Volume 113, Issue 17, 07 May 2013
Index of content:
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4803059View Description Hide Description
The effect of axial energy spread on the radiation of third harmonic is studied in the free electron laser with planar wiggler and ion-channel guiding. Spread in the longitudinal momentum and so in the initial energy of electron beam, without any spread in the transverse velocity, is assumed in the form of Gaussian distribution function. The technique that is employed is a one-dimensional and steady-state simulation. A set of self consistent nonlinear differential equations that describes the system is solved numerically by Runge-Kutta method. Due to the sensitivity of harmonics to thermal effects, gain improvement of third harmonic radiation is achieved by using ion-channel guiding technique and efficiency enhancement is applied by tapering the magnetic field of wiggler to optimize radiation. The bunching parameter of the electron beam is also studied. It is found that the growth of the magnitude of the bunching parameter that is caused by the ponderomotive wave stops before the saturation point of the radiation. This means that ponderomotive wave saturates at a shorter distance compared to the radiation.
- Plasmas and Electrical Discharges
Temporal and spatial effects of ablation plume on number density distribution of droplets in an aerosol measured by laser-induced breakdown113(2013); http://dx.doi.org/10.1063/1.4803677View Description Hide Description
We proposed and experimentally demonstrated a novel method of evaluating the number density of droplets in an aerosol by laser-induced breakdown. The number density of droplets is evaluated from the volume in which the laser intensity exceeds the breakdown threshold intensity for droplets, and the number of droplets in this volume, which is evaluated by the experimentally observed breakdown probability. This measurement method requires a large number of laser shots for not only precise measurement but also highly temporally and spatially resolved density distribution in aerosol. Laser ablation plumes ejected from liquid droplets generated by breakdown disturb the density around the measurement points. Therefore, the recovery time of the density determines the maximum repetition rate of the probe laser irradiating a fixed point. The expansion range of the ablation plume determines the minimum distance at which the measurement points are unaffected by a neighboring breakdown when multiple laser beams are simultaneously irradiated. These laser irradiation procedures enable the measurement of the number density distribution of droplets in an aerosol at a large number of points within a short measurement time.
Effect of laser light polarization on generation of relativistic ion beams driven by an ultraintense laser113(2013); http://dx.doi.org/10.1063/1.4803709View Description Hide Description
The effect of laser light polarization on properties of proton and carbon ion beams generated from a CH target irradiated by a 130 fs laser pulse of ultra-relativistic intensity (∼1022–1023 W/cm2) is investigated using particle-in-cell simulations. It is shown that only circular light polarization ensures the production of quasi-monoenergetic relativistic beams of both protons and carbon ions from such a target while using the linear one results in the generation of quasi-monoenergetic protons accompanied with carbon ions of complex and broad energy spectrum. The influence of the target thickness and laser intensity on the ion energy spectrum and the laser-ions energy conversion efficiency is examined.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
113(2013); http://dx.doi.org/10.1063/1.4803124View Description Hide Description
We have studied the structural and electronic properties of Ce2Ti2O7 (CeTO) and Pr2Ti2O7 (PrTO) by first-principles density functional theory calculations. The computed structural parameters are in fairly good agreement with the available experimental findings. Band structure calculations using the GGA+U approach predict an insulating ground state for the herein studied compounds. The insulating band gaps of 2.00 eV and 2.83 eV are found for CeTO and PrTO, respectively. The analysis of the density of states reveals that the strongly localized RE 4f levels act as charge-trapping sites, predicting a lower photocatalytic activity for CeTO. We have also calculated the optical properties for both CeTO and PrTO. Based on these properties, it is predicted that these titanates are insensitive to ultra-violet radiation, while they are more sensitive to frequencies of the radiation in visible and early UV regions.
