Volume 108, Issue 2, 15 July 2010
Index of content:
- Lasers, Optics, and Optoelectronics
108(2010); http://dx.doi.org/10.1063/1.3460278View Description Hide Description
Spectrally-, polarization-, and time-resolved photoluminescence (PL) experiments have been performed on 2.5 nm thick -plane single InGaN quantum wells. It has been found that PL decay is mainly determined by nonradiative recombination through several types of recombination centers, while PL rise is largely affected by exciton transfer into localization minima. Prolonged PL rise times and time-dependent spectral shift were used to study exciton transfer into the localization centers. Characteristic time of the exciton transfer is 80–100 ps at lower temperatures and about 50 ps at room temperature, which corresponds to the exciton diffusion length of 200–500 nm. Degree of PL linear polarization was found to decrease at a similar rate. Decreased PL polarization for the localized excitons suggests that the localization centers are related to areas with relaxed strain.
108(2010); http://dx.doi.org/10.1063/1.3464257View Description Hide Description
The glasses of the composition and with x varying from 2 to 10 mol % have been synthesized. Optical absorption and fluorescence spectra (in the spectral range 350–2100 nm were studied at ambient temperature. The Judd–Ofelt theory was applied to characterize the absorption and luminescence spectra of and ions in these glasses. Following the luminescence spectra, various radiative properties like transition probability , branching ratio , and the radiative life time for blue (B), green (G), and red (R) emission levels of these glasses have been evaluated. The radiative life times for these transitions of and have also been measured. The variations observed in these parameters were discussed in the light of varying coordinations (tetrahedral and octahedral positions) of ions in the glass network. The energy transfer between the two rare earth ions ( and ) in codoped glass system in the visible and near infrared (NIR) regions has also been investigated as a function of varying concentration of . A significant enhancement in the intensities of B, G, and R lines has been observed due to codoping. The quantitative analysis of these results (with the aid of the data on IR and Raman spectral studies) has indicated that the glasses mixed with around 6.0 mol % of is the optimum concentration for yielding the highest quantum efficiency and for maximum energy transfer with low phonon losses.
Role of and concentration on the tunability of green-yellow-red upconversion emission of codoped nanocrystals108(2010); http://dx.doi.org/10.1063/1.3465325View Description Hide Description
Strong green and red visible emissions were obtained from nanocrystals synthesized by sol-gel method and annealed at for 5 h. The average crystallite size was with tetragonal phase for total concentration lower than 3 mol % and cubic phase for concentration higher than 5 mol %. The color coordinate of the upconverted signal was tailored by controlling the dopant composition that change the red/green ratio dominated by the cross relaxation and energy back transfer process as was demonstrated theoretically and confirmed experimentally. Both coefficients were calculated, and , from the theoretical model based on the rate equations. The highest energy transfer efficiency was for 2 mol % of Yb and 2 mol % of . However, for the highest upconverted signal was only obtained for 2 mol % Yb and 1 mol % Er with effective decay time for red and for green band.
Strain relaxation effect on electronic properties of compressively strained InGaAs/InP vertically stacked multiple quantum wires108(2010); http://dx.doi.org/10.1063/1.3456073View Description Hide Description
Electronic properties of compressively strained InGaAs/InP vertically stacked multiple quantum wires were investigated using an six-band strain-dependent Hamiltonian. The strain tensor is found to relax from its initial strain. The amount of relaxation is dependent on the number of wire layers in the vertical stack and increases with the number of wire layers. The interband transition energy also decreases with the number of wire layers. This is mainly attributed to the decrease in the conduction band energy because subband energies in the valence band are nearly independent of the strain. The matrix element is shown to slightly decrease with increasing number of the wire layer in the vertical stack.
108(2010); http://dx.doi.org/10.1063/1.3462400View Description Hide Description
Germanium is a promising material for electrically pumped light emitters integrated on silicon. In this work, we have investigated the room temperature electroluminescence of pure germanium diodes grown by metal organic chemical vapor deposition. The dependence of the optical response of the diodes is studied as a function of the injected current. Both direct and indirect band gap recombinations are observed at room temperature around 1.6 and . The amplitude of the direct band gap recombination is equivalent to the one of the indirect band gap.
