Volume 113, Issue 8, 28 February 2013
Index of content:
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4793269View Description Hide Description
A theoretical model of the absorption coefficient of quantum dot devices is presented. Both of bound to bound absorption and bound to continuum absorption are taken into consideration in this model which is based on the effective mass theory and the nonequilibrium Greens function formalism. The results of the model have been compared with a published experimental work and a good agreement is obtained. The effects of the dot dimensions and electron filling on the bound to continuum absorption coefficient are also investigated. In general, increasing the dot filling increases the absorption and decreasing the dots dimensions will increase the absorption and move the absorption peak towards longer wavelengths.
Orientation dependence of electronic structure and optical gain of (11N)-oriented III-V-N quantum wells113(2013); http://dx.doi.org/10.1063/1.4793279View Description Hide Description
A ten-band k·p Hamiltonian for III-V-N dilute nitride semiconductor quantum wells (QWs) grown on the (11N)-oriented substrates is presented. The energy dispersion curves, optical transition matrix elements, internal piezoelectric field, and optical gain of InGaAsN/GaAs on the (110), (111), (113), and (11∞)-oriented substrates are investigated including band-anti-crossing, strain, and piezoelectric field effects. The band structures and optical gain are sensitive to the substrate orientation. The fundamental transition energy is the largest for the (111)-oriented QW and the smallest for (11∞)-oriented QW. The absolute values of internal piezoelectric field in the well and barrier layers reach the maximum for the (111)-QW, and zero for the (110) and (11∞)-oriented QWs. There exists an injection current density turning point. When the injection current density is below the turning point, the (111)-oriented QW has the largest peak gain. At the larger injection current density, the (11∞)-oriented QW has the largest peak gain.
Method of extreme surfaces for optimizing geometry of acousto-optic interactions in crystalline materials: Example of LiNbO3 crystals113(2013); http://dx.doi.org/10.1063/1.4792304View Description Hide Description
We suggest a method for optimizing geometry of acousto-optic (AO) interactions in anisotropic crystalline materials. Within the framework of this method, one gets global maximums of AO figure of merit M 2 and their spatial orientations, proceeding from so-called “extreme” indicative surfaces, which are obtained after finding such an acoustic wave propagation direction that maximizes the M 2 parameter for each propagation direction of the incident electromagnetic wave. The method improves earlier indicative surface-based techniques in several aspects, particularly in properly accounting for the momentum conservation condition for the AO diffraction, and yields a higher accuracy in assessing spatial anisotropy of the AO effect. We have constructed the extreme surfaces of LiNbO3 crystals for all possible cases, including those of isotropic/anisotropic AO diffractions and longitudinal/transverse acoustic waves. The anisotropy of the AO figure of merit for LiNbO3 is analyzed for the acoustic frequencies 0.01–2.0 GHz and the light wavelengths 405–1444 nm. The absolute M 2 maximums refer to ‘indirect crystal cuts' and are equal to 26.3 × 10−15 s3/kg at 2 GHz and 405 nm, and 15.4 × 10−15 s3/kg at 0.4 GHz and 1444 nm.
113(2013); http://dx.doi.org/10.1063/1.4793265View Description Hide Description
The Er3+, Eu3+, and Yb3+ codoped Y2O3 phosphors have been synthesized by combustion synthesis process. For the structural information, the XRD analysis of the developed phosphor has been done. The frequency upconversion (UC) emissions in the codoped Y2O3 phosphor on excitation with 980 nm diode laser in the visible region have been performed and explained on the basis of excited state absorption and energy transfer process. The mechanism responsible in UC emissions was observed to involve two photon absorption and efficiency of the UC luminescence is significantly enhanced by introducing the Yb3+ ions. The tunability in colour of emitted radiation has been visualized by chromaticity diagram on increasing power of excitation source. The temperature sensing behaviour of developed phosphor material has been investigated using fluorescence intensity ratio technique.
