Effect Of Radiation Trapping Research Articles

Fluorescence reabsorption and its effects on the local effective excitation lifetime

A spectral theory of fluorescence reabsorption is presented that allows us to describe the effects of radiation trapping, which especially occurs in barrel-polished laser rods, as an effective change of the excitation lifetime. This effect causes a local fluorescence lifetime difference in the laser medium and thus a local change in the quantum efficiency. The theory is compared to experimental results obtained with an Er3+:YAG rod with good agreement.

Spectroscopic properties and thermal stability of Er 3+-doped germanate–borate glasses

Er 3+-doped germanate–borate glass samples xGeO 2–(80 − x)B 2O 3–10ZnO–10BaO ( x = 0–80) have been prepared. Their thermal stability against crystallization and spectroscopic properties were investigated. The obtained data indicated that germanate–borate glass could be used as potential host material for developing optical amplifiers. However, OH-groups, introduced from boric acid, enhanced the radiation trapping effect in glasses samples and resulted in overestimate of the spectral bandwidth of 4I 13/2 level. The mechanism of OH-groups leading to enhancing the radiation trapping was discussed.

Growth and characterization of Er-doped single crystal lithium niobate fibers

Single crystal lithium niobate fibers with diameters ranging from 1to3mm have successfully been grown by the micro-pulling-down technique. The fibers are doped with various Er doping levels from 0to0.7mol% and appear to be transparent and crack-free. The study includes x-ray structural investigation and complete spectroscopic characterization. The analysis shows a good homogeneity of composition, and the comparison with bulk crystals demonstrates the good optical quality of the samples. Particular attention has been devoted to the measurements of the fluorescence decay time of the I13∕24 Er level, which has been carried out with a particular technique in order to avoid radiation trapping effects. Results are in agreement with the literature both in the case of bulk crystals and of fibers. Reliable polarized emission cross section in the 1.5μm region are presented.

Thermal and Optical Properties of ${\hbox {Yb}}^{3+}$- and ${\hbox {Nd}}^{3+}$-Doped Phosphate Glasses Determined by Thermal Lens Technique

In this work, we study the thermal and optical properties of ion-doped phosphates glasses using the thermal lens (TL) technique. Three samples were characterized: Nd3+-doped Q-98; Nd3+-doped Q-100; and Yb3+-doped QX. We report multiwavelength TL measurements for a more accuracy determination of the fluorescence quantum efficiency and temperature coefficient of the optical path length change (ds/dT). In Nd-doped glasses, it was carried out using four discrete excitation wavelengths (between 514 and 872 nm) chosen to match with the ion absorption lines. In Yb-doped glass, the spectrum of heat generated along the Yb3+ transition (2 F 5/2rarr2 F 7/2) was obtained. In addition, parameters as thermal diffusivity and conductivity, thermal loading, etc were achieved. The advantages to obtain fluorescence quantum efficiency using the TL technique, mainly in Yb3+ doped materials, which are normally overestimated due to radiation trapping effect, are presented. The accuracy knowledge of these parameters is very important for design of high-power solid-state lasers, since these properties are directly related to the heat generation.

Effect of radiation trapping on the emission properties of Er 3+: 4I 13/2 → 4I 15/2 transition in oxide glasses

The effect of radiation trapping on the emission properties of Er 3+-doped tellurite and phosphate glasses has been investigated as the function of sample thickness and doping concentration. It was found that radiation trapping exists generally in two glass matrices, even at low doping concentration (⩽0.1 mol% Er 2O 3). The larger effect of radiation trapping in tellurite glasses compared with phosphate glasses is due to its larger emission cross-section at 1.5 μm band and the spectral overlap between the emission and absorption spectra of Er 3+: 4I 13/2 ↔ 4I 15/2transition. Due to radiation trapping, the measured lifetime of the Er 3+: 4I 13/2 level in tellurite glasses increases by about 11–37% with increasing the sample thickness at the different erbium doping concentration, while 6–17% for phosphate glasses. And the full-width at half maximum of fluorescence (FWHM) of Er 3+: 4I 13/2 → 4I 15/2 transition in tellurite glasses increased by about 15–64% with increasing the sample thickness, while 11–55% for phosphate glasses. It caused a high overestimation on the figure of merits (FOM) for amplifier bandwidth ( σ e × FWHM).

