A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce³⁺ doped SrAlF₅ single crystals. Several Ce³⁺ centers emitting near 4 eV due to 5d-4f transitions of Ce³⁺ ions substituting for Sr²⁺ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.     

Temperature dependence of the PbWO4:F,Eu luminescence

Luminescence of the PbWO₄:F,Eu single crystal was investigated in the temperature region of 10–300 K. Besides two well known “blue” (2.80 eV) and “green” (2.45 eV) luminescence bands an additional band at 2.25 eV was observed in the whole temperature region and was assigned to the WO₃F defect centres. Europium dopant evinced as a narrow weak luminescence band at 2.02 eV only at 300 K. Temperature dependence of the excitation spectra was simulated assuming existence of the two defect absorption bands located near the fundamental absorption edge.     

Auger-free luminescence due to interatomic p–d transition and self-trapped exciton luminescence in Rb2ZnCl4 crystals

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb₂ZnCl₄. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p–d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

Characterization of luminescent chromophore obtained from silica spheres

Blue light emitting chromophores have been separated from silica spheres by soaking them into acetone for 120 days. The luminescent chromophores were not obtained from other solvents, including ether, methanol, ethanol, 2-propanol, chloroform and tetrahydrofuran. According to the Fourier transform infrared spectrum, the luminescent material is composed of C–OH, –CH₂, –CH₃, C=O, and Si–O–Si. UV–visible absorption peak of the chromophore is at 5.17 eV (240 nm). Field emission scanning electron microscope images show small cracks on the surface of aged spheres. The luminescence peak was at 2.81 eV (441 nm) for excitation energy between 3.88 and 3.35 eV and slightly shifted toward lower energy for excitation energy lower than 3.35 eV. The deconvoluted luminescent spectrum shows two emission bands at 3.08 and 2.74 eV, which are well-matched the oxygen deficient center model. Compared to the absorption peak (5.17 eV) and the emission peak (2.81 eV), large Stokes shift (2.36 eV) is observed.     

Behaviour of scintillators under XUV free electron laser radiation

Free electron lasers (FEL) are new generation accelerator-based short wavelength light sources providing high pulse intensity and femtosecond pulse duration, which enable investigation of interaction of elementary excitations in solids under extreme conditions. Using the FLASH facility of HASYLAB at DESY (Hamburg, Germany), we investigated the response of different materials with scintillating properties based on intrinsic emissions to the 25.6 and 13.8 nm FEL radiation by means of time-resolved luminescence spectroscopy. FLASH delivered single pulses of 25 fs duration having energy per pulse up to 30 μJ resulting in power densities of ～10¹² W/cm² on crystals. As a function of excitation density we observed the shortening of lifetime and non-exponential behaviour of emission decays in CaWO₄, while the emission spectra recorded are comparable to those obtained at conventional excitation sources.     

Intrinsic excitons in 12CaO·7Al2O3

Unusual crystal structure of 12CaO·7Al₂O₃ is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ～5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ₁ = 3.7 ns and τ₂ = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens.     

Relaxation of electronic excitations in beryllium oxide: A time-resolved vacuum-UV spectroscopy study

Beryllium oxide crystals are studied by time-resolved optical and luminescence vacuum-UV spectroscopy. The low-temperature luminescence spectra and the luminescence decay kinetics (2.5–10 eV, 1–500 ns) upon selective photoexcitation, and also the luminescence excitation and reflectivity spectra (8–35 eV), are analyzed for BeO crystals with the optic axis aligned parallel and perpendicular to the electric vector of exciting polarized synchrotron radiation. It is found that the radiative relaxation of electronic excitations proceeds through a large number of channels. The excited states of self-trapped excitons are characterized by different multiplicity depending on the excitation energy and the sample orientation.     

Growth,characterization and upconversion properties of erbium-doped potassium lithium tantalate niobate single crystals under 975 nm laser excitation

Potassium lithium tantalate niobate single crystals doped with erbium ions are grown by top-seeded solution growth method. The crystals are characterized by X-ray diffraction and differential thermal analysis. The refractive indices of the crystal are measured using ellipsometry method and fitted by Sellmeier equation. The as-grown crystals are tetragonal phase tungsten bronze-type structure with Curie temperature of 271.3 °C. Characteristic Er^3 + absorption bands are observed from 350 to 1100 nm in ultraviolet–visible-near infrared absorption spectra. These crystals emit brightly green and red upconversion fluorescence under 975 nm LD laser excitation, and the steady state upconversion spectra are obtained at room temperature. The red emission intensity increases as the erbium ions concentration increases in crystals. Processes of excited state absorption and energy transfer are responsible for upconversion luminescence. The emission intensities are quadratic dependences on pump power from pump power dependence analyses and deduction of transition rate equation model.     

