Photoluminescence properties of Er-doped β-FeSi2 grown by ion implantation

Photoluminescence (PL) properties of Er-doped β-FeSi₂ (β-FeSi₂:Er) and Er-doped Si (Si:Er) grown by ion implantation were investigated. In PL measurements at 4.2 K, the β-FeSi₂:Er showed the 1.54 μm PL due to the intra-4f shell transition of ⁴I_13/2→⁴I_15/2 in Er³⁺ ions without a defect-related PL observed in Si:Er. In the dependence of the PL intensity on excitation photon flux density, the obtained optical excitation cross-section σ in β-FeSi₂:Er (σ=7×10⁻¹⁷ cm²) is smaller than that in Si:Er (σ=1×10^-15 cm²). In the time-resolved PL and the temperature dependence of the PL intensity, the 1.54 μm PL in β-FeSi₂:Er showed a longer lifetime and larger activation energies for non-radiative recombination (NR) processes than Si:Er. These results revealed that NR centers induced by ion implantation damage were suppressed in β-FeSi₂:Er, but the energy back transfer from Er³⁺ to β-FeSi₂ was larger than Si:Er.     

Persistent luminescence in Ca8Zn(SiO4)4Cl2:Eu2+

A new persistent, green emission is reported for europium activated Calcium Zinc Chlorosilicate (CZCS). The phosphor was prepared by the solid state reaction and characterized using XRD, PL and TL techniques. The decay was characterized by admixture of two exponential components with decay constants 20.2 and 101 s.     

Morphology dependent luminescence properties of rare-earth doped lanthanum fluoride hierarchical microstructures

Here, we demonstrate an ionic liquid-assisted hydrothermal method for preparing Tb³⁺ and Eu³⁺ doped LaF₃ hierarchical microstructures and the morphology is modified by hydrothermal reaction time, temperature of heating and ionic liquid concentration. The mechanism related to morphology control is proposed and discussed. It is also found that PL intensity, decay time and quantum efficiency are sensitive to the morphology. The average decay times are 2.9 ms and 4.8 ms for Eu³⁺ doped LaF₃ microstructures prepared at 10 min and 3 h reaction time, respectively. The average decay time is increased from 4.8 ms to 5.8 ms after heating the sample at 500 °C. The quantum efficiency varies from 34% to 67% with changing morphology. Analysis suggests that morphology plays an important role on efficiency of rare-earth doped materials.     

Photoluminescence properties of Er-doped AlN films prepared by magnetron sputtering

Er-doped aluminum nitride films, containing different Er concentrations, were obtained at room temperature by reactive radio frequency magnetron sputtering. The prepared samples show a nano-columnar microstructure and the size of the columns is dependent on the magnetron power. The Er-related photoluminescence (PL) was studied in relation with the temperature, the Er content and the microstructure. Steady-state PL, PL excitation spectroscopy and time-resolved PL were performed. Both visible and near infrared PL were obtained at room temperature for the as-deposited samples. It is demonstrated that the PL intensity reaches a maximum for an Er concentration equal to 1 at% and that the PL efficiency is an increasing function of the magnetron power. Decay time measurements show the important role of defect related non-radiative recombination, assumed to be correlated to the presence of grain boundaries. Moreover PL excitation results demonstrate that an indirect excitation of Er³⁺ ions occurs for excitation wavelengths lower than 600 nm.     

Adjustable EDFA gain equalization filter for DWDM channels based on a single LPG excited by flexural acoustic waves

A fast adjustable gain equalization filter for dense wavelength division multiplexing (DWDM) system is reported. The method is based on a single long period fiber grating (LPG) which is excited by means of flexural acoustic waves. The equalization of a typical erbium doped fiber amplifier (EDFA) gain spectrum with a gain flatness of <0.3 dB over a 32 nm bandwidth is demonstrated. The filter adjustment is obtained by choosing different acoustic loads applied to the acousto-optic modulator, which presents a switching time of ～17 μs. A maximum power penalty of 0.84 dB, relatively to the back-to-back signal, was achieved.     

