Continuous-wave tunable Cr2+:ZnS laser

We report the first continuous-wave tunable over ∼280 nm around 2.3 μm room-temperature operation of a chemical vapor transport-grown and diffusion-doped Cr²⁺:ZnS laser, pumped by a Co:MgF₂ laser at 1.67 μm and generating over 100 mW of output power at 16% slope efficiency. The self-consistent results of the laser and spectroscopic analysis demonstrate a large potential of this crystal as an active medium for diode-pumped tunable mid-infrared sources. Received: 7 January 2002 / Revised: 14 March 2002 / Published online: 2 May 2002     

Electrical transport and specific heat of a Cr+2.2 at% Al single crystal

Electrical resistivity, thermoelectric power and specific heat measurements are reported for a Cr+2.2 at% Al alloy single crystal, having an Al content close to the triple point on the magnetic phase diagram of the Cr–Al alloy system. A weak resistivity component of spin-density-wave origin, not previously observed in alloys near the triple point concentration, is isolated at low temperatures. Its presence is confirmed by the thermoelectric measurements. The specific heat measurements suggest a maximum in the density of states at the Fermi energy for alloy concentrations close to the triple point, a point that appears to be a particular type of critical point for the Cr–Al alloy system.     

Precession of μ+ in single crystal nickel

We have observed long-lived μ⁺ precession in single crystal nickel at 300 and 77K. Inhomogeneous local fields which cause rapid depolarization in polycrystalline samples are avoided. Muon precession in a magnetically saturated sample is reported.     

Afterglow of manganese centres in pulse-excited ZnS:Mn

The afterglow of self-activated and manganese centres in ZnS-Mn by excitation with a pulsed nitrogen laser is investigated. Assuming that a time-independent part of the excited “blue” centres returns to the ground state under energy transfer to the “yellow” centres, an equation describing the decay kinetics is derived. The comparison with the experimental data shows satisfactory agreement in the time interval up to 120 μs. A maximum in the afterglow curve arises within this limit.     

Optical and paramagnetic properties of titanium centres in ZnS

ZnS:Ti crystals grown by different methods are investigated at low temperatures (2 K K) by electron paramagnetic resonance (EPR) and optical spectroscopies, mainly with regard to their photoluminescence properties, including excitation and sensitisation spectra. The more familiar lattice-neutral Ti²⁺(d ² ) ion on a cubic site (AN) presents an isotropic EPR signal with g=1.928 observable up to 80 K. Signals of axial PN and AS sites are detected in the same temperature range. This Ti²⁺ ion exhibits the transition in emission, structured by no-phonon lines (NPL) of centres in various environments, with the AN site represented at 3613 cm^-1, and the ³ T ₁ (F) and ³ T ₁ (P) bands in excitation spectra. Substitutional Ti³⁺(d ¹ ) is identified by an anisotropic EPR spectrum at T = 3.5 K, indicating a quasistatic Jahn-Teller effect at the doubly degenerate ² E(D) ground state, while in the first excited vibronic state a quasidynamic JT effect in the strain-split vibronic ² E/² A ₂ manifold is found. A new luminescence band centred at 4500 cm^-1 with NPL structures near 5000 cm^-1 represents AN, PN and AS sites in the transition. Charge-transfer processes are described in a one-particle model, depicting the Ti³⁺/Ti²⁺ donor level at 12 900 cm^-1 below the conduction-band edge. Received: 3 October 1997 / Revised: 17 December 1998 / Accepted: 18 May 1998     

Structural heterogeneities in the cubic ZnS crystal doped with Fe2+ ions

The structural state of the cubic Zn_0.999Fe_0.001S single crystal grown by the chemical transport method has been investigated using thermal neutron diffraction at 300 K for the first time. It has been found that the diffraction patterns of the crystal contain diffuse scattering regions with wave vectors q = 0 and $q = (\bar 1/3, 1/3 , 0)2\pi /a$ . The experimental results have been discussed in the context of the available information on neutron diffraction in the Zn_0.999Fe_0.001Se compound. It has been shown that the effects of diffuse scattering are caused by local shear deformations of the metastable cubic lattice of the compound under investigation.     

Double-clad Cr4+:YAG crystal fiber amplifier

A novel double-clad Cr4+:YAG crystal fiber is demonstrated by use of a codrawing laser-heated pedestal growth method. Up to 10 dB of gross gain at a wavelength of 1.52 microm is achieved at a pump power of 0.83 W, which, to our knowledge, is the first Cr4+-doped fiber amplifier in the optical fiber communication band.     

Luminescent Ni+ centres and changes of the charge state of nickel ions in ZnS and ZnSe

In addition to Ni²⁺(3d ⁸) with its known internal transitions, Ni⁺(3d ⁹) is identified here by detecting its²E(D)?² T ₂(D) transition in ZnS and ZnSe both in absorption and emission. An analysis of vibronic satellites in these spectra indicates a moderate Jahn-Teller coupling in the² T ₂ ground state. Thermal annealing procedures which raise the Fermi level allow to increase the intensity ratio between the characteristic low-temperature optical bands of Ni⁺ and Ni²⁺. Ionisation and capture processes at these centres are studied by using the intensity of internal transitions as a probe for the concentration of the respective charge state of nickel. They are induced by additional optical irradiation of the samples in various spectral ranges. These photoionisation and radiative recombination processes manifest themselves in absorption, excitation and luminescence spectra as well as in the spectral response of photosensitive EPR signals. In addition to the threshold for the process [Ni²⁺]^x+hv→[Ni⁺]′+e _vb ^/ which had previously been determined, the experiments yield another threshold for the reaction [Ni⁺] +hv→[Ni²⁺]^x+e _cb ^/′ . A well-known but hitherto unsettled emission band of ZnS:Ni is attributed to the radiative recombination which is the reverse of this photoionisation. The corresponding transition in ZnSe:Ni is observed as well.     