108(2010); http://dx.doi.org/10.1063/1.3462438View Description Hide Description
We describe some experimental aspects required for the implementation of a few-order high-harmonic source for coherent diffractiveimaging in the extreme-ultraviolet region. Polychromatic high-angle diffractiveimages have been successfully processed for both periodic and aperiodic inorganic samples using a modified Gerchberg–Saxton algorithm and maximum-entropy-method refinement and reconstructions yielding a resolution of 100 nm can now be achieved at harmonic wavelengths around 32 nm.
108(2010); http://dx.doi.org/10.1063/1.3452376View Description Hide Description
Photonic crystal fiber(PCF) modal interferometer is studied for sensing applications. There is a trade-off between high sensitivity and wide sensing range due to the intrinsic two-beam interference. We propose and experimentally demonstrate a three-beam path reflective interferometer by exciting mode conversion at the end surface of a dual-mode PCF. Approximately 5% mode conversion is taken place at the PCF end surface according to our model. In comparison with the normal dual-mode interference in a PCF, the mode conversion induced three-beam interference generates sharper spectrum and expanded sensing range. Gas sensing experiment is implemented with a sensitivity of 3019 nm/RIU. The advantage of expanded sensing range is also experimentally demonstrated.
108(2010); http://dx.doi.org/10.1063/1.3460645View Description Hide Description
(YAP) crystal with large size and good optical quality has been grown by the Czochralski method. Thermal properties of the as-grown Ho:YAP crystal have been investigated by measuring the temperature-dependent, anisotropicthermal expansion,specific heat,thermal diffusion, and thermal conductivity. The results show that Ho:YAP crystal possesses a large anisotropicthermal expansion and good thermal conductivity. The calculated average thermal expansion coefficients along , , and axis are , , and from 293.15 to 770.15 K. The thermal conductivities along , , and axis are up to 11.6, 9.9, and at 298.15 K. Compared with Ho: (YAG), Ho:YAP crystal has a larger thermal conductivity along axis from 298.15 to 568.15 K.
108(2010); http://dx.doi.org/10.1063/1.3465615View Description Hide Description
The ultraviolet (uv) absorption spectra, representing transitions to all energy levels below of trivalent erbium , have been analyzed for the crystal-field splitting of the multiplet manifolds of in symmetry cation sites in single-crystal cubic and . A solid solution, without a change in the local symmetry, exists between the two compounds, allowing us to identify the weaker transitions in from the stronger transitions observed in the uv spectrum of . As a result, we have identified a complete set of energy (Stark) levels for the electronic configuration up to the absorption band-edge of these crystals. A total of 134 Stark levels representing 30 multiplets with energies as high as have been modeled using a parameterized Hamiltonian defined to operate within the electronic configuration. The crystal-field parameters were determined through use of a Monte Carlo method in which 14 independent crystal-field parameters, , were given random starting values and optimized using standard least-squares fitting between calculated and experimental levels. The final fitting standard deviations between 134 calculated-to-experimental Stark levels are (rms error ) and (rms error ) for in and for in , respectively. The excellent and consistent agreement between the experimental and calculated Stark levels in both crystals, together with the predicted sets of wave functions, are important for the ongoing analyses of intensity data and magneto-optical studies on these crystals.
The contribution of sidewall light extraction to efficiencies of polygonal light-emitting diodes shaped with laser micromachining108(2010); http://dx.doi.org/10.1063/1.3456445View Description Hide Description
The light extraction efficiencies of InGaN/GaN blue light-emitting diodes(LEDs) of different geometries ranging from a triangle to a decagon have been simulated by ray-tracing. The conventional rectangular LED was found to be the most inefficient among the investigated polygons, and light extraction through the device sidewalls was the key factor. The results were experimentally verified by fabricatingLEDs shaped into polygons by nanosecond-pulsed laser micromachining, which proved the simulated results. The mechanism of light extrac-tion in polygonal LEDs is discussed in detail.
108(2010); http://dx.doi.org/10.1063/1.3460806View Description Hide Description
The luminescent properties of the or singly doped and codoped in novel phosphors were investigated, which are prepared by a sol-gel method. dopedphosphor showed a brighter and broader violet-blue color emission band with a maximum peak centered at 389 nm, which is attributed to the parity and spin allowed transition. Photoluminescence spectra reveals that the white color emission is originated from the mixtures of two characteristic emission bands of ion, viz., the 473 nm blue emission and the 580 nm yellow emission . codoping of has enhanced the luminescence of quite significantly upon the UV excitation wavelength (242 nm) and the optimized codopant concentration of is found to be 3 mol %. The mechanism involved in the energy transfer between and has been elucidated by an energy level diagram. The structure and morphology of the prepared samples have been analyzed by x-ray diffraction and transmission electron microscope.
