Diode-pumped colour-centre lasers tunable in the 1.5 µm range

Under pumping with a 990 nm, 1 W laser diode, continuous-wave (cw) tunable laser emission in the 1.5 µm wavelength range was obtained from two different colour centres: Tl⁰(1) in NaCl:Tl⁺ and (F ₂ ⁺ )_H in NaCl:OH^–. The results are compared to those recently obtained with a similar apparatus and Tl⁰(1) centres in KCl:Tl⁺. The highest output power (30 mW) and the broadest tuning range (1.48–1.68 µm) were achieved with (F ₂ ⁺ )_H centres.     

Optical properties and stable,broadly tunable cw laser operation of new FA-type centers in Tl+-doped alkali halides

A new group of complex color centers with F_A-type properties, involving simple center production and high thermal and optical stabilities, has been found in six Tl⁺-doped alkali halides: NaCl, KCl, RbCl, KBr, RbBr and RbI. In its first tested examples, KCl and KBr, broadly tunable cw laser operation over the 1.4 to 1.7 μm range has been obtained, with output powers in the 100 mW range. In contrast to already existing F⁺₂ and F⁺₂-like centers, operating in the same wavelength range, the new F_A(Tl⁺) lasers are optically stable and do not show any bleaching effects under laser operation.     

Electronic structure of thallous centres and Tl+ - Vk recombination in KCl crystal

Parameters of impurity Tl ion required for the calculation of different thallous centres in alkali halides are obtained for the semiempirical method of the intermediate neglect of differential overlap (INDO). The electronic structure of Tl⁺, Tl²⁺ centres in KCl is calculated. The potential energy curves for the recombination of nearest Tl⁺, V_$\text{k}$ centres against the breathing vibrational mode of the V_$\text{k}$ centre are calculated. This recombination (hole trapping) is found to be nonradiative tunneling with small activation energy rather than radiative one. A number of experimental data is also discussed in the light of the present calculations.     

Decay modes of the 3000 Å emission band in KCl: Tl+

The time behavior of the 3000 Å luminescence signals of KCl: Tl crystals is complex. Three different components can be resolved in the temperature range investigated (80–340K). These results that are typical for the A_x emission of phosphors like Kl:Tl⁺ and KCl:Pb²⁺, cannot be explained with the available theoretical models for KCl:Tl system.     

FA(Ga+,In+,Tl+) tunable laser activity and interaction of halogen atoms (F,Cl,Br,I,At) at the (001) surface of KCl crystal: ab initio calculations

An attempt has been made to examine F_A(Ga⁺,In⁺,Tl⁺) tunable laser activity and adsorptivity of halogen atoms (F,Cl,Br,I,At) at the (0 0 1) surface of KCl crystal using an embedded cluster model, CIS and density functional theory calculations with effective core potentials. The ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field at the host surface. The nearest neighbor ions to the defect site were then allowed to relax to equilibrium. Based on the calculated strength of electron–phonon coupling and Stokes-shifted optical transition bands, The F_A(Tl⁺) center was found to be the most laser active in agreement with the experimental observation that the optical emissions of F_A(In⁺) and F_A(Ga⁺) centers were strongly quenched. The disappearance of the anisotropy and np splitting observed in the absorption of F_A(Ga⁺,In⁺,Tl⁺) centers were monotonically increasing functions of the size of the impurity cation. The F_A(Ga⁺,In⁺,Tl⁺) defect formation energies followed the order F_A(Ga⁺)>F_A(In⁺)>F_A(Tl⁺). The Glasner–Tompkins empirical relationship between the principal optical absorption of F centers in solids and the fundamental absorption of the host crystal was generalized to include the positive ion species. As far as the adsorptivity of the halogen atoms is concerned, the F and F_A(In⁺,Tl⁺) centers were found to change the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred from the defect-free surface. The calculated adsorption energies were explainable in terms of the electron affinity, the effective nuclear charge and the electrostatic potentials at the surface. The spin pairing mechanism played the dominant role in the course of adsorbate–substrate interactions and the KCl defect-free surface can be made semiconducting by F or F_A(In⁺,Tl⁺) surface imperfections.     