113(2013); http://dx.doi.org/10.1063/1.4803152View Description Hide Description
“Black silicon” layers were formed by catalytic etching of Au/Si(100) wafers in HF−H2O2−H2O solutions at room temperature. The structural and optical properties of the catalytic-etched Si layers were investigated by scanning electron microscopy (SEM), wettability observations, Fourier-transform infrared (FTIR) spectroscopy analysis, near-IR−UV transmittance, Raman scattering, photoluminescence (PL), PL excitation, and PL decay measurements. The SEM observation suggested that the vertically well-aligned Si nanowires can be formed in the limited synthesis conditions (H2O2 concentration, deposited Au film thickness, and etching time). FTIR and near-IR−UV transmittance spectra revealed that the catalytic-etched Si layers show optical absorbance about two orders higher in the far-IR−UV region than that for the single-crystalline Si substrate. The Raman scattering spectra were found to be clearly different from those for the bulk single-crystalline Si and were analyzed using a newly developed model. All the catalytic-etched Si samples showed efficient visible emission at ∼2 eV. This emission can be explained by the quantum-mechanical confinement effect, i.e., a relaxation of the momentum conservation at and above the indirect-absorption edge of Si (supra- emission).
Delayed emission from InGaAs/GaAs quantum dots grown by migration-enhanced epitaxy due to carrier localization in a wetting layer113(2013); http://dx.doi.org/10.1063/1.4803493View Description Hide Description
Wetting layer (WL) photoluminescence (PL) at 10 K dominated the PL spectra of low-density quantum dots (QDs) grown by migration-enhanced epitaxy (MEE), even at very low excitation powers. Long PL rise time at the ground state (GS) of QDs was observed, when carriers are generated in the WL, indicating suppressed carrier capture from the WL into the QDs. Fluctuations in the WL thickness due to WL thinning in the MEE-grown QDs produced strong localization effects. Temperature dependence of the WL PL intensity and the GS PL rise time agreed well with this interpretation.
113(2013); http://dx.doi.org/10.1063/1.4803053View Description Hide Description
Eu3+ doped and Eu3+, Yb3+ co-doped Gd3Ga5O12 phosphors have been developed by facile solid state reaction method which can be easily scaled-up in large quantity. The synthesis has been optimized to get a single phase material at 1300 °C. The phase and crystal parameters have been analyzed by using X-ray diffraction measurement. Photoluminescence excitation (monitored for the 5D0 → 7F1 transition of Eu3+) depicts that the active ion (Eu3+) can be excited through direct excitation into 4f band of Eu3+, through charge transfer band (Eu3+-O2−) excitation and also through the excitation into 8S7/2 → 6IJ intra f–f transition of Gd3+ ion, which significantly all together cover a broad excitation region in 200–420 nm. In addition, in the presence of Yb3+ ions, the emission is also achieved by near infrared excitation (976 nm), through a typical upconversion (UC) process. Thus, the material efficiently behaves as a dual mode emitting phosphor (emission is achieved both through normal fluorescence and through UC process). The conversion efficiency of silicon solar cells is only 15% of terrestrial solar energy for 200–400 nm region and also the sub-band gap energy (in infrared region) is lost as heat; therefore, this kind of dual mode phosphors may be used to overcome the above mentioned incomplete utilization of the solar spectrum and can open realm of new possibilities for energy harvesting.
Exothermic phenomena and hazardous gas release during thermal oxidation of mesoporous silicon powders113(2013); http://dx.doi.org/10.1063/1.4803680View Description Hide Description
We report on the occurrence of exothermic phenomena during the thermal ramping of mesoporous silicon powders in ambient air. For furnace set temperatures of up to 800 °C, discrete exotherms occur during the initial ramp-up stage from room temperature. With an onset around 200 °C, the powder temperature rapidly self-elevates to significantly beyond the concurrent furnace baseline value and cools thereafter, in most cases over a period of a few minutes. A number of periodically spaced exotherms can occur, depending on both the weight and layout of the powder batch. A broadening and amalgamation of exotherms are observed for large batch sizes, indicating a longer-term retention of induced thermal energy, in one case with the powder temperature rising well beyond 1000 °C and being maintained for up to 80 min. We associate these exotherms with dehydrogenation processes, some of which may involve N–H as well as Si–H bonds. Oxidation is accompanied by the release of silanes and hydrogen, care therefore being required to avoid combustion of these pyrophoric gases.
