Time-resolved Luminescence Spectra Research Articles

Optical properties of hybrid quantum dot/quantum well active region based on GaAs system

We experimentally investigate the optical properties of a novel hybrid material/structure consisting of a GaInNAs quantum well and stacked InAs/InGaAs quantum dot layers on GaAs substrate. We demonstrate that the strong quantum confined Stark effect within the quantum well can effectively control well-dot detuning when reverse bias voltage is applied. With a combination of low- and room-temperature time resolved luminescence spectra we infer device absorption recovery time under 30 ps. These properties could be utilized in high-speed optoelectronics devices, in particular electro-absorption modulated lasers and reconfigurable multisection devices, where the hybrid quantum dots – quantum well material system could offer easily and rapidly interchangeable function, i.e., emission gain or variable attenuation, of each section depending on the external bias.

Specific features of fluorescence kinetics of Pr+3 doped BiB3O6 glasses

Praseodymium-doped BiB3O6 glass was synthesized and a wide range of its optical properties was studied, including absorption, excitation and high resolution time resolved luminescence spectra. Particular interest was devoted to study of their time decay kinetics of induced fluorescence under influence of the third harmonic Nd:YAG laser illumination (355nm). Role of borate glass matrices contributing through the electron–phonon interaction to the time-decay kinetics of fluorescence is discussed. Comparison with other glass matrices is performed both following the Judd–Ofelt analysis as well as experimental decay times for principal fluorescence lines.

Emission characteristics of SPAN-80 activated ZnS nanocolloids

Quantum surface effects (new emission bands, blueshifts, intensity enhancement) were observed in SPAN-80 activated ZnS nanocolloids and explained in terms of time-dependent density functional theory. The experimental evidences were demonstrated for both undoped and Cu, Mn-doped colloidal phases. The photoluminescence spectra of these materials showed a new green band at 520nm (ZnS:Cu) and a yellow-orange band at 576nm (ZnS:Mn) besides a blue band at 465nm. All bands lie in the visible region and are blueshifted, show sharp emissions with narrow widths and have approximately 20-times stronger intensities in comparison with those of the bulk samples. The time-resolved luminescence spectra showed that the life-times of free electrons were 0.12μs and 1.9ms in ZnS:Cu and ZnS:Mn correspondingly.

Color and Luminescence Stability of Selected Dental Materials In Vitro

To study luminescence, reflectance, and color stability of dental composites and ceramics. IPS e.max, IPS Classic, Gradia, and Sinfony materials were tested, both unpolished (as-cast) and polished specimens. Coffee, tea, red wine, and distilled water (control) were used as staining drinks. Disk-shaped specimens were soaked in the staining drinks for up to 5 days. Color was measured by a colorimeter. Fluorescence was recorded using a spectrofluorometer, in the front-face geometry. Time-resolved fluorescence spectra were recorded using a laser nanosecond spectrofluorometer. The exposure of the examined dental materials to staining drinks caused changes in color of the composites and ceramics, with the polished specimens exhibiting significantly lower color changes as compared to unpolished specimens. Composites exhibited lower color stability as compared to ceramic materials. Water also caused perceptible color changes in most materials. The materials tested demonstrated significantly different initial luminescence intensities. Upon exposure to staining drinks, luminescence became weaker by up to 40%, dependent on the drink and the material. Time-resolved luminescence spectra exhibited some red shift of the emission band at longer times, with the lifetimes in the range of tens of nanoseconds. Unpolished specimens with a more developed surface have lower color stability. Specimens stored in water develop some changes in their visual appearance. The presently proposed methods are effective in evaluating the luminescence of dental materials. Luminescence needs to be tested in addition to color, as the two characteristics are uncorrelated. It is important to further improve the color and luminescence stability of dental materials.

Photo- and radioluminescence of lithium hafnate Li2HfO3

Photoluminescence, excitation and pulse cathode luminescence spectra, and decay kinetics of lithium hafnate Li2HfO3 were investigated at temperatures 300 and 7.5K. Time-resolved luminescence spectra were excited with synchrotron radiation in UV–VUV range. Strong emission with the peak position at 4.42eV and with the decay time in μs range was revealed at T=7.5K. The emission was attributed to radiative decay of self-trapped excitons (STE). The features observed in the excitation spectra at 6.6eV were likely to take place because of the exciton formation, with inter-band transitions dominating at 6.7eV. Crystal structure defects underpin the luminescence spectra formation at room temperature in the frame of vacancy model.