Aggravated efficiency droop in vertical-structured gallium nitride light-emitting diodes induced by high temperature aging113(2013); http://dx.doi.org/10.1063/1.4790594View Description Hide Description
The present work demonstrates that aging at higher temperatures significantly aggravates “efficiency droop” in the n-side-up vertical-structured GaN-based light-emitting diodes (LEDs). The observed luminous efficiency droop is over 40% at the measuring current of 350 mA. This phenomenon closely relates with creeping of Au 80Sn20 eutectic bonds. On one hand, the plastic deformation accumulated during creeping at higher aging temperatures will make the LED epilayers tensile strained at room temperature. The tensile strain induces a change of the internal quantum efficiency (IQE). The maximum variation of IQE related with strain states was around 20%. On the other hand, creeping under the thermal-mismatching induced tensile stress activates voids' nucleation and growth in the solder bonds. The distribution profile of voids in solder bonds will be mapped on the multiple quantum-well structure in vertical-structured LED chips. Local current densities can be much higher than the average current density used in the calculation of LED's efficiencies. Therefore, the efficiency roll-off value will shift toward the smaller bias direction and the total internal quantum efficiency will decrease as current increases.
113(2013); http://dx.doi.org/10.1063/1.4793592View Description Hide Description
At present, solid thin films are recognized by their well established and mature processing technology that is able to produce components which, depending on their main characteristics, can perform either passive or active functions. Additionally, Si-based materials in the form of thin films perfectly match the concept of miniaturized and low-consumption devices—as required in various modern technological applications. Part of these aspects was considered in the present work that was concerned with the study of optical micro-cavities entirely based on silicon and silicon nitride thin films. The structures were prepared by the sputtering deposition method which, due to the adopted conditions (atmosphere and deposition rate) and arrangement of layers, provided cavities operating either in the visible (at ∼670 nm) or in the near-infrared (at ∼1560 nm) wavelength ranges. The main differential of the work relies on the construction of optical micro-cavities with a reduced number of periods whose main properties can be changed by thermal annealing treatments. The work also discusses the angle-dependent behavior of the optical transmission profiles as well as the use of the COMSOL software package to simulate the micro-cavities.
- Plasmas and Electrical Discharges
113(2013); http://dx.doi.org/10.1063/1.4792665View Description Hide Description
This work presents a physics based circuit model for calculating the total energy dissipated into neutral species for nanosecond pulsed direct current (DC) dielectric barrier discharge (DBD) plasmas. Based on experimental observations, it is assumed that the nanosecond pulsed DBD's which have been proposed for aerodynamic flow control can be approximated by two independent regions of homogeneous electric field. An equivalent circuit model is developed for both homogeneous regions based on a combination of a resistor, capacitors, and a zener diode. Instead of fitting the resistance to an experimental data set, a formula is established for approximating the resistance by modeling plasmas as a conductor with DC voltage applied to it. Various assumptions are then applied to the governing Boltzmann equation to approximate electrical conductivity values for weakly ionized plasmas. The developed model is then validated with experimental data of the total power dissipated by plasmas.
113(2013); http://dx.doi.org/10.1063/1.4793280View Description Hide Description
The ferromagnetic inductively coupled plasma (FICP) source, which is a version of the common inductively coupled plasma sources, has a number of well known advantages such as high efficiency, high level of ionization, low minimal gas pressure, very low required driver frequency, and even a possibility to be driven by single current pulses. We present an experimental study of such a FICP source which showed that above a certain value of the driving pulsed power the properties of this device changed rather drastically. Namely, the plasma became non-stationary and non-uniform contrary to the stationary and uniform plasmas typical for this kind of plasma sources. In this case, the plasma appeared as a narrow dense spike which was short compared to the driving pulse. The local plasma density could exceed the neutral atoms density by a few orders of magnitude. When that happened, the afterglow plasma decay time after the end of the pulse was long compared to an ordinary case with no plasma spike. Experiments were performed with various gases and in a wide range of pressures which enabled us to understand the physical mechanism and derive the parameters responsible for such plasma behavior. A qualitative model of this phenomenon is discussed.
113(2013); http://dx.doi.org/10.1063/1.4793561View Description Hide Description
Axial-oriented and azimuthal-distributed grooves are formed on channel walls of a Hall thruster after the engine undergoes a long-term operation. Existing studies have demonstrated the relation between the grooves and the near-wall physics, such as sheath and electron near-wall transport. The idea to optimize the thruster performance with such grooves was also proposed. Therefore, this paper is devoted to explore the effects of wall grooves on the discharge characteristics of a Hall thruster. With experimental measurements, the variations on electron conductivity, ionization distribution, and integrated performance are obtained. The involved physical mechanisms are then analyzed and discussed. The findings help to not only better understand the working principle of Hall thruster discharge but also establish a physical fundamental for the subsequent optimization with artificial grooves.