113(2013); http://dx.doi.org/10.1063/1.4803153View Description Hide Description
Folded graphene nanoribbons (FGNRs) have attracted great attentions because of extraordinary properties and potential applications. The atomic structure, stacking sequences, and electronic structure of FGNRs are investigated by first-principle calculations. It reveals that the common configurations of all FGNRs are racket-like structures including a nanotube-like edge and two flat nanoribbons. Interestingly, the two flat nanoribbons form new stacking styles instead of the most stable AB-stacking sequences for flat zone. The final configurations of FGNRs are greatly affected by the initial interlayer distance, stacking sequences, and edge styles. The stability of folded graphene nanoribbon depends on the length, and it can only be thermodynamically stable when it reaches the critical length. The band gap of the folded zigzag graphene nanoribbons becomes about 0.17 eV, which provides a new way to open the band gap.
Efficient 2.0 μm emission in Nd3+/Ho3+ co-doped tungsten tellurite glasses for a diode-pump 2.0 μm laser113(2013); http://dx.doi.org/10.1063/1.4803043View Description Hide Description
We report on an efficient 2.0 μm emission from Ho3+ via Nd 3+ sensitization in tungsten tellurite glasses upon excitation with an 808 nm laser diode. The underlying mechanism is analyzed by means of photoemission spectroscopy and lifetime measurement. Judd-Ofelt intensity parameters, radiative properties, and emission cross section of Ho3+ are evaluated. Nd 3+ with high absorption cross section near 808 nm (2.64 × 10−20 cm2) plays an important role in 2.0 μm emission by transferring pump energy to Ho3+:5I5 level. Meanwhile, only very weak upconversion are obtained. The use of Nd 3+ as a sensitizing ion could open up a possibility to realize high efficient 2.0 μm lasing from Ho3+, which could operate at an 808 nm diode laser pump.
113(2013); http://dx.doi.org/10.1063/1.4803683View Description Hide Description
Photo darkening was observed in epitaxial InGaAs/GaAs quantum dots (QDs). The photoluminescence (PL) intensity of the QDs showed a non-reversible decrease under continuous laser irradiation. The time constants varied from tens of minutes to several hours, depending on the applied laser power. Based on the spectral evolution, it was concluded that the observed phenomenon should originate from laser induced structural damage and a sustained increase of non-radiative recombination rate in the wetting layer. Additionally, according to the PL decay dynamics at different laser powers, it is argued that there should exist other processes that hinder PL degradation at a high laser power.
Nucleation rate reduction through stress relief of thermally annealed hydrogenated amorphous silicon films113(2013); http://dx.doi.org/10.1063/1.4803686View Description Hide Description
The effect of film stress on crystallite nucleation is investigated in 0.11 μm thick, thermally annealed hydrogenated amorphous silicon films. Using a recently developed optical method, the crystallite density is measured as the films are isochronally annealed at 600 °C, which enables the determination of the crystallite nucleation rate. This rate is significantly suppressed around scratches, cleaved film edges, and laser ablated areas, extending laterally as much as 100–150 μm from these regions where the film connectivity is disrupted. μ-Raman measurements of the transverse optical mode of Si demonstrate an accompanying reduction in tensile stress in the regions where nucleation is suppressed. The first measurements of nucleation rate in stress and in stress relieved areas in the same film are presented.