Dislocation spectroscopy of crystals

A new method of studying the energy characteristics of dislocations is proposed, which is based on the investigation of the interaction of moving dislocations with purposefully introduced electronic and hole centers. A study has been made of KCl, NaCl, KBr, LiF, and KI alkali halide crystals containing electronic F and hole V _K and Me ⁺⁺ (Cu⁺⁺, Ag⁺⁺, Tl⁺⁺, In⁺⁺) centers. Investigation of the temperature dependence of the dislocation interaction with the F centers permitted determination of the position of the dislocation-induced electronic band (DEB) in the band diagram of the crystal. In KCl, the DEB is separated by ≈2.2 eV from the conduction-band minimum. It is shown that dislocations transport holes from the centers lying below the dislocation-induced hole band (DHB) (X ⁺, In⁺⁺, Tl⁺⁺, V _K) to those above the DHB (the Cu⁺ and Ag⁺ centers). Such a process is temperature independent. The DHB position in the crystal band diagram has been determined; in KCl it is separated by ≈1.6 eV from the valence-band top. The effective radii of the dislocation interaction with the electronic F and hole X ⁺, V _K, and Tl⁺⁺ centers have been found. Fiz. Tverd. Tela (St. Petersburg) 41, 2139–2146 (December 1999)     

Electron-lattice interaction in the absorption spectra of thallium doped alkali halides

The electron-lattice interaction of NaCl:Tl⁺, KCl:Tl⁺, KBr:Tl⁺, and KI:Tl⁺ is discussed using the moments of the absorption bands. The discussion is based on a theory ofToyozawa andInoue andHonma. Consistency of the data is found for the absorption measurements. An analysis of the band shift under applied stress shows for theA-band in KCl and KBr and for theA- andB-band in KI that the electronlattice coupling constants derived from these data assuming next neighbour interaction differ considerably from those derived from the second moments of the bands.     

Electrical conductivity and colour centres in Tl-doped KCl crystal

The electrical conductivity and optical absorption of potassium chloride crystals doped with different concentrations of thallium have been measured before and after X-irradiation. The optical absorption spectrum exhibits the characteristic Tl⁺ ion band at 247 nm. The extrinsic conductivity and the absorption coefficient at 247 nm increase with impurity addition upto a certain concentration. Further increase in impurities decreases them. Room temperature X-irradiation decreases the Tl⁺ ion band and produces the F band. F centre concentration is higher in lightly doped crystal compared to pure or heavily doped KCl. These results have been interpreted in terms of formation of interstitial potassium ions and positive ion vacancies in the Tl-doped KCl lattice due to large ionic radius of Tl⁺ ions. The impurity ions precipitate into TlCl phase when the doping is heavy.     

Polarized infrared reflectivity study in charge density wave conductor Tl0.3MoO3

Polarized infrared reflectivity measurements between 300 and 10 K have been carried out on charge density waves (CDW) conductor blue bronze Tl_0.3MoO₃. Three important features are observed: (i) A bump at 1155 cm⁻¹ in the reflectivity spectra of Tl_0.3MoO₃ at 300 K is a precursor of the Peierls gap due to optical excitations across a pseudogap, and this kind of Peierls-like gap opens gradually with decreasing temperature from 180 to 160 K. (ii) The three sharp modes as “triplet” of infrared reflectivity between 800 and 1000 cm⁻¹ of Tl_0.3MoO₃ along [1 0 2] axis show red shift compared to K_0.3MoO₃ and Rb_0.3MoO₃, which is assigned to the increase of the distance of Mo-O bond with the substitution of thallium ions. (iii) Two peaks at about 514 and 644 cm⁻¹ in the far-infrared reflectivity spectra of Tl_0.3MoO₃ along [1 0 2] direction are suggested to be the electronic transitions from the valence band to the midgap state and from occupied midgap state to the conduction band, respectively.     

Untersuchung von Neutronenreaktionen mit Emission von α-Teilchen an Na23 im Energiebereich von 12,6 bis 18,7 MeV

TheA band of Tl⁺ centers in mixed crystals of KCl—KBr, KBr—KI, and KCl—KI is split into several subbands at low temperatures. The degree of the splitting increases with the difference of the anion radii. The relative size of the subbands strongly depends on the mixture ratio and on the thermal history. Each of the subbands gives its own fluorescence spectrum. As in pure crystals it consists of two subbands. The shorter wavelength band dominates at low temperatures. We interpret the results by considering complexes of the general form TlX_nY_6?n (n=1, ..., 5). Their symmetry is lower than in a uniform surrounding (n=0; 6). As a consequence the degeneracies of the excited electronic states are lifted. In KCl with a sufficient low content of KI we get only TlICl₅ in addition to TlCl₆. From the equilibrium at different temperatures we find 0.07 eV for the energy of association of I^? to Tl⁺. TlICl₅ centers absorb in theA ₀ band, which is equivalent to theA band of the pure TlCl₆, and in anA ₁ band at longer wavelength. TheA _n1 fluorescence can be linearly polarized up to 60% in [100] direction. By applying uniaxial stress to the crystal in [100] direction at higher temperatures the TlICl₅ complexes can be partially oriented parallel to [100]. This gives a slight dichroism in theA ₁ band.     

