Effect Of Radiation Trapping Research Articles

Dissipation-induced stationary entanglement in dipole-dipole interacting atomic samples

The dynamics of two two-level dipole-dipole interacting atoms coupled to a common electro-magnetic bath and closely located inside a lossy cavity, is reported. Initially injecting only one excitation in the two-atom cavity system, loss mechanisms asymptotically drive the matter sample toward a stationary maximally entangled state. The role played by the closeness of the two atoms, with respect to such a cooperative behavior, is carefully discussed. Stationary radiation trapping effects are found and transparently interpreted.

Effects of radiation and charge trapping on the reliability of high-κ gate dielectrics

The radiation response and long term reliability of alternative gate dielectrics will play a critical role in determining the viability of these materials for use in future space applications. The total dose radiation responses of several near and long term alternative gate dielectrics to SiO2 are discussed. Radiation results are presented for nitrided oxides, which show no change in interface trap density with dose and oxide trapped charge densities comparable to ultra thin thermal oxides. For aluminum oxide and hafnium oxide gate dielectric stacks, the density of oxide trapped charge is shown to depend strongly on the film thickness and processing conditions. The alternative gate dielectrics discussed here are shown to have effective trapping efficiencies that are up to ∼15 to 20 times larger than thermal SiO2 of equivalent electrical thickness. A discussion of single event effects in devices and ICs is also provided. It is shown that some alternative gate dielectrics exhibit excellent tolerance to heavy ion induced gate dielectric breakdown. However, it is not yet known how irradiation with energetic particles will affect the long term reliability of MOS devices with high-κ gate dielectrics in a space environment.

Spectroscopic properties of Yb 3+ in bismuth borate glasses

Absorption spectra, fluorescence spectra and fluorescence decay curves have been measured at room temperature. Spectroscopic and laser performance parameters of Yb 3+ in bismuth borate glasses with different compositions and Yb 3+ concentrations were calculated. The effects of the host glasses compositions and Yb 3+ concentration on the spectroscopic and laser performance parameters have been analyzed. The effect of radiation trapping on the spectroscopic properties has also been discussed. The results show that the Yb 3+-doped bismuth borate glass is a promising laser material.

Sensitive detection of radiation trapping in cold-atom clouds

In this paper, we calculate the effect of radiation trapping on the photon statistics of the light scattered from optical molasses. We propose that an intensity correlation function measurement may be sensitive to the presence of radiation trapping at an on-resonance optical depth as low as 0.1, more than an order of magnitude less than where effects of multiple scattering in cold-atom clouds have been previously observed [T. Walker, D. Sesko, and C. Wieman, Phys. Rev. Lett. 64, 408 (1990); D. Sesko, T. Walker, and C. Wieman, J. Opt. Soc. Am. B. 8, 946 (1991)].

Accurate measurement of 1.5 μm lifetime of Er3+ in LiNbO3 crystals and waveguides

Two optimized methods for accurately measuring the 1.5 μm lifetime of Er3+ in LiNbO3 crystals and waveguides are proposed. The first method, suitable for measurement of bulk material, was suggested by forming an annealed proton exchanged sandwich structure and fluorescence was collected through an adjustable aperture. This excitation collection scheme was used to investigate bulk Er-doped LiNbO3 crystals with eight different Er3+ doping levels, three different crystal thicknesses, and three different cut orientations. The 1.5 μm lifetime was investigated in three configurations: with both a sandwich and an aperture, with a sandwich but without an aperture, and without not only a sandwich but also without an aperture. In addition, the effect of the aperture was also studied. The second method, suitable for a waveguide structure, was proposed on the basis of transversely gathering the fluorescence aided by an outcoupling fiber. It was optimized by comparing three different fluorescence collection schemes. Using this method, the lifetimes of some Ti:Er:LiNbO3 strip waveguides were measured. In addition, the effect of radiation trapping on the polarized fluorescence spectrum was also investigated. The experimental results are discussed and summarized.

Effect of radiative trapping on measurement of the spectroscopic properties of Yb 3+:phosphate glasses

The effect of radiative trapping on measurement of the spectroscopic properties of Yb 3+-doped phosphate glasses was investigated as a function of Yb 3+ concentration at different thicknesses. It was found that radiative trapping exists generally in Yb 3+:phosphate glasses, even at low concentration. As a result, the measured lifetime of Yb 3+ in phosphate glasses is usually larger than the calculated one. The maximum discrepancies between them at high concentration are found to be <42%. The calculated lifetime should be used as a reference in determining the true value of the measured lifetime because of it being lengthened largely by radiative trapping. On the other hand, the shape of fluorescence spectrum exhibits remarkable changes due to the radiative trapping. What is more, the intensity increase of Δ λ eff at high concentration is greater than that of low doping. The Δ λ eff increases 36% from 53 to 72 nm with thickness increasing from 0.3 to 4 mm at 6 mol% Yb 2O 3 content, while Δ λ eff increases only 11% when Yb 2O 3 content was fixed at 0.2 mol%.

