and self-stimulated Raman laser applications. PACS 78.20.-e; 42.70.Hj     

New class of holographic materials based on CdF<Subscript>2</Subscript> semiconductor crystals with bistable centers. I. Role of covalence in the formation of bistable centers

Calculations of the electronic structure of In, Ga, and Al impurity centers in a CdF₂ crystal in the cluster approximation using the method of scattered waves are made. The first two impurities form in additively colored crystals bistable centers having a ground two-electron (deep) state and a metastable hydrogen-like (shallow) state. A change in the nature of the chemical bond on doping a crystal with these impurities is traced, which consists in a considerable increase of its covalent component. A change for deep In and Ga centers is shown to be caused by the reconstruction of centers in their ground state, and a conclusion about the character of reconstruction is made. This conclusion agrees with recent calculations made for the center structure using the pseudopotential method. Conditions of formation of bistable centers in CdF 2 and their structure in different charge states are discussed.     

PbWO<Subscript>4</Subscript> crystal for laser energy accumulation and transformation

The picosecond interband two-photon laser excitation of PbWO₄ crystals at a temperature of 10 K leads to electronic excitation energy accumulation, which results in almost 100% induced absorption in the 450–750 nm spectral range. The relaxation time of this induced absorption exceeds 100 min. The electronic excitation energy accumulated in the PbWO₄ crystal at T = 10 K excites the intrinsic luminescence with a decay time longer than 45 min. The decay kinetics and the spectra of the intrinsic luminescence of the PbWO₄ crystal at a temperature of 10 K were measured under two-photon and single-photon excitation. The luminescence under two-photon and single-photon excitation revealed a difference in the structure of the spectra.     

Spectral-kinetic characteristics of crystals and nanoceramics based on BaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript>: Ce

Optical characteristics of BaF₂ and BaF₂: Ce single crystals and nanoceramic materials prepared from these single crystals by uniaxial hot pressing have been studied. It has been shown that the introduction of Ce₃₊ ions into BaF₂ hardly affects the ultrafast (∼0.9 ns) luminescence component. The integrated luminescence intensity of the BaF₂: Ce nanoceramics is higher than that of the corresponding single crystal and considerably higher that the intensity of the undoped BaF₂ crystal. It has been demonstrated that the slow (several hundred nanoseconds) component of the luminescence decay of BaF₂: Ce is due to the energy transfer from excitons to Ce³⁺ ions.     

IR photoluminescence of Bi<Superscript>+</Superscript> impurity centers in the RbY<Subscript>2</Subscript>Cl<Subscript>7</Subscript> ternary chloride

Single-crystal samples of RbY₂Cl₇ ternary chloride containing an Bi⁺ isomorphic impurity are prepared for the first time. The optical absorption spectra in the visible region and photoluminescence spectra in the near-IR associated with the presence of Bi⁺ impurity center are studied. The kinetics of the photoluminescence decay is investigated. The influence of the symmetry of the crystal field on the arrangement of the energy levels and on the characteristics of the absorption and photoluminescence spectra of Bi⁺ is demonstrated.     

Optical phase conjugation in nonresonant and saturable absorptive/resonant systems

Recent results obtained at Alabama A&M University on optical phase conjugation in electrically unpoled BaTiO₃, BANANA and SBN crystals are presented. The decay curves of the transient gratings and phase conjugation in these crystals indicate the involvement of more than one exponential process, possible reasons for which are indicated. Self induced gratings have been studied in electrically unpoled SBN crystal using a He−Ne Laser. The oscillations in a ring passive phase conjugator (RPPC) and in a unidirectional ring resonator using an electrically poled BaTiO₃ crystal have, been found to be bistable. It is also found that bistable oscillations are possible in a geometry in which the crystal is used simultaneously in a RPPC and as a linear passive phase conjugate (LPCC) mirror. The auxiliary oscillations pumped by the RPPC are bistable with the oscillations in LPCC. A brief review of the recent literature on phase conjugation in saturable absorptive/resonant systems along with the experiments planned to be undertaken by the authors on such systems are presented.     

Growth and optical properties of Pr<Superscript>3+</Superscript>:KLu(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> laser crystal: a candidate for red emission laser

A Pr³⁺:KLu(WO₄)₂ crystal with dimension of 30 × 30 × 15 mm³ was grown in the K₂W₂O₇ flux. A slice was cut from the crystal, and the polarized absorption and fluorescence spectra were measured at room temperature. Based on the J-O theory, the oscillator intensity parameters Ω _t (t = 2, 4, 6), spontaneous emission probabilities and branch ratios were estimated and good results had been obtained. Furthermore, the crystal has a relatively large emission cross-section in the region of 615–630 nm with the highest value of 14.5 × 10^?20 cm², which indicates that the crystal is good for the application in red emission laser. The emission decay time for ¹D₂ and ³P₀ multiplets was discussed. By adapting the I-H model to fit the emission decay curves, the lifetime for ¹D₂ at 607 nm and ³P₀ at 615 nm are 19.72 μs and 8.95 μs, respectively. Then the corresponding fluorescence quantum efficiencies of the two multiplets reach 83.7 % and 87.9 %, respectively. All the studies illustrate that this crystal is potential in red emission laser application.     