113(2013); http://dx.doi.org/10.1063/1.4792033View Description Hide Description
The ablation plume dynamics arising from ablation of silver with a 500 fs, 248 nm laser at ∼2 J cm−2 has been studied using angle-resolved Langmuir ion probe and thin film deposition techniques. For the same laser fluence, the time-of-flight ion signals from femtosecond and nanosecond laser ablation are similar; both show a singly peaked time-of-flight distribution. The angular distribution of ion emission and the deposition are well described by the adiabatic and isentropic model of plume expansion, though distributions for femtosecond ablation are significantly narrower. In this laser fluence regime, the energy efficiency of mass ablation is higher for femtosecond pulses than for nanosecond pulses, but the ion production efficiency is lower.
Effects of finite beam and plasma temperature on the growth rate of a two-stream free electron laser with background plasma113(2013); http://dx.doi.org/10.1063/1.4793503View Description Hide Description
A fluid theory is used to derive the dispersion relation of two-stream free electron laser (TSFEL) with a magnetic planar wiggler pump in the presence of background plasma (BP). The effect of finite beams and plasma temperature on the growth rate of a TSFEL has been verified. The twelve order dispersion equation has been solved numerically. Three instabilities, FEL along with the TS and TS-FEL instabilities occur simultaneously. The analysis in the case of cold BP shows that when the effect of the beam temperature is taken into account, both instable bands of wave-number and peak growth rate in the TS instability increase, but peak growth of the FEL and TS-FEL instabilities decreases. Thermal motion of the BP causes to diminish the TS instability and it causes to decrease the FEL and TS-FEL instabilities. By increasing the beam densities and lowering initial velocities (in the collective Raman regime), growth rate of instabilities increases; however, it has opposite behavior in the Campton regime.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
113(2013); http://dx.doi.org/10.1063/1.4792727View Description Hide Description
By applying non-equilibrium molecular dynamics simulation, it is demonstrated that the thermal conductivity of the reported Si/Ge core-shell nanowires (NWs) can be further reduced by shell doping. The thermal conductivity of Si/Ge0.6Si0.4 core-shell NWs is only about 66% of that of Si/Ge core-shell NWs. By analyzing the participation ratios of eigenmodes, it is revealed that the large reduction in the thermal conductivity of Si/Ge0.6Si0.4 core-shell NWs stems from the strong localization of the phonon modes from 1.0 THz to 2.0 THz and the modes from 9.0 THz to 16.0 THz due to both impurity scattering and interface scattering associated with peculiar structure of shell doped silicon NWs.
113(2013); http://dx.doi.org/10.1063/1.4792731View Description Hide Description
Based on the particle swarm optimization algorithm on crystal structural prediction, we first predict that TaN undergoes a phase transition from the experimental θ-TaN to a hexagonal P63 /mmc structure at 87.5 GPa with volume drop of 1.6%. This hexagonal P63 /mmc structure is isostructural with anti-NiAs and can be quenchable to ambient pressure by further phonon dispersions calculations. The Young's modulus E and shear modulus G as a function of crystal orientation for TaN have thus been systematically investigated. The calculated mechanical properties suggest that the P63 /mmc-TaN is ultra-incompressible and hard due to its high bulk modulus (336 GPa), large shear modulus (214 GPa), originating from a staking of “N-Ta-N” sandwiches layers linked by strong covalent Ta-N bonding.