Spectroscopic studies on U(VI)-salicylate complex formation with multiple equilibria

Abstract This study investigates multiple equilibria related to the formation of the U(VI)-salicylate complex in a pH range of 3.0–5.5 using UV-Vis absorption and fluorescence measurement techniques. The absorbance changes at the characteristic charge-transfer bands of the complex were monitored, and the results indicated the presence of multiple equilibria and the formation of both 1:1 and 1:2 (U(VI):salicylate) complexes possessing bi-dentate chelate structures. The determined step-wise formation constants (log K 1:1 and log K 1:2) are as follows: 12.5 ± 0.1 and 11.4 ± 0.2 for salicylate, 11.2 ± 0.1 and 10.1 ± 0.2 for 5-sulfosalicylate, and 12.4 ± 0.1 and 11.4 ± 0.1 for 2,6-dihydroxybenzoate, respectively. The molar absorptivities of the complexes are also provided. Furthermore, time-resolved laser-induced luminescence spectra of U(VI) species demonstrate the presence of both a dynamic and static quenching process upon the addition of a salicylate ligand. Particularly for the luminescent hydroxouranyl species, a strong static quenching effect is observed. The results suggest that both the UO2(HSal)+ and the U(VI)–Sal chelate complexes serve as ground-state complexes that induce static quenching. The Stern–Volmer parameters were derived based on the measured luminescent intensity and lifetime data. The static quenching constants (log K S) obtained are 3.3 ± 0.1, 4.9 ± 0.1, and 4.4 ± 0.1 for UO2 2+, (UO2)2(OH)2 2+ and (UO2)3(OH)5 +, respectively.

Quantum cutting mechanism in Tb3+-Yb3+ co-doped oxyfluoride glass

Rate equations were created to describe cooperative quantum cutting phenomena, which incorporated the interactions between donor Tb3+ and acceptor Yb3+ ions. Two judgment criteria were developed for the excitation power dependence and time-resolved luminescence spectra of donor and acceptor ions, which can be used to verify the proposed mechanism. Under the excitation of a 473 nm continuous wave laser, the emission intensities of Tb3+ and Yb3+ increased linearly with the excitation power. The decay curve of Yb3+ indicated two distinct contributions: the fast decay time of its own lifetime, and the slow decay time representing the lifetime of the 5D4 energy level of Tb3+. The experimental results meet the two judgment criteria, which confirmed the proposed cooperative quantum cutting mechanism in Tb3+-Yb3+ co-doped oxyfluoride glass.

Luminescence study of self-trapped excitons in CdMoO 4

Luminescence properties of CdMoO 4 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 (MoO 4) 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 (MoO 4) 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 CdMoO 4 luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.

Flow injection analysis of volatile phenols in environmental water samples using CdTe/ZnSe nanocrystals as a fluorescent probe

On the basis of flow injection analysis technology, a simple, accurate, and sensitive method has been developed for the determination of volatile phenols in environmental water samples by using CdTe/ZnSe nanocrystals as a fluorescent probe. The influences of coexisting metal ions and volatile phenol substitutes were also investigated. The method developed for analysis of volatile phenols displayed very good linearity in the range from 1.0 × 10(-8) to 4.0 × 10(-7) g L(-1), with a correlation coefficient greater than 0.995 and a detection limit down to 2.7 × 10(-9) g L(-1) (signal-to-noise ratio 3). The proposed method was successfully applied to determine the content of volatile phenols in environmental water samples, and the quantitative recoveries were 93.4-106.1%. A possible reaction mechanism for the quenching of fluorescence is discussed using UV-vis absorption spectra, fluorescence spectra, and time-resolved luminescence spectra of volatile phenols obtained by titrating a CdTe/ZnSe nanocrystal aqueous solution and zeta potential data.

Laser-induced time-resolved luminescence of orange kyanite Al 2SiO 5

Manganese is a very important microelement performing a large number of biological functions in human body. We have detected by spectroscopic measurements manganese in mineral kyanite. In this paper we present laser-induced time-resolved luminescence and optical absorbance spectra of orange, Mn containing kyanite. It was proven the orange color is caused by Mn 3+. Several luminescence lines and bands were found and ascribed to Mn 4+ and Mn 3+, emission centers. The spectroscopic technique can be utilized for detection of small amounts of manganese in minerals.