Sensitivity study of the urban heat island intensity to urban characteristics

AbstractA detailed urban surface exchange parameterization, implemented in a meso‐scale atmospheric model, has been used to study the urban heat island (UHI) intensity during a summer period in the city of Basel, Switzerland. In this urban parameterization, the city is represented as a combination of three urban classes (road, roof and wall), characterized by the size of the street canyon and the building and is thus able to take into account the momentum sink over the entire height of the building, as well as the shadowing and the radiation trapping effects. A control experiment including all the urban parameters describing the city centre of Basel produced a canyon air temperature that compared well with observations. A series of experiments was then conducted in which successively each of the urban parameters characterizing the city centre was changed providing the basis for an assessment of its effect on UHI mitigation. Copyright © 2007 Royal Meteorological Society

X-ray diffraction, optical absorption and emission studies on Er3+- and Er3+/Yb3+-doped Li3NbO4 powder

Er3+(/Yb3+)-doped Li3NbO4 powder were prepared by thermally sintering mixtures of Er2O3 (0.5, 1.0 mol%), Yb2O3 (0, 0.5, 1.0 mol%), Li2CO3 (48–49 mol%) and Nb2O5 (50 mol%) at 1125, 1150 and 1450 °C over the durations of 8–22 h. The crystalline phases contained in these samples were determined by using X-ray diffraction and discussed in comparison with a vapor-transport-equilibration-treated (VTE-treated) Er(2.0 mol%):LiNbO3 single crystal and ErNbO4 powder previously reported. The results show that the X-ray patterns of the rare-earth-doped samples reveal little difference each other, but large differences with those of the VTE crystal and ErNbO4 powder. The doped rare-earth ions Er3+ (and Yb3+) present in the powder as the ErNbO4 (and YbNbO4) phase(s). The possibility that the highly Er-doped LiNbO3 crystal contains Li3NbO4 precipitates is small. Optical absorption and emission studies show that the only Er-doped Li3NbO4 powder shows similar absorption and emission characteristics with the pure ErNbO4. The codopant Yb3+ ion enhances the 980-nm-upconversion emissions of Er3+ ions, results in remarkable spectral alterations at 0.98 μm region, and causes the alterations of relative absorbance and relative emission intensity of individual peaks or bands at 1.5 μm region. On the other hand, the Yb-codoping hardly affects the Er3+ energy structure and the lifetime of Er3+ ion at 1.5 μm. The measured lifetimes at 1.5 μm of Er3+ ions in the singly Er3+- and doubly Er3+/Yb3+-doped mixtures have a nearly same value of ∼ 1.5 ms. For the pure ErNbO4 powder, the lifetime is prolonged to ∼2 ms perhaps due to radiation trapping effect.

Radiation trapping effects in Cu, Mn, Ca, Sr, and Ba vapor lasers

The radiation trapping effect was investigated in Cu, Mn, Ca, Sr, and Ba vapor lasers with a cylinder discharge tube. Compared with the negligible impact induced by the gas temperature, the lower level population density of radiation trapping was found to dominate the effective spontaneous emission probability. Meanwhile a tube radius scanning from 1 mm to 20 mm revealed that the threshold parameters of radiation trapping effect were affected little by the tube radius when the radius changed from 20 mm to 5 mm, but they varied visibly when the tube radius decreased from 5 mm to 1 mm. Furthermore, the threshold temperature and the lower level population density of the radiation trapping effect in six resonant lines in Cu, Mn, Ca, Sr and Ba vapor lasers were calculated, and the threshold temperature was found to be close to the initial lasing temperature of the corresponding resonance level to metastable level transition.

Resonance radiation trapping in Ca, Sr, Ba and Hg ion lasers

According to Holstein's theory about radiation trapping effect, the influence of gas temperature and ground-state population density on the effective lifetime of the resonance level is discussed. The results show that the ground-state population density is the dominating factor in the radiation trapping effect. The threshold parameters of radiatoin trapping have been calculated for seven resonance metastable transition laser lines in Ca+, Sr+, Ba+ and Hg+ in different ionization ratios. The threshold temperature of radiation trapping was found to be consistent with the initial lasing temperature for the ionization ratio of 5%, which matches the ionization ratio of 3% to 5% observed in experiment. This consistency reveals that the threshold condition of radiation trapping effect is an important factor for population inversion in lastrs based on the ionic resonance to metastable transition.

Iterative numerical method for the inclusion of radiation trapping into a collisional-radiative model

The effect of radiation trapping is included in the standard collisional-radiative model for atomic hydrogen under a typical condition of divertor plasmas The population of the first excited level of the ionizing plasma component is strongly enhanced by the absorption of resonance line photons produced by the recombining plasma component, and the effective ionization rate coefficient is also strongly enhanced.

Radiative-trapping and fluorescence-concentration quenching effects of Yb:YAl_3(BO_3)_4 crystals

The effects of radiative-trapping and fluorescence-concentration quenching on the emission spectra and fluorescence lifetimes of Yb3+-doped YAl3(BO3)4 crystals with a series of Yb3+ concentrations were studied. The shift of barycenters in emission spectra, Δλ, was proposed as an appraisement of the radiative-trapping effect on the spectra, and an empirical relation between Δλ and Yb3+ concentration in the Yb:YAl3(BO3)4 crystals was deduced. The model of excitation random walk was used to analyze the effects of radiative trapping and concentration quenching on the fluorescence lifetimes. The relation between fluorescence lifetime and Yb3+ concentration was obtained.