Multifunctional tunable ultra-broadband visible and near-infrared luminescence from bismuth-doped germanate glasses113(2013); http://dx.doi.org/10.1063/1.4791698View Description Hide Description
Here, we present three facile approaches to achieve wavelength tunable luminescence in the same host material with single dopant, i.e., by modulating doping level, preparation temperature, and atmosphere. Based on these methods, ultra-broadband tunable near-infrared luminescence with the largest full width at half maximum of about 500 nm covering the whole windows of optical communication has been obtained in bismuth-doped germanate glasses. Wavelength tunable luminescence is also observed with the change of excitation wavelength. Systematical strategy was followed to approach the physical origin of the near-infrared luminescence and we proposed that three different bismuth active centers contribute to the near-infrared luminescence in the germanate glasses. A comprehensive explanation for the tunable luminescence is given, combining the concentration, energy transfer, and chemical equilibrium of these active centers in the glasses. With the increase of melting temperatures and the increase of reducing extent of the preparation atmosphere, bismuth species transform from Bi3+ to Bi2+, Bi+, Bi0 and bismuth clusters, and then to bismuth colloid. Of particular interest is that red tunable luminescence was also observed by modulating doping level, preparation atmosphere, and excitation wavelength. Besides, the trapped-electron centers in germanate glasses can interact with bismuth species of high valence states leading to the formation of bismuth active centers of low valence states and the decrease of trapped-electron centers. This tunable ultra-broadband luminescence is helpful for a better understanding of the origin of the near-infrared luminescence in Bi-doped glasses and may have potential applications in varieties of optical devices.
In-plane optical anisotropy induced by asymmetrically δ-doping in (001) GaAs/AlGaAs quantum wells studied by reflectance difference spectroscopy113(2013); http://dx.doi.org/10.1063/1.4790577View Description Hide Description
In-plane optical anisotropy (IPOA) in modulation-doped (001) GaAs/AlGaAs quantum wells (QWs) has been studied by reflectance difference spectroscopy (RDS). By changing the position of the δ-doping layer, we introduce an asymmetric potential into the quantum well system, which results in an additional IPOA. Compared to symmetrically doped and undoped structure, the asymmetrically doped QWs exhibit larger IPOA, which is clearly demonstrated both by the RDS results measured at 80 K and the linear extrapolation of the RDS signal under uniaxial strain measured at room temperature. Numerical calculations within the envelope function framework show that the asymmetric potential induced by asymmetrically doping will introduce additional hole-mixing coefficients. This work demonstrates that the IPOA of QWs can be tailored by changing the delta-doping position.
Density functional theory calculation of the optical properties and topological analysis of the electron density of MBi2B2O7 (M = Ca,Zn) compounds113(2013); http://dx.doi.org/10.1063/1.4792733View Description Hide Description
The topology of the electron density for congruent melting oxyborate Bi2ZnOB2O6 and CaBi2B2O7 is studied in light of the theory of atoms in molecules. All the electron density critical points in the unit cell are systematically calculated. What makes these compounds most interesting is a rich collection of B–O long-distance bond paths. We focus on the study of the asymmetric bonds and basins forming the anisotropic B2O5 groups in these compounds. B2O5 shows transferable contributions to the crystal, with long bond paths. We relate these observations to the strong behavior, which favorites its application to the second harmonic generation field. Wherefore, the analyses of bonding and related optical properties as well as the multipole moments of the CaBi2B2O7 compounds are predicted for the first time. CaBi2B2O7 exhibits some uniaxial dielectric anisotropy resulting in a strong birefringence. We also report calculations of the complex second-order optical susceptibility dispersion for the principal tensor components and evaluate their intra-and inter-band contributions.
113(2013); http://dx.doi.org/10.1063/1.4790629View Description Hide Description
A general approach to determine the acoustic reflection and transmission coefficients of multilayered panels is proposed in this paper. Contrary to the Transfer Matrix Method (TMM), this method does not become unstable for high frequencies or large layer thicknesses. This method is shown to be as general as the TMM and mathematically equivalent. Its principle is to consider a so called Information Vector which contains all the information necessary to deduce the State Vector through a Translation Matrix. The Information Vector is of reduced length compared to that of the State Vector and can be propagated in any layer without involving exponentially growing terms. In addition, this method enables the coupling between any type of physical media as far as proper boundary relations can be written. Moreover, the method does not lead to an enlargement of the systems’ size in the case of interfaces between media of different physical type. Finally, this method can be easily implemented in numerical codes. The method is validated on three cases classically encountered in acoustic problems. However, it is general enough to model any type of multilayered problems in any field of applied physics.