Material properties and modeling characteristics for materials for application in magnetic refrigeration113(2013); http://dx.doi.org/10.1063/1.4803495View Description Hide Description
Compounds of have received attention recently for their use in active magnetic regenerators (AMR) because of their relatively high isothermal entropy change and adiabatic temperature change with magnetization. However, the materials also generally exhibit a significant magnetic and thermal hysteresis, and it is not well understood how the hysteresis will affect performance in a practical AMR device. The amount of hysteresis shown by a material can be controlled to an extent by tuning the processing conditions used during material synthesis; therefore, knowledge of the practical impact of hysteresis is a key element to guide successful material development and synthesis. The properties of a magnetocaloric compound are characterized as a function of temperature and applied magnetic field, and the results are used to assess the effects of hysteresis on magnetocaloric properties. Different methods of building property functions from the measured specific heat, magnetization, and adiabatic temperature change are presented. It is shown that model predictions can be highly dependent on how the properties that are used by the AMR model are calculated.
Assessment of the Holland model for silicon phonon-phonon relaxation times using lattice dynamics calculations113(2013); http://dx.doi.org/10.1063/1.4803514View Description Hide Description
We assess the ability of the Holland model to accurately predict phonon-phonon relaxation times from bulk thermal conductivity values. First, lattice dynamics calculations are used to obtain phonon-phonon relaxation times and thermal conductivities for temperatures ranging from 10 K to 1000 K for Stillinger-Weber silicon. The Holland model is then fitted to these thermal conductivities and used to predict relaxation times, which are compared to the relaxation times obtained by lattice dynamics calculations. We find that fitting the Holland model to both total and mode-dependent thermal conductivities does not result in accurate mode-dependent phonon-phonon relaxation times. Introduction of Umklapp scattering for longitudinal modes resulted in improved prediction of mode-dependent relative contributions to thermal conductivity, especially at high temperatures. However, assumptions made by Holland regarding the frequency-dependence of phonon scattering mechanisms are found to be inconsistent with lattice dynamics data. Instead, we introduce a simple method based on using cumulative thermal conductivity functions to obtain better predictions of the frequency-dependence of relaxation times.
Influence of defect reduction and strain relaxation on carrier dynamics in InGaN-based light-emitting diodes on cone-shaped patterned sapphire substrates113(2013); http://dx.doi.org/10.1063/1.4803515View Description Hide Description
This study investigates optical properties and carrier dynamics of InGaN-based light-emitting diodes grown on cone-shaped patterned sapphire (CSPS) and planar sapphire substrates. Edge-type threading dislocations were dramatically reduced in InGaN multiple quantum wells (MQWs) on CSPS substrates compared to the case of planar substrates. We observed a smaller Stokes shift and enhanced quantum efficiency for CSPS substrates. From time-resolved optical analysis, we found that the non-radiative (radiative) recombination rate of MQWs on CSPS is lower (higher) than that of MQWs on planar substrates, which is consistent with improved crystal quality (strain relaxation) of the MQWs on CSPS.
Multiscale modelling on the shock-induced chemical reactions of multifunctional energetic structural materials113(2013); http://dx.doi.org/10.1063/1.4803712View Description Hide Description
Multifunctional energetic structural materials (MESMs) are usually granular mixtures, which release energy due to exothermic chemical reaction initiated under shock loading conditions. The mesostructure, in terms of the size, shape, and distribution of granular mixture, plays a significant role in chemical reaction and the energy release characteristics of MESMs. However, it is difficult to model such a complex process involving thermal-mechanical-chemical responses, especially the effects of the initial mesostructures. In this paper, a multiscale modelling approach is proposed to simulate the chemical reaction of MESMs under a shock compression. The thermal-mechanical response of MESMs is first obtained from mesoscale simulations. Then, the macroscale thermochemical model for a shock-induced chemical reaction is given, in which the extent of reaction is considered. Finally, the spatial profiles of temperature and pressure from the mesoscale heterogeneous simulation are homogenized into cells as an initial state for chemical reaction and further combined with the thermochemical model in macroscale. Hence this provides insight into thermal-mechanical-chemical responses based on the initial mesostructures. Aluminum/Tungsten/Polytetrafluoroethylene granular mixture is selected to demonstrate the method and the effects of volume fraction and impact velocity on the shock-induced chemical reaction. The multiscale approach developed, which combines the mesoscale simulation and macroscale thermochemical modelling, can be used to predict the shock-induced chemical reaction of MESMs with different mesoscale characteristics over a wide range of impact velocities.