Effect of radiation trapping on spectroscopic properties of Yb3+-dope d phosphate glasses

The absorption and emission properties of the concentrations and thickness of Yb3+-doped phosphate glasses have been investigated. The integrated cro ss section, absorption cross section, emission cross section, the spontaneous emission probability, and the effective line width of fluorescence of Yb3+- doped phosphate glasses were determined. The effect of the radiation trapping on spectroscopic properties was discussed. It was found that the effect of radiation trapping increases remarkably with the concentration and thickness of Yb3+-doped phosphate glasses. Radiation trapping causes significant lengthening of the measured fluorescence lifetime τf which is longer than the radia tive lifetime τrad. The error in measuring the fluorescence lifetime unde r low Yb3+ ion concentration (0.2mol%Yb2O3) is 3 0% and the error is 43% under high Yb3+ ion concentration (6mol%Yb2O3 ) 6mol%. Radiation trapping also significantly increases the effective line width Δλeff. I t will be increased by 14% and 30% larger under low and high Yb3+ ion concentration respectively.

Magnetic coherence transfer induced by discharge radiation and the effect of radiation trapping on the lifetime of the hidden alignment of Ne terms

The dichroism of a neon gas discharge plasma in a weak magnetic field is investigated by scanning the 3s2-2p4 transition by monochromatic laser radiation. Magneto-optical resonances of the intrinsic macroscopic alignment and hidden alignment of the 2p4 level are separated. Against the background of these resonances, resonances attributed to coherence transfer from the 1s2, 4, 5 levels and the birefringence of the wings of the nearest absorption lines from the 1s4, 5 levels are observed. A new type of alignment is revealed—integral hidden alignment, whose lifetime, in contrast to the case with macroscopic alignment, increases without bound with increasing radiation trapping.

An Urban Surface Exchange Parameterisation for Mesoscale Models

A scheme to represent the impact of urban buildings on airflow in mesoscale atmospheric models is presented. In the scheme, the buildings are not explicitly resolved, but their effects on the grid-averaged variables are parameterised. An urban quarter is characterised by a horizontal building size, a street canyon width and a building density as a function of height. The module computes the impact of the horizontal (roof and canyon floor) and vertical (walls) surfaces on the wind speed, temperature and turbulent kinetic energy. The computation of the shortwave and longwave radiation, needed to compute the temperature of the urban surfaces, takes into account the shadowing and radiation trapping effects induced by the urban canyons. The computation of the turbulent length scales in the TKE equation is also modified to take into account the presence of the buildings. The parameterisation is introduced into a mesoscale model and tested in a bidimensional case of a city over flat terrain. The new parameterisation is shown to be able to reproduce the most important features observed in urban areas better than the traditional approach which is based only on the modification of the roughness length, thereby retaining the Monin-Obukhov similarity theory. The new surface exchange parameterisation is furthermore shown to have a strong impact on the dispersion characteristics of air pollutants in urban areas.

Two-dimensional self-consistent radiation transport model for plasma display panels

A two-dimensional radiation transport model is coupled with a fluid simulation to incorporate the resonance radiation trapping effect in a plasma display panel cell. Compared with the conventional trapping factor approach, this model has an advantage in describing the spatial evolution of the radiative excited-state density. Compared with a Monte Carlo model, it also takes advantage of its fast computation to couple the radiation transport self-consistently with the time-dependent fluid model. The effect of the spatial evolution of the resonant excited state on the light emission is investigated for the variations of system sizes and the gas mixture ratio, and the results are compared with those of the conventional trapping factor approach. The discrepancy between the two methods increases as the gap size between the dielectrics increases, but does not change significantly for the variation of the gas mixture ratio.