The Excitation of N2H+in Interstellar Molecular Clouds. I. Models

We present large velocity gradient (LVG) and nonlocal radiative transfer calculations involving the rotational and hyperfine structure of the spectrum of N2H+, with collisional rate coefficients recently derived by us. The goal of this study is to check the validity of the assumptions made to treat the hyperfine structure and to study the physical mechanisms leading to the observed hyperfine anomalies. We find that the usual hypothesis of identical excitation temperatures for all hyperfine components of the J = 1-0 transition is not correct within the range of densities existing in cold dense cores, i.e., a few 104 < n(H2) < a few 106 cm-3. This is due to different radiative trapping effects in the hyperfine components. Moreover, within this range of densities and considering the typical abundance of N2H+, the total opacity of rotational lines has to be derived taking into account the hyperfine structure. The error made when only considering the rotational energy structure can be as large as 100%. Using nonlocal models, we find that, due to saturation, hyperfine anomalies appear as soon as the total opacity of the J = 1-0 transition becomes larger than 20. Radiative scattering in less dense regions enhances these anomalies and particularly induces a differential increase of the excitation temperatures of the hyperfine components. This process is more effective for the transitions with the highest opacities for which emerging intensities are also reduced by self-absorption effects. These effects are not as critical as in HCO+ or HCN, but should be taken into account when interpreting the spatial extent of the N2H+ emission in dark clouds.

Passive optical diagnostic of Xe-propelled Hall thrusters. I. Emission cross sections

This paper presents a set of xenon apparent emission excitation cross sections for emission lines that have diagnostic value in the analysis of Xe-propelled Hall thruster plasmas. Emission cross sections are presented for three excitation processes involving ground-state xenon atoms: e−+Xe, Xe++Xe, and Xe2++Xe. The cross sections are derived from luminescence spectra produced at single-collision conditions. Apparent emission excitation cross sections are tabulated for 12 visible and 8 near-infrared lines for electron energies ranging from 10to70eV. In case of the near-infrared lines, radiation trapping effects are accounted for by measuring the detailed pressure dependence of the apparent emission cross sections and extrapolating to zero pressure. A semiempirical expression for the pressure dependence is derived that allows zero-pressure extrapolation from threshold to 70eV. Ion-induced cross sections are reported for the same emission lines at an energy per unit charge E∕q of 300eV, chosen for typical Hall thruster operating voltages. Radiation trapping effects are negligible for the ion emission excitation cross sections between 0.1 and 2.0mTorr in the present luminescence experiment.

Effect of radiation trapping on spectroscopic properties of Er3+-doped oxide glasses

The absorption spectra, fluorescence spectra and the lifetimes of 4I13/2 level of Er3+-doped phosphate and tellurite glasses ha ve been measured for sample with different concentrations and thickness. The abs orption cross-section (σa), emission cross-section(σe), spontaneous emission probability(Arad), radiative lifetime(τrad ) and quantum efficiency(η) of fluorescence around 1.53 μm of Er3+-doped phosphate and tellurite glasses were determined. The effect of radia tion trapping on the spectral properties and parameters of Er3+-doped tellurite and phosphate glasses has been investigated. It was found that radiat ion trapping exists generally in erbium-doped glass hosts, even at low Er3+ -doping concentration (0.1 mol% Er2O3). Due to radia tion trapping, the values of the τrad of the Er3+∶4 I13/2 level in tellurite glasses increased about 11%—37％ with the sample thickness and erbium doping concentration, while in phosphate glasse s τrad increased by 6%—17%. The full-width at half maximum (FWHM)of fluorescence in tellurite glasses increased about 15%—64%, with 11%—55% for p hosphate glasses. It caused a high overestimation on the figure of merits (FOM) for amplifier bandwidth (σe×FWHM). The spectral overlap between the emission and absorption spectra in 1.53 μm band is relatively larger in tellur ite than in phosphate glasses, and values of the emission cross-section in tellu rite glasses are larger than that in phosphate glasses. Consequently, the radiat ion trapping in tellurite glasses is more severel than that in phosphate glasses .