Ionic conduction and effect of cation doping in Tl<Subscript>4</Subscript>HgI<Subscript>6</Subscript>

The ionic conduction properties of undoped and doped Tl₄HgI₆ were investigated using electrical conductivity, dielectrics, differential scanning calorimetry, and X-ray diffraction techniques. The heavy Tl⁺-ions diffusion was activated at high temperature, whereas low conductivity at the lower temperature suggested electronic contribution in undoped Tl₄HgI₆. The partial replacement of heavy Tl⁺ ion by suitable cations (Ag⁺ and Cu⁺) enhanced the conductivity by several orders of magnitude, whereas diminution in conductivity results with increasing dopants’ concentration in Tl₄HgI₆. These results can be interpreted in terms of a lattice contraction and vacancy–vacancy interaction (leading to the cluster formation), respectively. The dielectric values of undoped Tl₄HgI₆ system gradually increasing with temperature, followed by a sharp change, were observed around 385 K and can be explained on the basis of increasing number of space charge polarization and ions jump orientation effects. The activation energy of undoped and doped Tl₄HgI₆ systems were calculated, and it was found that ionic conductivity activation energy for 5 mol% of cation dopants is much lower than that of undoped one, and also 10 mol% doped Tl₄HgI₆ systems.     

Systematic study of the relationships between energy levels of Tl(1), Tl centers and the crystalline environment

In this paper, we present the first systematic study of the relationships between energy levels of Tl⁰(1) and Tl²⁺ centers in alkali halides and the crystalline environment by using dielectric theory of chemical bond for complex crystals. It is found that the coordination number of the central ion, the bond volume polarizability, and the fractional covalence of the chemical bond between the central ion and the nearest anion are the three determinative parameters for the absorption band energy. Four empirical formulas are proposed. The calculated results are in good agreement with the experimental values. The current model can serve as a prediction tool and can be applied to assign the absorption band energy of Tl⁰(1) and Tl²⁺ centers.     

Structural analysis of Fe3+ centers in Tl2MgF4 and Tl2ZnF4 fluorine compounds

Theoretical analysis of the Fe³⁺ centers observed in Tl₂MgF₄ fluorine crystals have been carried out by means of semi-empirical approaches. The most appropriate models are proposed by matching the theoretically predicted zero-field splitting parameters (ZFSPs) with the experimental ones obtained by EPR spectroscopy. Compression on the MF₆ octahedron of tetragonal (TE) center I is indicated in both Tl₂MgF₄ and Tl₂ZnF₄. A structural model for monoclinic (MO) center II and orthorhombic (OR) Fe³⁺ center IV in Tl₂ZnF₄ is proposed by assuming that the substitution of Fe³⁺ induces both ligand length and angular distortions.     

On the origin of the visible emission in KBr:Tl and KCl:Tl crystals

The intensity of the visible emission of KBr:Tl at 440 nm is shown to increase linearly with the absorption in the A₁ band at 4.64 eV (267 nm). The latter band increased with the square of the Tl⁺ ions concentration, which supports its assignment to thallium dimers. Similarities in behavior between the 440 nm emission of KBr:Tl and the 475 nm emission of KCl:Tl suggest that the latter is also related to dimers. This is supported by the superlinear dependence of this emission on the Tl⁺ ion concentration. Deviations ofthe superlinearity from the square dependence expected for dimers is explained by the overlap between the weak dimer absorption and the very strong A-band.The 440 nm emission of KBr:Tl was found to be excited only in the A₁ component of the dimer absorption and not in the stronger A₂ component. This indicates that the emission process may be compound.     