Franck–Hertz effect in cathodo- and photoluminescence of wide-gap materials

A brief overview of previously obtained and novel data on the manifestations of an analogue of Franck–Hertz effect in photo- and cathodoluminescence of wide-gap inorganic materials is presented. On the example of NaCl:Tl⁺ and MgO:Cr³⁺ single crystals, the excitation processes of the luminescence of 6s² Tl⁺ ions and 3d³ Cr³⁺ ions by 5–15 keV electrons or 5–20 eV photons at 6–420 K have been studied. The rapid processes of the direct energy transfer to Tl⁺ by hot conduction electrons or to Cr³⁺ centers by hot electrons and/or hot valence holes have been separated from rapid excitonic and more inertial electron–hole processes.     

Spectroscopic analysis of Cr,Er:Gd3Ga5O12 crystals as candidates of potential xenon lamp pumped ~ 2.7 μm laser

A series of Cr,Er:Gd₃Ga₅O₁₂ crystals with high concentrations of Er^3 + were grown by Czochralski method. The absorption spectra, the up-conversion, near infrared (NIR) and mid-infrared (Mid-IR) luminescence spectra as well as the luminescence decay curves of Er: ⁴I_13/2 and ⁴I_11/2 levels were measured at room temperature. The spectroscopic properties of Cr,Er:Gd₃Ga₅O₁₂ crystals and Cr–Er energy transfer processes were investigated. The spectroscopy of the Er^3 +:⁴I_11/2 → ⁴I_13/2 transition was centralized to discuss, and the important optical parameters including luminescence lifetimes and the Cr–Er energy transfer efficiency are presented. Based on the comprehensive spectral analyses, 0.6 at.%Cr/50 at.%Er:GGG crystal is preferred as candidate of potential xenon lamp pumped ~ 2.7 μm laser in this work.     

Time-resolved luminescence of complex wide-gap oxide crystals under inner-shell excitation

In order to investigate the soft X-ray energy transformation in oxide detectors the optical spectra of several wide-gap oxide crystals were analyzed. The time-resolved luminescence (2.5–10.5 eV) and luminescence excitation spectra (50–200 and 500–630 eV) as well as decay kinetics of luminescence at 10 and 295 K were recorded using the synchrotron radiation from BW3 channel (HASYLAB, DESY). Several analogous features were discovered in the excitation spectra of both intrinsic self-trapped exciton luminescence and recombination luminescence for BeO, BeAl₂O₄, Be₂SiO₄ and AlPO₄ crystals under inner-shell excitation. Simultaneously, the excitation of Ce³⁺-luminescence in scintillating Be₂La₂O₅-Ce crystals significantly differs.     

The luminescence and energy transfer in Pr3+–Ce3+ co-doped Lu0.8Sc0.2BO3 crystals

Lu_0.8Sc_0.2BO₃ crystals doped with 1 at%Ce³⁺ and co-doped 0.1 at% and 0.5 at%Pr³⁺ were grown by the Czochralski method. The concentrations of Pr³⁺ and Ce³⁺ in crystals were measured by the ICP-AES method. Absorption spectra, VUV–UV spectra, fluorescence decay time and X-ray excitation luminescence spectra were investigated at room temperature. The excitation luminescence spectra of Ce³⁺ emission and decay curves from the lower excited state levels of the 4f¹5d¹ and 5d¹ electronic configurations of the Pr³⁺ and Ce³⁺ conspicuously indicated the non-radiative energy transfer from Pr³⁺ to Ce³⁺. The detailed pathways were shown in the energy level diagram of the respective Ce³⁺ and Pr³⁺ in Lu_0.8Sc_0.2BO₃ host. In addition, the scintillation efficiency data indicated that the energy transfer effect is directly associated with the Pr³⁺ concentration.     

Time-resolved spectroscopy of natural and synthetic BeO crystals

The results of comparative luminescence investigation of natural and synthetic BeO crystals are presented. Time-resolved luminescence (2.5–8 eV) and luminescence excitation spectra, and the kinetics of glow decay were measured using ultraviolet-vacuum-ultraviolet (VUV) synchrotron radiation (5–22 eV) or x-radiation (50–620 eV or 3–62 keV) ranges. X-ray and thermostimulated luminescence of natural BeO crystals were compared to the glow of additively colored synthetic crystals. The characteristic luminescence of F and F ⁺ centers was found in natural crystals. In synthetic crystals similar luminescence is observed only after additive or radiation coloration by virtue of the creation of F ⁻ and F ⁺ centers on anion vacancies. The defects found in the crystal lattice of a natural BeO crystal testify to the degree of mineral metamictization of the given deposit.     