Numerical Study on Gain Coefficient of a Capillary-Discharged Neon-Like Argon X-Ray Laser

X-ray laser gain of J = 0 → 1 transition (λ = 46.9nm) in collisionally pumped neon (Ne)-like argon (Ar IX) ion has been investigated considering 37 levels of the 2p6, 2p53s, 2p53p, 2p53d, 2s2p63s, 2s2p63p, and 2s2p63d configurations. The effect of radiative trapping of resonance lines on the gain coefficient is studied using a collisional radiative model and a one-dimensional single-fluid, two-temperatures magnetohydrodynamic (MHD) equations. Results indicate that the gain value is significantly affected by the plasma opacity, and the velocity gradient allows us to obtain a higher gain in collisionally pumped Ne-like ions excited in fast capillary-discharged plasmas.

Loss induced collective subradiant Dicke behaviour in a multiatom sample

The exact dynamics of N two-level atoms coupled to a common electromagnetic bath and closely located inside a lossy cavity is reported. Stationary radiation trapping effects are found and very transparently interpreted in the context of our approach. We prove that initially injecting one excitation only in the N atoms-cavity system, loss mechanisms asymptotically drive the matter sample toward a long-lived collective subradiant Dicke state. The role played by the closeness of the N atoms with respect to such a cooperative behavior is brought to light and carefully discussed.

Comparing the short and ultrashort pulsed laser ablation of LiF

Pulsed laser ablation of LiF was studied using both nanosecond (ns) and femtosecond (fs) pulses at 248nm. Optical emission from electronically excited Li and F atoms in the plume of ejected material was investigated by wavelength, time and spatially resolved imaging methods. Careful analysis of images of species selected optical emission yielded estimates of the mean velocities of the Li+ ions arising in both excitation schemes (∼11 and ∼13km∕s, respectively), and highlighted the dramatic effects of radiation trapping, most notably by the reabsorption of Li(2p→2s) emission by ground state Li atoms in the ns ablation studies. Plumes formed by fs excitation are found to contain a higher fraction of energetic∕electrically excited components, including excited F atoms and ions, indicative of an explosive boiling mechanism, whereas the ablation plume resulting from ns ablation is deduced to arise primarily from thermal evaporation of the transiently heated target surface. The amount of target material removed per shot is significantly less in the case of fs excitation. The density (and size) of unwanted droplets in films grown by fs ablation is much smaller than in the case of ns ablation, especially on substrates mounted in an off-axis ablation geometry, implying that hydrodynamic sputtering is much reduced by the use of short pulses and that fs ablation must be the preferred route to forming very thin LiF coatings.

GeO2–PbO–Bi2O3 glasses doped with Yb3+ for laser applications

Glasses of heavy metal oxide GeO2–PbO–Bi2O3 doped with different concentrations of Yb3+ were prepared as materials for IR emission. Absorption and fluorescence emission spectra were measured. The spectroscopic properties and laser parameters are calculated; the disagreement between the reciprocity method and Fuchtbauer–Ladenburg formula in calculations of Yb3+ emission cross-sections is discussed using the radiation trapping effect. GeO2–PbO–Bi2O3 glass doped with Yb3+ is a promising material for laser action at 1021nm, with properties similar to other known glasses (phosphate and tellurite laser glasses) used as active laser media; the high absorption cross-section (of 1.5×10−20 cm2) and the large emission bandwidth (of 70nm) are properties appropriate for short pulse generation.

Spectroscopic properties of Er3+ ions in La2(WO4)3 crystal

Abstract Er 3+ -doped La 2 (WO 4 ) 3 single crystals were grown by the Czochralski technique. The absorption spectra, fluorescence spectra and fluorescence decay curves of the crystals were measured at room temperature. The spectroscopic parameters, including intensity parameters Ω t ( t = 2, 4, 6), spontaneous emission probability, fluorescence branching ratio, radiative lifetime, and stimulated emission cross-section were estimated. The fluorescence decay curves of fluorescence manifolds 4 I 13/2 , 4 I 11/2 , and 4 S 3/2 were measured for crystal and powder samples, respectively. The effect of radiation trapping on the spectroscopic parameters was discussed. Green up-conversion fluorescence bands centered at wavelengths of 530 nm and 550 nm were observed when the crystal was excited at 977 nm. The possible up-conversion mechanisms were proposed.

Spectroscopic properties of Yb 3+ ions in La 2(WO 4) 3 crystal

Yb 3+-doped La 2(WO 4) 3 single crystals were grown by the Czochralski technique. Absorption and fluorescence spectra of the crystal were recorded at the room temperature. The stimulated emission cross-sections of Yb 3+ ions were calculated using the reciprocity method and Fuchtbauer–Ladenburg formula, respectively. The fluorescence decay curves of 2F 5/2 manifold of Yb 3+ ions were recorded at room temperature for both crystal and powder samples. The effect of radiation trapping on the spectroscopic properties is discussed. Comparison with other Yb 3+-doped laser crystals is made. The results show that Yb 3+:La 2(WO 4) 3 crystal is a promising laser material.