Impact of doping on carrier recombination and stimulated emission in highly excited GaN:Mg

Light-induced transient grating and photoluminescence measurements were employed for carrier recombination studies in variously Mg doped GaN layers. Carrier lifetime and ambipolar diffusion coefficient were found to decrease with doping from 210 to 20 ps and from 2.0 to 0.9 cm²/s,respectively, which proved the degradation of electrical quality of the layers. A threshold of stimulated emission was found to depend non-monotonously on doping and had the lowest value of 0.19 mJ/cm² in the most doped layer. This dependence was explained in terms of degeneracy of the hole system.     

Energy transfer induced Eu3+ photoluminescence enhancement in tellurite glass

In this work, structural, thermal and optical properties of Eu³⁺ doped TeO₂–La₂O₃–TiO₂ glass were investigated. The differential scanning calorimetry (DSC) measurements reveal an important stability factor ΔT=143.52 K, which indicates the good thermal and mechanical stabilities of tellurite glass. From the absorption spectrum, the optical band gap was found to be direct with E_g=3.23 eV. The temperature dependences of photoluminescence (PL) properties of Eu-doped and Eu–Tb codoped tellurite glass are investigated. As the temperature increases from 7 to 300 K, both the PL intensity and the PL lifetime relative to the ⁵D₂→⁷F₀ are nearly constant below 230 K and then an enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from charged intrinsic defects states to Eu³⁺ energy levels.By co-doping tellurite glasses with Eu and Tb, a strong Eu³⁺ PL enhancement is shown due to excitation transfer from Tb³⁺ and intrinsic defects to Eu ions.     

Identification of the photoluminescence response in the frequency domain modulated infrared radiometry signal of ZnTe:Cr bulk crystal

In this work we investigated the photoluminescence response in the frequency domain modulated infrared radiometry signal observed of ZnTe:Cr bulk crystal. In mid-infrared range, three characteristic phenomena are observed in ZnTe:Cr crystal: absorption and emission of IR photons (2–3 μm) and the free carrier absorption. This implies that the modulated infrared radiometry signal yields information about the effective infrared absorption coefficient (photothermal response) as well about the recombination lifetime of carriers related with the infrared photoluminescence emission. In this paper, the frequency equivalence of the two-term independent exponential photoluminescence decay model in order to explain the measured frequency characteristics is proposed. The measured recombination lifetimes (2.3 μs for two exponential decay model and 1.5 μs for one exponential decay model) are in good agreement with the values given by other authors (about 2.5–3.0 μs). Moreover, we found that the photothermal response is uncorrelated with the photoluminescence one, in contrast, to the photocarrier response.     

Acceptors related to group I elements in ZnO ceramics

ZnO ceramics doped with Li, Na or K were sintered in air for 4 h at 1000 °C. Electrical conductivity as well as photoluminescence (PL), PL excitation and photoconductivity spectra were measured and compared with those in undoped samples. The influence of both fast and slow cooling of the samples from 1000 °C on measured characteristics was investigated. The yellow–orange PL bands associated with the deep acceptors Li_Zn, Na_Zn and K_Zn were observed and the corresponding PL excitation spectra were determined. These acceptors were found to form some complexes with other lattice defects.     

1.53 μm electroluminescence from ErF3-doped organic light-emitting diodes

Near-infrared (NIR) organic light-emitting devices (OLEDs) are demonstrated by employing erbium fluoride (ErF₃)-doped tris-(8-hydroxyquinoline) aluminum (Alq₃) as the emitting layer. The device structure is ITO/N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPB)/Alq₃: ErF₃/2,2′,2″-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole) (TPBI)/Alq₃/Al. Room-temperature electroluminescence around 1530 nm is observed due to the ⁴I_13/2–⁴I_15/2 transition of Er³⁺. Full width at half maximum (FWHM) of the electroluminescent (EL) spectrum is ～50 nm. NIR EL intensity from the ErF₃-based device is ～4 times higher than that of Er(DBM)₃Phen-based device at the same current. Alq₃–ErF₃ composite films are investigated by the measurements of X-ray diffraction (XRD), absorption, photoluminescence (PL) and PL decay time. Electron-only devices are also fabricated. The results indicate that energy transfer mechanism and charge trapping mechanism coexist in the NIR EL process.     