Nonradiative Energy Losses and Radiation Trapping in Neodymium‐Doped Phosphate Laser Glasses

Fluorescence radiation trapping and nonradiative energy losses from the Nd3+4F3/2 state are reported for two widely used commercial phosphate laser glasses (LHG‐8 and LG‐770). The effects of hydroxyl‐group, transition‐metal (Cu, Fe, V, Co, Ni, Cr, Mn, and Pt), and rare‐earth (Dy, Pr, Sm, and Ce) impurities on the 4F3/2 nonradiative decay rate in these glasses are quantified. Nd concentration quenching effects are reported for doping levels ranging from about 0.5 × 1020 to 8.0 × 1020 ions/cm3. The results are analyzed using the Förster–Dexter theory for dipolar energy transfer. Quenching rates for transition‐metal ions correlate with the magnitude of spectral overlap for Nd emission (donor) and the metal ion absorption (acceptor). The nonradiative decay rates due to hydroxyl groups follow Förster–Dexter theory except at low Nd‐doping levels (≲2 × 1020 ions/cm3) where the quenching rate becomes independent of the Nd concentration. The data suggest a possible correlation of OH sites with Nd ions in this doping region. The effects of radiation trapping on the fluorescence decay are reported as a function of sample size, shape, and doping level. The results agree well with the theory except for samples with small doping‐length products; in these cases, multiple internal reflections from the sample surfaces enhance the trapping effect.

Escape factors for laser-plasmas

A method of parameterizing escape factors (transmission factors and net radiative brackets) for conditions typical of laser-produced plasmas is introduced. The assumptions of planar geometry, exponentially decreasing emissivity and absorption coefficient with distance with a step rise at a particular point, and spatially constant Doppler broadened line profiles have been made. The effect of velocity gradients in spectrally shifting the absorption and emission line profiles relative to each other is taken into account assuming linear velocity gradients with distance. A parameter R representing the ratio of the spatial scale-length of the absorption coefficient to the Doppler decoupling length is introduced. Fitting formulae for transmission factors and net radiative brackets are given which are valid for all R and all optical depths. In the limit of small R (large Doppler decoupling length), the escape factors asymptotically approach formulae developed originally by, for example, Holstein assuming negligible plasma velocities. For large R (small Doppler decoupling length), the escape factors have been shown to asymptotically approach the Sobolev approximation. The parameterized net radiative bracket has been used in the hydrodynamic and atomic physics code ‘EHYBRID’ for the calculation of the effect of radiation trapping on population densities in laser-produced plasmas. The output of the modified EHYBRID code has then been post processed using the parameterised transmission factors to simulate 123 Ne-like and 399 F-like germanium resonance line intensities emitted in typical X-ray laser experiments. We obtain an agreement between the simulated and experimental spectra.

Evaluation of spectroscopic properties of Yb 3+ in tetraphosphate glass

The tetraphosphate glasses with various Yb 3+ concentrations were prepared. The absorption and fluorescence emission spectra were measured. The spectroscopic properties and laser performance parameters of Yb 3+ in tetraphosphate glasses were calculated. The result shows that Yb 3+-doped tetraphosphate glass exhibits excellent spectroscopic properties. The disagreement between the reciprocity method (RE) and Fuchtbauer–Ladenburg formula in calculations of Yb 3+ spectroscopic properties was discussed using the radiation trapping effect. The effects of OH − contents on emission properties of Yb 3+ were investigated.

Relaxation mechanism of optically pumped Rb vapor of high density in strong magnetic fields

The relaxation rates of the polarized rubidium (Rb) vapor were measured by using a chopped pumping laser to investigate the depolarization mechanism of a high density Rb vapor in a strong magnetic field. The measurement was carried out in a range of the magnetic field from 1 to 5 T and the Rb cell temperature from 90 to 130 °C which corresponds to the Rb vapor density from 0.2 to 2.0×10 13 at./cm 3 . The position dependence of the relaxation rates was also measured along the Rb cell axis. The experimental results showed that two components, i.e. a fast component with the relaxation rate of ∼ 65 ms −1 and a slow component with the relaxation rate of ∼ 1 ms −1 contribute. The observed relaxation rates were analyzed by the wall relaxation model, the effusion model and the radiation trapping model. It was found that the slow component was well explained by the wall relaxation and the effusion effect. On the other hand, the fast component is due to a formation of the localized Rb polarization around the pumping laser by a radiation trapping effect and a subsequent relaxation process due to an effusion.

A radiation transport coupled particle-in-cell simulation. II. Simulation results in a one-dimensional planar model

The radiation-transport coupled particle-in-cell model is applied to one-dimensional planar model to investigate the radiation trapping phenomena in relatively high- and low-pressure Ar glow discharges. The radiation intensity spectra and the total radiation flux are calculated from the radiative excited-state profile. The simulation results for Doppler and resonance collision broadenings are compared with the eigenmode description of the Holstein equation, with good agreement. Observed are the effects of radiation trapping of photons, diffusion, collisional quenching, and step ionization from excited states on the discharge properties.