Synthesis,measurements, and theoretical analysis of carbazole derivatives with high-triplet-energy

In order to obtain the blue light-emitting organic materials with high triplet state energy, two 3,5-diphenyl-4H-1,2,4-triazole (Tz) containing carbazole (Cz) derivatives of 9-(4-(3,5-diphenyl-4H-1,2,4-triazol-4-yl)phenyl)-9H-carbazole (TzCz1) and 3,6-di-tert-butyl-9-(4-(3,5-diphenyl-4H-1,2,4-triazol-4-yl)phenyl)-9H-carbazole (TzCz2) are synthesized using Cz acting as the starting material, as well as characterized by the ¹H NMR spectra, ultraviolet–visible (UV–vis) absorption spectra, and the IR absorption spectra. The luminescence quantum yields (LQYs) of TzCz1 and TzCz2 are measured in CH₂Cl₂ solution to be 32.1% and 47.5%, respectively. The electrochemical analysis and the photophysical measurements suggest that the triplet energy levels and the energy gaps of the highest-occupied orbital and the lowest-unoccupied orbital are 2.83 eV and 3.59 eV for TzCz1, and 2.80 eV and 3.43 eV for TzCz2. At last, the theoretical analyses of their ground state geometries and the simulated UV–vis absorption spectra are carried out at B3LYP1/6-31G? level. The studies mentioned above indicate that both TzCz1 and TzCz2 are suitable for the host materials of blue light-emitting diodes.     

Electron spin resonance and luminescence in Ga2−xEuxS3 single crystals

In present work electron spin resonance (ESR) and luminescence have been studied in Ga_2−xEu_xS₃ single crystals. The ESR and photoluminescence (PL) spectra of Ga_2−xEu_xS₃ were observed and the intensity of these spectra has increased linearly with the Eu concentrations in the samples. At the low temperature of 3.8 K in Ga_2−xEu_xS₃ single crystals, when the magnetic field has been applied as the perpendicular to the direction of [110], the hyperfine structure of ESR spectra has been observed. The exchange interaction of Eu²⁺ atoms in Ga_2−xEu_xS₃ single crystals have been determined as, g=4.2. The PL spectrum of pure Ga₂S₃ single crystals was recorded at 4.2 K and consisted of a narrow high-energy band at 2.53–2.38 eV and a low-energy band at 2.14–1.59 eV. Ga_2−xEu_xS₃ resulted in complete quenching of the impurity luminescence bands gave rise to a green europium band (2.38–2.14 eV) in the luminescence spectrum. Crystals of Ga_2−xEu_xS₃ emitted bright green electroluminescence (EL) spectrum when they excited with static and alternating electric fields at 77 K. We have also determined the field and frequency dependences of the EL.     

Effect of erbium concentration on optical properties of Er:YLF laser crystals

Four Er-doped LiYF₄ crystals with different Er-concentrations were grown by Czochralski method. The laser crystals were characterized by measurements of ICP-AES, XRD, absorption spectra, up-conversion fluorescence spectra, near-infrared (NIR) and mid-infrared (Mid-IR) fluorescence spectra, as well as luminescence decays. It was found that the heavily 15 at% Er-doped YLF crystal is more proper in up-conversion or ∼3 μm laser applications; while the 5 at% Er-doped YLF is a better candidate for ∼1.5 μm lasers within these four crystals.     

Afterglow in bulk AlN single crystals under β-irradiation

Regularities of afterglow at room temperature and of thermoluminescence at further heating up to 673 K have been studied in bulk aluminum nitride single crystals. It has been established that after exposure to β-irradiation luminescence decay at RT may be described by superposition of two exponential components: fast (59 s) and slow (606 s) ones, caused by defects of the anion crystal sublattice O_N- and V_N-centers, respectively. The afterglow spectrum is shown to be characterized by the 3.43 eV band with FWHM=0.61 eV that dominates also in the thermoluminescence under study. From analysis of the TL curves in terms of the general order formalism it has been concluded that variation of the activation energy observed within the 0.46–0.85 eV range with increasing storage of the samples from 5 min to 3 days may be caused by energy distribution of traps on the basis of oxygen-related centers. For the first time the compensation effect has been found, and phenomenologically interpreted for the TL processes of the AlN single crystals. Isokinetic temperature has been estimated within the framework of empiric and non-empiric relations.