Scintillation kinetics and thermoluminescence of SrI2:Eu2+ single crystals

Single crystal of strontium iodide doped with 1% europium (SrI₂:1% Eu²⁺) was grown by Vertical Gradient Freeze technique. UV excited emission spectra were studied as a function of temperature. Results indicate the thermal quenching of Eu²⁺ emission starts from ～400 K with a thermal activation energy of 0.39 eV. Gamma and UV excited decay measurements indicate that the scintillation decay time of SrI₂:Eu²⁺ is longer than the lifetime of Eu²⁺ luminescence center in the SrI₂ host. The thermoluminescence glow curve revealed a highly concentrated charge carrier trap at 50 K. Elimination of this trap is expected to enhance the energy migration of charge carriers and result in faster scintillation decay.     

Luminescence enhancement by the reduction–oxidation synthesis in monoclinic RE2O3 (RE=Eu,Gd) phosphors containing Eu3+ activator

A novel synthesis was developed for enhanced luminescence in sesquioxide phosphors containing Eu³⁺ activator. It consisted of two annealing steps: reduction under vacuum with gaseous H₂ at 10 Torr and 1300 °C and re-oxidation at 300–1500 °C in air. The integrated luminescence intensity of the monoclinic Eu₂O₃ phosphor was enhanced ca. 21 times by this method compared with conventional processing. The photoluminescence (PL) intensity was maximized at re-oxidation temperatures of 500–1100 °C. The PL characteristics of monoclinic Eu₂O₃ and Gd₂O₃:0.06Eu samples were compared with a commercial cubic Y₂O₃:Eu phosphor. The evolution of physical characteristics during the two-step annealing was studied by Raman spectroscopy, XPS, XRD, PL decay analysis, and SEM. PL decay lifetime increased proportionally to the PL intensity over the range 0.5–100 μs. Additional vibrational modes appeared at 490, 497, and 512 cm^?1 after the two-step annealing. The increase in PL intensity was ascribed to the formation of excess oxygen vacancies and their redistribution during annealing. Resonance crossovers between the charge transfer state and the emitting ⁵D_J states are discussed in relation to reported luminescence saturation mechanisms for oxysulfides Ln₂O₂S:Eu³⁺ (Ln=Y, La).     

High index contrast Er:Ta2O5 waveguide amplifier on oxidised silicon

We report a high index contrast erbium doped tantalum pentoxide waveguide amplifier. 2.3 cm long waveguides with erbium concentration of 2.7 × 10²⁰ cm^? 3 were fabricated by magnetron sputtering of Er-doped tantalum pentoxide on oxidised silicon substrates and Ar-ion milling with photolithographically defined mask. A net on-chip optical gain of ~ 2.25 dB/cm at 1531.5 nm was achieved with 20 mW of pump power at 977 nm launched into the waveguide. The pump threshold for transparency was 4.5 mW.     

Crystal growth and optical properties of Cr3+, Er3+, RE3+: Gd3Ga5O12 (RE=Tm,Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr³⁺, Er³⁺, and RE³⁺ (RE=Tm, Ho, Eu) doped Gd₃Ga₅O₁₂ crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr³⁺ ion highly increases the absorption pump power and the three kinds of co-doped RE³⁺ ions depopulate the Er:⁴I_13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ～3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.     

