Krzysztof Wisniewski

Excited state absorption spectroscopy of ZBLAN:Ho3+ glass—experiment and simulation

The excited state absorption (ESA) spectra of ZBLAN glass activated by trivalent holmium ions have been measured in a wide spectral range (550–1750 nm) and simulated using such theoretical tools as the Judd–Ofelt formalism and McCumber theory of stimulated emission. We also propose a systematic approach for prediction of various types of up-conversion mechanisms in a given type of material. Experimental results on ESA up-conversion processes in ZBLAN:Ho3+ under red and infrared laser excitation, which confirm theoretical analysis, are presented. The optical linewidths were studied using high resolution laser spectroscopy at low temperatures and the existence of different crystallographic sites for Ho3+ ions was revealed.

Changes in optical properties of YAG:Ce single crystals due to codoping and ionizing radiation treatment

Absorption, luminescence, ESR and ESA spectra and TL glow curves of YAG:Ce crystals codoped with Mg2+ ions and subjected to various treatments have been measured. The variations induced by irradiations in gamma and proton beams, and by thermal annealings under oxidation and reduction conditions point to large variations of the cerium ion valency. The ESA spectra of Mg-free and Mg-codoped crystals are clearly different with the peak value in the YAG:Ce, Mg sample higher and shifted towards the shorter wavelengths. Although the TL glow curves reveal the existence of the same major traps in both materials, their role in the overall deexcitation process is strongly modified upon Mg-codoping.

Competition between two types of anti-Stokes emission in Ho3+-activated ZBLAN glass

A competition between two anti-Stokes emissions has been observed and interpreted in ZBLAN glass activated by Ho(3+) ions. The first anti-Stokes emission intensity was seen to increase with temperature, whereas another, upconverted emission, was seen to decrease under the same conditions. Both observed tendencies are believed to be caused by the same effect: the multiphonon anti-Stokes excitation of the state responsible for the first emission. Analysis of the kinetics and fits of the theoretical model to experimental data are presented.

New Eu2 + sites in KMgF3:Eu2 + crystal

The luminescence properties of KMgF(3):Eu(2 + ) are investigated at different pressures in the temperature range 25-292 K. Five new Eu(2 + ) luminescence (NEL) lines due to the [Formula: see text] transition are identified at 362.49 nm (L(1)), 362.53 nm (L(2)), 360.72 nm (L(3)), 360.15 nm (L(4)) and 359.59 nm (L(5)) together with the line at 359.32 nm (L(0)) which is well known in KMgF(3):Eu(2 + ). The emission lines under excitation at 325 nm show a strong dependence on temperature. At 25 K the emission spectrum consists of only two sharp lines, L(1) and L(2). Three additional lines (L(3), L(4) and L(5)) begin to appear with increasing temperature. With a further increase in temperature from 150 to 292 K all the lines disappear except for the single sharp line at 359.32 nm (L(0)). The zero-phonon transition of line L(0) is accompanied by vibronic sidebands. A pressure shift of five NELs is estimated to be about - 0.6 cm( - 1) kbar( - 1) similarly to the shift of line L(0), while the lifetimes of the NELs are about 0.7 ms which is shorter than that (5.2 ms) of L(0) at 80 K. The new luminescence lines are attributed to the Eu(2 + ) ions occupying the K( + ) sites with fluorine vacancy (F( - ) center) complexes.

High pressure luminescence studies of europium doped GaN

We reported on the high pressure luminescence spectra of polycrystalline Eu-doped GaN material synthesized in the reaction between alloys of gallium, bismuth and europium in ammonia atmosphere. The integrated luminescence intensity of the dominant Eu3+ ion transition (5D0→7F2) at 622 nm increased approximately one order of magnitude whereas its spectral position and line width did not change significantly between ambient and 6.8 GPa pressure, respectively. Moreover, material was characterized with photo- and cathodo-luminescence, and photoluminescence excitation spectra at different temperatures. It was found that the Eu3+ ions occupying substitutional Ga site created different centers which could be effectively excited with above band gap excitation and from excitons resonantly photoexcited at the I2 bound exciton energy. Furthermore, the less efficient Eu3+ ions excitation path existed through intrinsic impurities and defects generating shallow energy levels in the forbidden gap. It was proposed that reduction of the thermal quenching and consequent enhancement of Eu3+ ion emission intensity resulted from stronger localization of bound exciton on RESI trap induced by applied pressure.

Optical properties of Li2B4O7 glasses doped with rare-earth and transition-metal ions

Absorption and emission spectra of Cr, Eu and Dy ions in Li2B4O7 glasses melted in oxygen and hydrogen were measured for valency states and excited states analysis. It was stated that the presence of Cr6+ ion is limited by composition of the starting mixture and atmosphere of the melting and that this ions arises as Cr6+O4 complex.

Structure of the 2Eg-4A2 emission of the Cr3+: gahnite glass ceramics

The spectroscopic characteristics of chromium-doped gahnite glass ceramics have been evaluated. The material exhibits multi-site spectra, including a complicated emission spectrum and room temperature anti-Stokes emission. Careful analysis of the room and low temperature spectra, supported by luminescence decay measurements, proves the existence of a t least two high-field sites associated with single Cr3+ ions and pairs of Cr3+ ions. The R line of the highest field sites is accompanied by phonon side bands. Estimates of the associated phonon energies have also been made.

Spectroscopic evaluation of titanium-doped glass

We have evaluated a prototype glass doped with titanium from the point of view of its basic spectroscopic characteristics. Measurements of the luminescence lifetime at various wavelengths, luminescence spectra and time resolved luminescence spectra have been performed. Using the obtained characteristics we have attempted to interpret the de-excitation mechanism in the Ti:glass. For sufficiently high excitation energy the luminescence involves the following centers: Ti3+, emitting at 780 nm, Ti4+, emitting at 460 nm and an unidentified luminescence center emitting at approximately 560 nm.

Excited-state absorption in the Cr6+O2- center in Li2B4O7 glass

We present excited state absorption (ESA) measurements of the Cr:Li2B4O7 glass along with preliminary interpretation. The most distinct feature in the ESA characteristics is connected with the presence of chromium in its hexe-valence states. The Cr6+ ions appear to contribute in the deexcitation processes and their evidence can be seen in the ESA spectra under excitation wavelength at 308 nm. We interpret the ESA spectra detected with UV excitation in terms of transitions in the framework of Cr5+O- center which forms aster charge- transfer-type absorption in (CrO4)2- group. We assume the presence of the double-electron state of the 3d22p4 electronic configuration together with crystal field-split states of the 3d12p5 configuration, both allow to reproduce the obtained ESA spectra.