Radiation induced defects in KBr:Tl+ crystals

The defects produced in KBr:Tl⁺ crystals during x-irradiation at 77 K were studied using thermoluminescence (TL), thermally stimulated currents (TSC), and absorption and emission spectra. Three main glow peaks at 165, 193 and 258 K were observed both in the TL and in the TSC curves. A variety of irradiation induced absorption bands were observed in the UV, visible and infrared up to about 2 microns. The 165 K TL peak was found to emit only the 440 nm band assigned to thallium dimers, while the peaks at 193 and 258 K exhibited the UV bands at 310 and 365 nm as well as the 440 nm band.The defects produced during the irradiation were the V_k hole center, the Tl° and the Tl⁺₂ electron centers. Smaller concentrations of Tl²⁺ and (Tl⁺)⁺₂ centers were also produced.An analysis of the results including measurements on lightly and heavily doped crystals enabled to draw conclusions on the nature of the defects and on the recombination processes involved. A close correlation has been found between the temperatures at which changes in the various absorption bands take place and the temperatures of the TL peaks. The analysis enabled also a full classification of the absorption bands.     

67Cu(67Zn) Mössbauer emission spectroscopy of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8

The parameters of the crystal electric-field gradient at the copper sites of Tl₂Ba₂CuO₆ and Tl₂Ba₂CaCu₂O₈ have been determined by Mössbauer emission spectroscopy using the ⁶⁷Cu(⁶⁷Zn) isotope, and calculated in the point-charge approximation. The results obtained are analyzed using the available ⁶³Cu nuclear quadrupole resonance data. The experimental and calculated data can be brought in agreement if one assumes that the holes appearing after part of the thallium atoms lower their valency become localized primarily at the oxygen sites lying in the plane of the copper ions.     

Vibrational fluorescence of CN− centers in KCl by electronic excitation of neighbouring Tl+ centers

Abstract

Infrared emission from higher vibrational states of CN^? in KCl has been achieved by optically exciting nearby Tl⁺ ions with an excimer laser. The electronic vibrational energy transfer, already known from F_H(CN^?) in alkali halides, leads to population of vibrational states up to at least the tenth level.     

A study of the system Ag3−xTlxFe(CN)6, (x =0, 1, 2, 3) by Mössbauer spectroscopy

The ferricyanides Ag_3−xTl_xFe(CN)₆, (x = 0, 1, 2, 3) have been measured by Mössbauer spectroscopy. The results indicate a negative EFG for all samples except for x = 1, that is positive. The isomer shift for Tl₃Fe(CN)₆ is the more negative, indicating a strong contribution from the empty p-orbitals of Tl⁺ ions.     

Photoluminescence and EPR studies on gamma irradiated Ce doped potassium halide single crystals

Electron Paramagnetic Resonance(EPR), Photoluminescence(PL), Thermoluminescence (TL) and other optical studies of γ-irradiated KBr, KCl:Ce³⁺ single crystals. Cerium when doped into the KBr, KCl is found to enter the host lattice in its trivalent state and act as electron trap during γ-irradiation, thereby partially converting itself to Ce²⁺. The Photoluminescence(PL) spectra of both KCl and KBr crystals doped with Ce exhibit the strong blue emissions of Ce corresponding to ⁵d(²D)→²F_5/2 and ⁵d(²D)→²F_7/2 transitions. The defect centers formed in the Ce³⁺ doped KBr and KCl. Crystals are studied using the technique of EPR. A dominant TL glow peak at 374, 422 K and KCl:Ce³⁺ at 466, 475 K is observed in the crystal. EPR studies indicate the presence at two centers at room temperature. Spectral distribution under the thermoluminescence emission(TLE) and optically stimulated emission(OSL) support the idea that defect annihilation process to be due to thermal release of F electron in KBr, KCl:Ce³⁺ crystals. Both Ce³⁺ and Ce²⁺ emissions were observed in the thermoluminescence emission of the crystals.     

Mn2+ and Tl+ luminescence in β-aluminas

The compounds AGa₁₁O₁₇ (A = K, Rb, Cs) and AAl₁₁O₁₇ (A = Na, K, Rb) have the β-alumina structure. The Mn²⁺-activated gallates show efficient luminescence under 254 nm excitation with an emission peaking at 498 nm and a quantum efficiency up to 70%. The Tl⁺-activated aluminates show efficient luminescence under 254 nm excitation with an emission peaking at 380–390 nm and a quantum efficiency up to 70%. In AAl₁₁O₁₇: Tl⁺ energy is transferred from Tl⁺ to Mn²⁺, resulting in an emission peaking at 512 nm with a quantum efficiency up to 55%.