New phenomena of a.c. electroluminescent lines in ZnS:Cu single crystal

Observations under high magnification show that electroluminescent lines in ZnS:Cu single crystal may be in the form of intermittent arrays of tiny bright spots or segments instead of continuous luminescent lines. Three types of internal structures of segmented electroluminescent lines — linear segment type, bamboo joint type, and pearl string type — were recognized.     

Photoionization spectra of silver dimers adsorbed on the surface of a ZnS single crystal

We use the photostimulated luminescence flash method to measure photoionization spectra of Ag₂ clusters adsorbed on the surface of a ZnS single crystal. Adsorbed dimers of silver are prepared by treatment of the surface of a ZnS single crystal in high vacuum with a beam of molecular Ag ₂ ⁺ ions followed by their neutralization. We show that the adsorbed dimers create two types of electron traps at energy levels 1.63 and 1.82 eV below the bottom of the conduction band. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 545–547, July–August, 2007.     

Photoionization spectra of silver atoms adsorbed on the surface of a ZnS single crystal

We have used photostimulated flash luminescence to study deep electronic states arising when silver ions Ag⁺ are deposited under high vacuum onto the surface of a ZnS single crystal, followed by creation of the conditions for neutralization of the silver ions. The flux density of the silver ion beam was 10⁷ cm⁻²·sec⁻¹. We have observed the appearance of two types of deep electronic states with photoionization energies 1.60 eV and 1.80 eV, arising after depositing the silver ions onto the surface of the ZnS single crystal. We have hypothesized that there may be two different preferred sites for adsorption of silver atoms on the zinc sulfide surface. The corresponding photoionization spectra of the adsorbed silver atoms have maxima at 775 nm and 690 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 335–338, May–June, 2006.     

Optical spectroscopy of Cr3+ ions in LiF single crystals

This paper reports on the luminescence spectrum, its excitation-wavelength dependence and decaytime of Cr³⁺:LiF crystals. In contrast to Al₂O₃ and MgO, where the Cr³⁺ luminescence at low temperature is due to the R-line and its vibronic sideband, luminescence from Cr³⁺:LiF is assigned to the broad-band ⁴ T ₂⁴ A ₂ transition. A periodic structure in the vibronic sidebands of this transition occurs with energy separations of ca. 60 meV. The linear polarisation of the vibronic sidebands shows that their radiative transition takes place from the lowest excited level of the ⁴ T ₂ state, i.e. ⁴ B ₁, into the ⁴ A ₂ ground state of Cr³⁺ ions in orthorhombic symmetry sites. The crystal-field parameters calculated from the experimental data, and luminescence decay-time of the two polarized configurations of the ⁴ B ₁(⁴ T ₂)⁴ A ₂ transition are also reported.     

Electric-field effect in a mixed-valence ion pair Cr3+−Cr2+ in a KZnF3 crystal

The influence of an external axial electric field on the absorption spectrum of a Cr³⁺−Cr²⁺ mixed-valence pair center coupled by the double-exchange mechanism in a KZnF₃ crystal is investigated. It is shown experimentally that the Cr³⁺−Cr²⁺ pair has an electric dipole moment. The migration of an electron is accompanied by local lattice strain. At the minima of the adiabatic potential, the intermediate fluorine atom is displaced from the lattice site. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 147–149 (25 July 1998)     

Mechanical properties of ZnSe : Cr2+ single crystals

The microhardness of single-crystal samples of ZnSe: Cr²⁺ with a chromium concentration in the range from 3.3 × 10¹⁷ to 4.0 × 10¹⁹ cm^?3 has been studied. The microhardness as a function of the load on the indenter on the faces (111), (1 $\bar 1$ 0), and (001) of the ZnSe: Cr²⁺ and ZnSe samples has been measured. It has been established that doping of zinc selenide with chromium leads to a decrease in the anisotropy of the mechanical properties and stabilization of the cubic sphalerite structure.     

Single crystal EPR spectrum of Cr3+ in lanthanum lutecium gallium garnet

The Cr³⁺ EPR spectra of YAG type crystals normally consist of four peaks whose positions are dependent on the axial zero-field splitting D with axis along a 111 direction and angle between that direction and the applied field H ₀. In LLGG only three principal peaks are observed with zero-field axis nearly along x, y or z. This anomaly is attributed to the relatively small size of Cr³⁺ compared to Lu³⁺ which it displaces in an octahedral site. D varies slightly for the different zero-field axes: 0.480 cm^–1 along x, 0.429 cm^–1 along y and 0.470 cm^–1 along z. The spectroscopic splitting factor g=1.978±0.001 is essentially isotropic and independent of axes within the experimental error.     

Microstructure of pair centers of Cr3+-Cr2+ ions in the KZnF3 crystal

Studies involving the piezospectroscopy method have shown that the symmetry of the pair centers of Cr³⁺-Cr²⁺ ions in the KZnF₃ crystal is tetragonal. In this paper we develop a microscopic model of a pair center. We use the temperature dependence of the integrated intensity of the absorption line to find the effective hopping integral for an e _g electron, t _σσ=205 ± 10 cm⁻¹, and the polaron reduction factor, equal to 0.11. By analyzing the selection rules for exchange-induced electric dipole transitions under double-exchange conditions we identify all the absorption lines of Cr³⁺-Cr²⁺ pairs. Zh. éksp. Teor. Fiz. 114, 1421–1429 (October 1998)