The TL and room temperature OSL properties of the glow peak at 110 °C in natural milky quartz: A case study

The LM–OSL signal of quartz, while measured at room temperature, is dominated by an intermediate, broad and intense OSL component, so that its contribution and general characteristics are derived very accurately. Through a series of dose–response, bleaching and thermal decay at room temperature experiments, in conjunction with curve fitting studies, a component resolved analysis is carried out studying the correlation between this specific component, termed as LM–OSL component C₂ and the 110 °C TL glow peak in quartz. The dose–response of these two luminescence components behaves exactly similar being linear at low doses and saturating at almost 100 Gy. Both signals decay exponentially under illumination, providing identical optical detrapping cross-section values. Residual of both luminescence signals after thermal decay at room temperature follows an exponential law, yielding similar mean half-lives. All previous luminescence features provide strong evidence for the electron trap being the same for both the 110 °C TL trap and the LM–OSL component C₂. The results of the present work are very promising and clearly support the possibility of extrapolating the TL pre-dose methodology to the OSL pre-dose effect using only the LM–OSL component C₂.     

Spectroscopic study of singlet oxygen photogeneration by lipophilic photosensitiser in liposomes

The time- and spectrally-resolved phosphorescence measurements of protoporphyrin IX (PpIX), haematoporphyrin (HpD) and singlet oxygen in liposomal samples under different oxygen concentrations were performed. We observed two different phosphorescence lifetimes of two distinct groups of photosensitisers (PSs). The group with shorter lifetime is located deep inside the nonpolar lipid bilayer, whereas the group with longer lifetime is exposed to H₂O due to its localisation near the bilayer surface. When the oxygen concentration in H₂O is increased about five times, a significant change in the slower decay component of the group of PS near the surface was observed from 3.8 to 1.3 μs (HpD) and from 3.3 to 1.2 μs (PpIX). On the other hand, the shorter phosphorescence components exhibit less-pronounced changes in lifetimes from 0.42 to 0.39 μs (HpD) and from 0.28 to 0.25 μs (PpIX). The singlet oxygen decay time decreases from 7.3 to 3.5 μs (PpIX) and from 8.0 to 3.5 μs (HpD) in H₂O. The results are discussed in the frame of a model, where an increase of the oxygen concentration in the aqueous medium is accompanied by only a slight increase of the oxygen concentration inside the lipid bilayer.     

Morphology and optical properties of Mg and Sr doped CaF2 nanocrystals

Magnesium (Mg) and Strontium (Sr) doped Calcium fluoride nanocrystals were synthesized by co-precipitation method. The cubic structure of the samples was confirmed by Powder X-ray diffraction. The average crystallite size of Mg doped samples was found to be ~ 25 nm whereas in Sr doped one it was ~ 35 nm. The morphological features revealed that the nanocrystals were agglomerated, crispy and porous. The as-prepared samples showed the presence of hydroxyl groups. The optical absorption spectrum of as-prepared Mg doped samples showed a strong absorption band peaked at ~ 233 nm whereas the Sr doped one showed a prominent absorption peak at 248 nm. A strong PL emission was observed at ~ 300 nm in Mg doped samples. However, the Sr doped samples showed two prominent emissions at ~ 345 and 615 nm.     

Tantalum induced butterfly-like clusters on Si (111)-7 × 7 surface: STM/STS study at low coverage

The adsorption of the small amounts of tantalum on Si (111)-7 × 7 reconstructed surface is investigated systematically using scanning tunneling microscopy and tunneling spectroscopy combined with first-principles density functional theory calculations. We find out that the moderate annealing of the Ta covered surface results in the formation of clusters of the butterfly-like shape. The clusters are sporadically distributed over the surface and their density is metal coverage dependent. Filled and empty state STM images of the clusters differ strongly suggesting the existence of covalent bonds within the cluster. Tunneling spectroscopy measurements reveal small energy gap, showing semiconductor-like behavior of the constituent atoms. The cluster model based on experimental images and theoretical calculations has been proposed and discussed. Presented results show that Ta joins the family of adsorbates, that are known to form magic clusters on Si (111)-7 × 7, but its magic cluster has the structural and electronic properties that are different from those reported before.     