Photoinduced submillimeter-wave amplitude diffraction grating in a semiconducting CdF<Subscript>2</Subscript>:Ga crystal

It is shown that exposure of an additively colored CdF₂:Ga crystal with bistable DX centers that is slowly cooled to 150 K to blue-green light through a slotted mask produces a submillimeter-wave diffraction grating, which persists for a long time at temperatures of 160–240 K. It is also shown that the diffraction grating induced in a sample is an amplitude grating. The absorption of submillimeter waves in illuminated regions of the sample is associated with the conductivity due to the transition of impurity centers to a metastable donor state. In the n-i-n-i-type conducting structure obtained, the conductivity of n-type regions at 225 K amounts to σ ′ ≈ 0.24 Ω^?1 cm^?1.     

Growth and spectral and luminescent properties of a new Cr<Superscript>4+</Superscript>:LiAlGeO<Subscript>4</Subscript> nonlinear crystal

Single crystals of a new promising laser material, Cr⁴⁺-doped germanium eucryptite Cr⁴⁺:LiAlGeO₄, are grown from the melt for the first time. The crystals grown exhibit nonlinear optical properties. Absorption and luminescence spectra of the crystals and their luminescence decay are studied at various temperatures. The luminescence lifetime at 1.2 μ m at 300 K is 10 μ s. Active centers in the crystal are identified, and crystal field parameters are estimated.     

Quadrupole spin-echo envelopes for Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> single crystals doped with the atoms of transition and rare-earth elements

The influence of a weak (below 50 Oe) constant magnetic field on a quadrupole spin-echo envelope was studied for an undoped single crystal Bi₄Ge₃O₁₂, in which local magnetic fields on the order of 20–30 G were previously found, as well as for single Bi₄Ge₃O₁₂ crystals doped with the atoms of transition and rare-earth elements. In all of these cases, the spin-echo envelopes were strongly influenced. A considerable increase in the nuclear spin-spin relaxation time T ₂ was observed for the undoped sample upon the switching of weak external magnetic fields. For the doped samples, the spin-echo envelope decay became much slower already in the zero field. The external magnetic fields exhibited a markedly weaker influence on the spin-echo envelope for the doped samples. The text was submitted by the authors in English.     

Spectral and kinetic characteristics of CdI<Subscript>2</Subscript> and CdI<Subscript>2</Subscript>:Pb scintillators

We have studied the effect of lead dopant on the optical absorption, photoluminescence, and x-ray luminescence spectra, and the scintillation characteristics of CdI₂ at room temperature. The crystals for the study were grown by the Stockbarger-Bridgman method. Activation of CdI₂ from the melt by the compound PbI₂ leads to the appearance in the absorption spectra in the near-edge region of an activator band at 395–405 nm, which is interpreted as an A band connected with electronic transitions from the ¹S₀ state to the ³P₁ levels in the Pb²⁺ ion. For x-ray excitation, CdI₂:Pb²⁺ crystals with optimal dopant concentration (∼1.0 mol%) are characterized by a light yield with maximum in the 570–580 nm region that is an order of magnitude higher than for CdI₂ crystals in the 490–500 nm band. For α excitation, the radioluminescence kinetics for cadmium iodide is characterized by a very short (∼0.3 nsec) rise time and fast decay of luminescence, with τ₁ ≈ 4 nsec and τ₂ = 10–76 nsec. Depending on the conditions under which the crystals were obtained, the fast component fraction is 95%–99%. The crystal is characterized by a similar scintillation pulse in the case of excitation by x-ray pulses. The radioluminescence pulse shape for CdI₂:Pb in the decay stage is predominantly exponential, with luminescence decay time constants τ₁ ≈ 10 nsec and τ₂ = 200–250 nsec. This system is characterized by low afterglow, at the level for the Bi₄G₃O₁₂ scintillator. We have demonstrated the feasibility of using CdI₂:Pb as a scintillator for detecting α particles. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 825–830, November–December, 2008.     

Electrical properties,equivalent circuit,and dielectric relaxation studies on [(C<Subscript>3</Subscript>H<Subscript>7</Subscript>)<Subscript>4</Subscript>N]<Subscript>2</Subscript>Cd<Subscript>2</Subscript>Cl<Subscript>6</Subscript> polycrystalline

The Ac electrical conductivity and the dielectric relaxation properties of the [(C₃H₇)₄N]₂Cd₂Cl₆ polycrystalline sample have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures, 209 Hz–5 MHz and 361–418 K, respectively. The purpose is to make a difference between the electrical and dielectric properties of the polycrystalline sample and single crystal. Besides, a detailed analysis of the impedance spectrum suggests that the electrical properties of the material are strongly temperature-dependent. Plots of (Z" versus Z') are well fitted to an equivalent circuit model consisting of a series combination of grains and grains boundary elements. Moreover, the temperature dependence of the electrical conductivity in the different phases follows the Arrhenius law and the frequency dependence of σ (ω) follows the Jonscher’s universal dynamic law. Furthermore, the modulus plots can be characterized by full width at half height or in terms of a nonexperiential decay function φ(t) = exp(t/t)^β. Finally, the imaginary part of the permittivity constant is analyzed with the Cole–Cole formalism.     