Ligand exchange leads to efficient triplet energy transfer to CdSe/ZnS Q-dots in a poly(N-vinylcarbazole) matrix nanocomposite113(2013); http://dx.doi.org/10.1063/1.4793266View Description Hide Description
Upon exchanging long chain alkylamine ligands with a carbazole terminated fatty acid as 6-(N-carbazolyl)-hexanoic acid (C6) and 11-(N-carbazolyl) undecanoic acid (C11), efficient photoluminescence (PL) of CdSe/ZnS colloidal quantum dots (QDs) was observed upon excitation in the absorption band of the carbazole moiety at 330 nm. This effect, which occurred both in solution and in a poly(N-vinylcarbazole) (PVK) matrix doped with the QDs, is attributed to sensitization of the QDs by PVK and the ligands. More efficient energy transfer was observed in solution for the shorter ligand (C6) capped QDs, due to a shorter average distance between the donor (carbazole) and the acceptor (QD). The binding of C6 and C11 to the QDs was confirmed by 1H solution nuclear magnetic resonance, which showed line broadening of the carbazole signal due to a decrease of the mobility of the carbazoles upon binding to the QDs compared with the sharp lines observed for the free molecules in solution. In doped PVK films, the significant enhancement of the energy transfer to the QD core could also be related to a better miscibility between the QDs and the PVK as confirmed by optical transmission and confocal microscopy images. In contrast to the experiment in solution, the overall energy transfer in the doped films was found more efficient for QDs capped with C11. To study in more detail the energy transfer between the carbazole moieties and the QDs, time-resolved fluorescence measurements were performed for solutions of C6 and C11, capped QDs and PVK films doped with the QDs. In contrast to the large enhancement of the QD emission indicated by steady-state PL spectra, the latter experiments suggested only a relatively low efficiency (19.6% and 10.8%) for singlet transfer from the carbazole ligands to the QDs. This suggests that the enhancement of the QD emission must be largely due to triplet transfer.
113(2013); http://dx.doi.org/10.1063/1.4793281View Description Hide Description
Multifunctional energetic structural materials (MESMs) are a new class of energetic materials, which release energy due to exothermic chemical reactions initiated under shock loading conditions. In order to analyze the impact-initiated process of MESMs, a quasi-sealed test chamber, which was originally developed by Ames [“Vented chamber calorimetry for impact-initiated energetic materials,” in AIAA (American Institute of Aeronautics and Astronautics, 2005), p. 279], is used to study on shock-induced chemical reaction characters at various impact velocities. The impact initiated experiments are involving two typical MESMs, Al/PTFE (polytetrafluoroethylene), W/Zr and inert 2024 Al fragment. The video frames recorded from reactive and inert material impact events have shown the process of late-time after burn phenomena. The total pressure and shock wave reflection at the wall of the test chamber are measured using high frequency gauges. The quasi-pressures inside the test chamber, which is fitting from the total pressure curves, are used to determine the impact initiated reaction efficiencies of MESMs at different impact velocities. A thermochemical model for shock-induced reactions, in which the reaction efficiency is considered, is validated against the experimental data from impact initiation. The results show that the impact velocity plays a significant role in chemical reaction and the energy release characteristics of MESMs. The theoretical calculations correlate reasonably well to the corresponding experimental results, which can be used to predict the reaction results of MESMs over a wide range of pressure satisfactorily.
113(2013); http://dx.doi.org/10.1063/1.4792233View Description Hide Description
The pressure-induced structural transformations of CdS have been investigated using synchrotron x-ray diffraction in a diamond anvil cell up to 104 GPa and the density functional theory calculations. The x-ray diffraction experiments show that CdS is stable with the wurtzite-type structure at ambient conditions. The wurtzite-type phase transforms to NaCl-type structure at 3 GPa, which is followed by a second phase transition at 52.3 GPa. In the diffraction patterns, the peak-splitting is observed, indicating that the high-pressure phase appearing at 52.3 GPa is the Pmmn structure, rather than the Cmcm phase reported earlier. With increasing pressure, the lattice parameter a of the Pmmn phase increases abnormally, contrary to decrease of b and c axes. Our calculations reveal that the abnormal change of the a-axis could be related to the pressure-induced crystal structural change. The bulk modulus (B 0), is 64.3(9) GPa for wurzite-type phase, 105(2) GPa for NaCl-type phase, and 54(4) GPa for the Pmmn phase, respectively.