Spectroscopic properties of erbium-doped ultraphosphate glasses for 1.5 μm amplification

The spectroscopic properties of P2O5–Y2O3–Er2O3 and P2O5–Li2O–Al2O3–Y2O3–Er2O3 ultraphosphate glasses have been investigated. The concentration quenching rates of Er3+ in these two systems are lower compared with those in other oxide glasses. The spectral broadening of the measured fluorescence of 83.33P2O5–(16.67−x)Er2O3–xY2O3 (x=0.1 and 16.67) was observed by increasing temperature. Meanwhile, the lifetime of the I13/24 level shows the temperature dependence with increasing temperature, ascribed to the possible thermalization of higher-lying sublevels of the I13/24 level with faster radiative rates and the effect of radiation trapping. At room temperature, the deformation of the emission spectrum line shape has been observed with the increase of Er2O3 concentration due to the reabsorption effect. The 1.5-μm-amplification characteristics of P2O5–Y2O3–Er2O3 ultraphosphate glasses have been predicted from the calculated gain spectra at different pump powers.

Solution of the Holstein equation of radiation trapping by the geometrical quantization technique. III. Partial frequency redistribution with Doppler broadening

We introduce an analytical method to investigate radiation trapping problems with Doppler frequency redistribution. The problem is formulated within the framework of the Holstein-Biberman-Payne equation. We interpret the basic integro-differential trapping equation as a generalized wave equation for a four-dimensional (4D) classical system (an associated quasiparticle). We then construct its analytical solution by a semiclassical approach, called the geometrical quantization technique (GQT). Within the GQT, it is shown that the spatial and frequency variables can be separated and that the frequency part of the excited atom distribution function obeys a stationary Schr\"odinger equation for a perturbed oscillator. We demonstrate that there is a noticeable deviation of the actual spectral emission profile from the Doppler line in the region of small opacities. The problem of calculating the spatial mode structure and the effective radiation trapping factors is reduced to the evaluation of wave functions and quantized energy values of the quasiparticle confined in the vapor cell. We formulate the quantization rules and derive the phase factors, which allow us to obtain analytically the complete spectrum of the trapping factors in 1D geometries (layer, cylinder, sphere) and other (2D and 3D) geometries when the separation of space variables is possible. Finally, we outline a possible extension of our method to treat radiation trapping effects for more general experimental situations including, for instance, a system of cold atoms.

SPECTROSCOPIC PROPERTIES OF Yb3+-DOPED TETRAPHOSPHATE GLASS

The tetraphosphate glasses with various Yb3+ dopant were prepared. The absorption and fluorescence emission spectra were measured. The spectroscopic properties and laser performance parameters of Yb3+ in tetraphosphate glasses were calculated. The disagreement between the reciprocity method and Fuchtbauer-Ladenburg formula was discussed using the radiation trapping effect. The effects of Yb3+ and OH- contents on emission properties were investigated. The result shows that Yb3+-doped tetraphosphate glass exhibits excellent spectroscopic properties.

Spectroscopic study of ionizing and recombining plasma in a stationary plasma source

A hot cathode, which is composed of La2O3–W needles, has been constructed and tested in a modified arc discharge source in order to produce a high-density plasma by large discharge current. No damage of needles was found after 100 h at a discharge current of 300 A. Characteristics of generated helium plasmas have been investigated by spectroscopic observations. The electron density has been found to be ≧2×1014 cm−3 at a discharge current 100 A with this cathode. Experimental results were analyzed with calculations based on a collisional radiative model including an effect of radiation trapping. As a result, generated plasmas were shown to be classified into two groups by the pressure of neutral helium, that is, an ionizing plasma and recombining plasma. In low temperature recombining plasma, population inversions between He+ ion n=2 and the higher excited levels have been obtained.

Collisional quenching of CO B 1Σ+(v′=0) probed by two-photon laser-induced fluorescence using a picosecond laser

We report measurements of room-temperature, species-specific quenching cross sections of CO B 1Σ+ (v′=0) in collisions with He, Ne, H2, N2, Ar, CO, Kr, CH4, O2, Xe, CO2, C3H8, and H2O. The measured quenching cross sections (in Å2) were 0.25±0.02, 0.54±0.04, 11.0±0.4, 24.6±0.5, 27.7±0.5, 37±2, 42±2, 81±4, 85±5, 99±6, 133±5, 144±7, and 170±8, respectively. Two-photon excitation of the CO molecules via the Hopfield–Birge system (X 1Σ+→→B 1Σ+) was performed using the frequency-tripled 690 nm emission of a custom-built picosecond dye laser. Blue-to-green fluorescence in the Angström bands (B 1Σ+→A 1Π) was detected using a microchannel-plate photomultiplier tube and recorded with a digital storage oscilloscope. The quenching cross sections were directly obtained by time resolving the temporal decay of the fluorescence signal and observing its variation as a function of the quencher pressure. The effect of radiative trapping on the observed fluorescence was also quantitatively modeled.