Polarized spectral properties of Er<Superscript>3+</Superscript> ions in NaGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

Polarized spectral properties of Er³⁺:NaGd(WO₄)₂ single crystal are reported. The crystal was grown by the Czochralski method. The Judd–Ofelt theory was applied to analyze the polarized absorption spectra and then calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios. Fluorescence decay curves of the ⁴ I _13/2, ⁴ I _11/2, and ⁴ S _3/2 multiplets for the Er³⁺ ions were measured. Stimulated emission cross-sections of the ⁴ I _13/2→⁴ I _15/2 transition obtained by the Fuchtbauer–Ladenberg formula and the reciprocity method were compared. Multi-phonon relaxation rates of the crystal were estimated. Green up-conversion fluorescence around 531 and 552 nm was observed, and the possible up-conversion mechanisms were proposed. PACS 78.20.-e; 42.70.Hj     

Crystal growth,spectroscopic and CW laser properties of Nd<Subscript>0.03</Subscript>Lu<Subscript>2.871</Subscript>Gd<Subscript>0.099</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> crystal

Nd_0.03Lu_2.871Gd_0.099Al₅O₁₂ (Nd:LuGdAG) crystal was grown by the Czochralski method. The absorption, fluorescence spectra and fluorescence lifetime of Nd:LuGdAG crystal at room temperature were investigated for the first time. We reported the continuous-wave (CW) Nd:LuGdAG laser operation under diode pumping. Output power of 1.43 W at 1064 nm was achieved with a slope efficiency of 34.1%. All the results show that Nd:LuGdAG crystal is a promising laser material.     

Luminescence characteristics of the Pr<Superscript>3+</Superscript> ion in SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> and SrB<Subscript>6</Subscript>O<Subscript>10</Subscript>

Spectral and kinetic characteristics of the luminescence and luminescence excitation spectra of polycrystalline SrB₄O₇:Pr (1%) and SrB₆O₁₀:Pr (1%) samples are studied at 150–170 K. The samples show an intense luminescence band in the vicinity of 405 nm (¹ S ₀→¹ I ₆ transitions of Pr³⁺) and shorter wavelength bands also assigned to transitions from the ¹ S ₀ level. The main luminescence decay constant is ～2×10^?7 s. The excitation spectra of the ¹ S ₀ luminescence in these crystals are significantly different. The SrB₄O₇:Pr crystal shows three well-resolved bands at 6.14, 6.55, and 6.91 eV in the region of the 4f ²→4f ¹5d transitions and a complex structure in the region of interband transitions (7.1–20 eV), whereas the SrB₆O₁₀:Pr crystal shows a weakly structured band at 6.31 eV and no excitation in the region of the interband transitions. The physical mechanisms that may be responsible for the observed features of the spectra are discussed.     

Magnetic properties of the Nd<Subscript>0.5</Subscript>Gd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The magnetic properties of the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal have been studied in principal crystallographic directions in magnetic fields to 90 kG in the temperature range 2–300 K; in addition, the heat capacity has been measured in the range 2–300 K. It has been found that, below the Néel temperature T _N = 32 K down to 2 K, the single crystal exhibits an easy-plane antiferromagnetic structure. A hysteresis has been detected during magnetization of the crystal in the easy plane in fields of 1.0–3.5 kG, and a singularity has been found in the temperature dependence of the magnetic susceptibility in the easy plane at a temperature of 11 K in fields B < 1 kG. It has been shown that the singularity is due to appearance of the hysteresis. The origin of the magnetic properties of the crystal near the hysteresis has been discussed.     

Phase separation in a manganite La<Subscript>0.95</Subscript>Ba<Subscript>0.05</Subscript>MnO<Subscript>3</Subscript> crystal

The structure and magnetic states of a crystal of lightly doped manganite La_0.95Ba_0.05MnO₃ were studied using thermal-neutron diffraction, magnetic measurements, and electrical resistance data in a wide temperature range. It is shown that, in terms of its magnetic properties, the orthorhombic crystal is characterized by two order parameters, namely, antiferromagnetic (T _N = 123.6 K) and ferromagnetic (T _C = 136.7 K). The results obtained differ in detail from known information on the manganites La_0.95Ca_0.05MnO₃ and La_0.94Sr_0.06MnO₃. Two models of the magnetic state of the La_0.95Ba_0.05MnO₃ crystal are discussed, one of which is a model of a canted antiferromagnetic spin system and another is associated with the phase separation of the manganite. Arguments are advanced in favor of the coexistence in this crystal of the antiferromagnetic phase (about 87%) with a Mn⁴⁺ ion concentration of 0.048 and the 1/16-type charge-ordered ferromagnetic phase (about 13%) with a Mn⁴⁺ ion concentration of 0.0625. The specific features of the manganite studied are due to self-organization of the La_0.95Ba_0.05MnO₃ crystal lattice caused by the relatively large barium ion size.