A method for rapid measurement of laser ablation rate of hard dental tissue

The aim of the study reported here is the development of a new method which allows rapid and accurate in-vitro measurements of three-dimensional (3D) shape of laser ablated craters in hard dental tissues and the determination of crater volume, ablation rate and speed. The method is based on the optical triangulation principle. A laser sheet projector illuminates the surface of a tooth, mounted on a linear translation stage. As the tooth is moved by the translation stage a fast digital video camera captures series of images of the illuminated surface. The images are analyzed to determine a 3D model of the surface. Custom software is employed to analyze the 3D model and to determine the volume of the ablated craters. Key characteristics of the method are discussed as well as some practical aspects pertinent to its use. The method has been employed in an in-vitro study to examine the ablation rates and speeds of the two main laser types currently employed in dentistry, Er:YAG and Er,Cr:YSGG. Ten samples of extracted human molar teeth were irradiated with laser pulse energies from 80 mJ to the maximum available energy (970 mJ with the Er:YAG, and 260 mJ with the Er,Cr:YSGG). About 2000 images of each ablated tooth surface have been acquired along a translation range of 10 mm, taking about 10 s and providing close to 1 million surface measurement points. Volumes of 170 ablated craters (half of them in dentine and the other half in enamel) were determined from this data and used to examine the ablated volume per pulse energy and ablation speed. The results show that, under the same conditions, the ablated volume per pulse energy achieved by the Er:YAG laser exceeds that of the Er,Cr:YSGG laser in almost all regimes for dentine and enamel. The maximum Er:YAG laser ablation speeds (1.2 mm³/s in dentine and 0.7 mm³/s in enamel) exceed those obtained by the Er,Cr:YSGG laser (0.39 mm³/s in dentine and 0.12 mm³/s in enamel). Since the presented method proves to be easy to use and allows quite rapid measurements it may become a valuable tool to study the influence of various laser parameters on the outcome of laser ablation of dental tissues.     

Growth and characterization of pure and doped bis thiourea zinc acetate: Semiorganic nonlinear optical single crystals

Metal–organic coordination complex single crystals bis thiourea zinc acetate (BTZA) and Cd²⁺ doped BTZA have been synthesized and grown successfully by slow-cooling technique from their aqueous solutions. Single crystals of pure and Cd²⁺ doped BTZA with dimensions of 35 × 4 × 2 mm³ and 10 × 5 × 6 mm³, respectively were obtained with well defined morphology. The as grown single crystals are characterized by single crystal XRD studies and melting point measurements which reveal the incorporation of metallic dopants has not changed the structure of the parent crystal. The powder X-ray diffractogram of the grown crystals has been recorded and the various planes of reflection identified shows shift in the peak positions. The metal coordination with thiourea through sulphur in pure and Cd²⁺ doped BTZA were ascertained by FTIR studies and optical absorption study to identify the UV cut-off range. The presence of metals in pure and Cd²⁺ doped BTZA crystal lattice were confirmed by atomic absorption spectroscopy (AAS). The thermal decomposition of pure and Cd²⁺ doped BTZA crystals were investigated by thermo gravimetric analyses (TGA) and differential thermal analysis (DTA) indicate that doped crystals are more stable than pure crystals. The dielectric response of the crystals were studied in the frequency range 100 Hz–5 MHz at different temperatures and the results are discussed. Second harmonic generation (SHG) measurement confirms that the pure and Cd²⁺ doped BTZA have nonlinear optical (NLO) property. Laser damage threshold value of 12.44 MW/cm² has been determined using Q-switched Nd:YAG laser operating at 1064 nm and with 8 ns pulses in single shot mode for pure BTZA single crystal is reported for the first time.