High pressure and time resolved luminescence spectra of Gd 3Ga 5O 12:Pr 3+ crystal

Luminescence spectra and time resolved luminescence spectra of GGG crystal doped with Pr 3+ were measured at high hydrostatic pressure from ambient to 220 kbar. Effect of pressure results in the red shift of all luminescence lines related to Pr 3+ ion emission equals from −0.32 to −1.02 cm −1/kbar and in the diminishing of the luminescence lifetimes. The luminescence decay related to emission from 3P 0 state was single-exponential and diminished with pressure from 23 μs at ambient pressure to 6.5 μs at 165 kbar. Luminescence decay related to transition form 1D 2 state was two-exponential with longer decay equal to 162 μs at ambient pressure and 120 μs at 165 kbar. We discussed effect of pressure on the 1D 2 → 3H 4 luminescence and emission from 3P 0 state in the context of non-radiative processes that depopulate the 3P 0 and populate the 1D 2 state, considering mainly multiphonon relaxation processes and depopulation via the praseodymium trapped exciton state.

Positron and luminescence lifetimes in annealed synthetic quartz

The amount of luminescence that can be measured from quartz changes significantly as do the corresponding luminescence lifetimes if the quartz is annealed before pulsed optical stimulation. Owing to its importance in retrospective dosimetry, natural quartz has been the subject of most such thermal studies. In this work, the influence of annealing and beta irradiation on luminescence lifetimes determined from time-resolved luminescence spectra and positron lifetimes evaluated from positron annihilation spectra has been investigated in synthetic quartz, a material with eclectic applications including dosimetry and defect-physics studies. Measurements on the influence of stimulation temperature on luminescence lifetimes and luminescence intensity are also reported. Luminescence lifetimes are independent of annealing up to about 500 °C but increase thereafter suggesting the increasing importance of higher-lifetime luminescence centres. This interpretation is based on the energy band model for quartz used to explain luminescence lifetimes in which annealing induces the transfer of holes from a non-radiative luminescence center to and between radiative luminescence centres as is discussed in the text. Regarding the influence of measurement temperature on luminescence lifetimes, it was observed that lifetimes are strongly affected by measurement temperature particularly between about 120 and 200 °C in a manner consistent with thermal quenching. On the other hand, the luminescence intensity determined from the same time-resolved luminescence spectra goes through a peak as a function of measurement temperature. The initial increase in luminescence intensity is attributed to thermal assistance to optical stimulation whereas the subsequent decrease is ascribed to thermal quenching of the luminescence. Values of the activation energy for thermal assistance and thermal quenching are given. Positron lifetimes in unannealed quartz and quartz annealed at 500 °C are similar and independent of dose showing that irradiation or annealing did not produce new types of defects. The aim of this study is to improve understanding of the physical processes responsible for luminescence in synthetic quartz.

A time-correlated photon counting system for measurement of pulsed optically stimulated luminescence

Abstract A versatile light-emitting-diode system for measurement of time-resolved luminescence spectra is described. The instrument is an improvement on a previously reported version [1] also based on pulsed light-emitting diodes. The present configuration offers substantially improved performance in measurement of time-resolved optically stimulated luminescence spectra and records the luminescence with better dynamic throughput. Measurements made on sedimentary quartz annealed at 800 °C as an exemplar show that the luminescence obtained after the light pulse as a proportion of the total measured signal falls within the usable dynamic range of 0.1–98% for pulse-widths between 3 and 198 μ s , a range of pulse-widths and so luminescence yield that is sufficient for most contemporary applications of time-resolved optically stimulated luminescence.

Optical properties of Bismuth-doped silica core photonic crystal fiber

Optical properties of a Bismuth-doped pure silica sol-gel core photonic crystal fiber (PCF) were investigated. We report on the absorption, CW luminescence and time resolved luminescence spectra at different excitation wavelengths at room temperature. Complex structure of the energy levels of Bismuth-connected centers in pure silica glass is put in evidence.

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.

Optical spectroscopy investigation on distribution of Eu3+ in nanostructured glass ceramics

The time-resolved luminescence spectra of the Eu3+ doped SiO2–Al2O3–NaF–YF3–EuF3 precursor glass reveal that Eu3+ ions locate in both the oxygen-coordination environment and the fluorine-coordination one. After crystallization induced by heat treatment, the orthorhombic YF3 nanocrystals with mean size of 22 nm embedded homogeneously in the glassy matrix. The Stark splitting emission, the low electric dipole D50→F72 transition, the disappearance of the O2−–Eu3+ charge transfer band and the reduction in Ω2 value indicate the partition of Eu3+ into the YF3 lattice. Moreover, the nearly single-exponential luminescence decay curves of the Eu3+:D50 and D51 levels for the 0.1% Eu3+ doped glass ceramic evidence that Eu3+ ions mainly occupy the Y3+ sites.