Strain-induced phase transformations under compression, unloading, and reloading in a diamond anvil cell113(2013); http://dx.doi.org/10.1063/1.4803851View Description Hide Description
Strain-induced phase transformations (PTs) in a sample under compression, unloading, and reloading in a diamond anvil cell are investigated in detail, by applying finite element method. In contrast to previous studies, the kinetic equation includes the pressure range in which both direct and reverse PTs occur simultaneously. Results are compared to the case when “no transformation” region in the pressure range exists instead, for various values of the kinetic parameters and ratios of the yield strengths of low and high pressure phases. Under unloading (which has never been studied before), surprising plastic flow and reverse PT are found, which were neglected in experiments and change interpretation of experimental results. They are caused both by heterogeneous stress redistribution and transformation-induced plasticity. After reloading, the reverse PT continues followed by intense direct PT. However, PT is less pronounced than after initial compression and geometry of transformed zone changes. In particular, a localized transformed band of a weaker high pressure phase does not reappear in comparison with the initial compression. A number of experimental phenomena are reproduced and interpreted.
Measurement of the specific heat and determination of the thermodynamic functions of relaxed amorphous silicon113(2013); http://dx.doi.org/10.1063/1.4803888View Description Hide Description
The specific heat, cp, of two amorphous silicon (a-Si) samples has been measured by differential scanning calorimetry in the 100–900 K temperature range. When the hydrogen content is reduced by thermal annealing, cp approaches the value of crystalline Si (c-Si). Within experimental accuracy, we conclude that cp of relaxed pure a-Si coincides with that of c-Si. This result is used to determine the enthalpy, entropy, and Gibbs free energy of defect-free relaxed a-Si. Finally, the contribution of structural defects on these quantities is calculated and the melting point of several states of a-Si is predicted.
Optical absorption and photoreflectance spectroscopy of the single-crystalline chalcopyrite semiconductor AgGaSe2113(2013); http://dx.doi.org/10.1063/1.4803892View Description Hide Description
Optical absorption and photoreflectance (PR) spectra have been measured on the single-crystalline chalcopyrite semiconductor AgGaSe2 for light polarization perpendicular ( E ⊥ c) and parallel to the c-axis ( E ‖ c) at T = 15–300 K. Optical absorption measurements suggest that AgGaSe2 is a direct-gap semiconductor having an optical band gap of E 0 ∼ 1.8 eV at T = 15–300 K. The temperature-dependent PR spectra are obtained at T = 20–300 K in the 1.8–2.5 eV spectral ranges. The lowest band-gap energy E 0 of AgGaSe2 shows unusual temperature dependence at T ≤ 80 K. The resultant temperature coefficients dE 0/dT are positive at T ≤ 70 K and negative above 70 K, and are explained by considering the effects of thermal expansion and electron-phonon interaction. The spin-orbit and crystal-field splitting parameters are also determined to be Δso = 327 meV and Δcr = −288 meV at T = 20 K, respectively.
- Electronic Structure and Transport
Structure, band gap, and Mn-related mid-gap states in epitaxial single crystal (Zn1−xMgx)1−yMnyO thin films113(2013); http://dx.doi.org/10.1063/1.4803141View Description Hide Description
Epitaxial (Zn 1−x Mg x)1−yMnyO thin films were grown on c-Al2O3 substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn 1−x Mg x)1−yMnyO films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn 1−x Mg x)1−yMnyO films can create free carriers and contribute to charge transfer transitions. The conduction band offset ΔEC = 0.13 eV and valence band offset ΔEV = 0.1 eV were obtained for ZnO/Zn0.8 Mg 0.2O heterostructures, which increase to ΔEC = 0.21 eV and ΔEV = 0.14 eV for ZnO/Zn0.7 Mg 0.3O heterostructures.