Time-Resolved Photoluminescence Spectra, Lifetime Distributions and Decay-Associated Spectra of Lanthanide's Exchanged Microporous–Mesoporous Materials

Time-resolved luminescence spectra of europium in parent and silylated microporous-mesoporous materials were analyzed by using the maximum entropy method and global multi-exponential fitting, providing lifetime distributions and decay-associated spectra. Silylation was used as a method for the hydrophobization of the materials surface in order to inhibit the moisture intrusion at the lanthanide's sites. Due to the well-known sensitivity of the europium photoluminescence properties to the local environment, our approach can simplify the description of the complex structure-photoluminescence relationships in terms of the individual europium species that contribute to the total emission of the system.

Color-tunable luminescence of Eu 3+ in LaF 3 embedded nanocomposite for light emitting diode

Eu 3+-doped transparent nanocomposite of SiO 2–Al 2O 3–NaF–LaF 3 was fabricated by melt-quenching and subsequent heating. X-ray diffraction and transmission electron microscopy analyses evidenced that hexagonal LaF 3 nanocrystals were homogeneously precipitated among the aluminosilicate glass matrix. The distribution of Eu 3+ ions in the nanocomposite was investigated by energy dispersive X-ray spectroscopy, photoluminescence and time-resolved luminescence spectra. The nanocomposite exhibited intense blue and green emissions originating from the 5D 1,2,3 levels of Eu 3+ incorporated in the low-phonon-energy LaF 3 nanocrystals, and red emission corresponding to the Eu 3+: 5D 0 → 7F J ( J = 0, 1, 2, 3, 4) transitions, under the excitation of single wavelength light at 394 nm. By adjusting the Eu 3+ doping content, various luminescent colors, including perfect white light, were easily tuned through cross relaxation processes between Eu 3+ ions. The results suggest that the Eu 3+-doped transparent nanocomposite could be potentially applicable as a white-light-emitting material under UV chip excitation.

Observation of polarized luminescence from Jahn–Teller split states of self-trapped excitons inPbWO4 by time-resolved spectroscopy

Time-resolved luminescence spectra of orientedPbWO4 crystals have been measured for polarizations with the electric vector parallel to the crystallographicc axis and perpendicular to it . All the three emission bands (I, II and III) due to the Jahn–Tellersplit states of a self-trapped exciton (STE) located on the(WO4)2− complex anion are found to exhibit significant polarizations in the ac plane. The mainband II peaking at 425 nm is polarized with at low temperatures (T<100 K), but changes its polarization from to at T>150 K. Band III at 450 nm is polarized with at T = 230 and 300 K. The degree of polarization of these two bands does not depend on thepolarization of excitation light. On the other hand, band I at 407 nm is polarized with () under the photoexcitation polarized parallel (perpendicular) to thec axis, in thetemperature range of T = 150–300 K. The observed polarization characteristics of three emissionbands invoke further lowering of symmetry of tetrahedral(WO4)2− ions due to the uniaxially compressive crystal field, in addition to theJahn–Teller distortion. The sublevels of the STEs responsible for thePbWO4 luminescence are assigned on the basis of a group-theoretical consideration.

Broadband spectral modification from visible light to near-infrared radiation using Ce3+–Er3+ codoped yttrium aluminium garnet

Broadband spectral conversion from visible light to near-infrared radiation in Ce(3+)-Er(3+) codoped yttrium aluminium garnet is reported. Excitation, emission, time-resolved luminescence spectra and fluorescence decay curves have been measured to prove the energy transfer from Ce(3+) to Er(3+). The broadband and strong absorption of Ce(3+), efficient energy transfer from Ce(3+) to Er(3+), and three-photon quantum cutting process in Er(3+) ions result in efficient near-infrared emission when excited by 400-500 nm broad visible light. The energy of the Er(3+) ((4)I(13/2)→(4)I(15/2)) transition matches well with the band gap of germanium (Eg ∼ 0.785 eV). Ce(3+)-Er(3+) codoped yttrium aluminium garnet is promising for the enhancement of conversion efficiency of germanium solar cells.