- Plasmas and Electrical Discharges
108(2010); http://dx.doi.org/10.1063/1.3465316View Description Hide Description
The properties of kinetic ablation models are considered in this paper. The widely used kinetic ablation model (model-K) only considers monatomic vapor. A revised model (model-Z) was introduced by taking into account the polyatomic vapor’s internal degrees of freedom, as well as the temperature dependence of the average particle mass. In this work, both temperature and pressure dependence of average particle mass and the specific heat ratio are taken into account, producing an improved version of model-Z (denoted model-). Ablation data calculated by model-K and model- for two typical capillary materials are presented. Compared to model-K, model- predicts an increased ablation rate at lower plasma temperature and higher plasma density, and a decreased rate for the opposite conditions. Finally, based on the plasma parameters in a typical discharge cycle, all three models are used to calculate the time-dependent ablation rate and the integrated ablated mass. It is found that the main difference between their results arises because of the different average particle masses near the wall surface, and model- is the most accurate for the discharge cycle considered. Further, it is found that the ablation parameters are highly sensitive to the pressure, in particular, through the pressure dependence of average particle mass.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
108(2010); http://dx.doi.org/10.1063/1.3445262View Description Hide Description
A combination of microcompression experiments on single crystal micron-scaled pillars of NiTi of known orientations and in situneutron diffraction during loading of the same NiTi but in bulk, polycrystalline form are carried out to understand the stress-induced transformation associated with superelasticity at reduced length scales. At the length scales investigated, there is evidence through this work of a fully reversible stress-induced transformation from to NiTi that does not involve additional dislocation activity or irrecoverable strains. The orientation dependence of the elastic deformation of the phase, the onset of its transformation to the phase, the gradient and the hysteresis in the stress-strain response during transformation, the elastic modulus of the stress-induced phase and the onset of plasticity are quantified and analyzed in these experiments by examining the crystallography of the to transformation.
108(2010); http://dx.doi.org/10.1063/1.3459884View Description Hide Description
We report on the microstructure of silicon coimplanted with hydrogen and helium ions at moderate energies. X-ray diffraction investigations in as-implanted samples show the direct correlation between the lattice strain and implanted ion depth profiles. The measured strain is examined in the framework of solid mechanics and its physical origin is discussed. The microstructure evolution of the samples subjected to intermediate temperature annealing is elucidated through transmission electron microscopy. Gas-filled cavities in the form of nanocracks and spherical bubbles appear at different relative concentration, size, and depth location, depending on the total fluence. These different microstructure evolutions are connected with the surface exfoliation behavior of samples annealed at high temperature , determining the optimal conditions for thick layer transfer. thick Si films are then obtained onto glass substrates.
108(2010); http://dx.doi.org/10.1063/1.3457790View Description Hide Description
Heterogeneous precipitation of nanocrystallites of metallic Bi and anatase was observed in glass-ceramics. Addition of AlN reduced the to Bi metalnanoparticles, which were uniformly dispersed in the glass. After heat-treatment of the Bi-precipitated glass around the glass transition temperature, nanocrystalline anatase precipitated out without aggregation of the Bi metal particles. It was found that the anatase nanocrystal size was affected by the distance between a nanocrystal and a precipitated Bi nanoparticle. The glass-ceramic produced is a functional material containing a random dispersion of different types of nanoparticles with different dielectric constants.
Equivalency principle for magnetoelectroelastic multiferroics with arbitrary microstructure: The phase field approach108(2010); http://dx.doi.org/10.1063/1.3428438View Description Hide Description
A phase field method is proposed to determine the equilibrium fields of a magnetoelectroelastic multiferroic with arbitrarily distributed constitutive constants under applied loadings. This method is based on a developed generalized Eshelby’s equivalency principle, in which the elastic strain, electrostatic, and magnetostatic fields at the equilibrium in the original heterogeneous system are exactly the same as those in an equivalent homogeneous magnetoelectroelastic coupled or uncoupled system with properly chosen distributed effective eigenstrain, polarization, and magnetization fields. Finding these effective fields fully solves the equilibrium elasticity,electrostatics, and magnetostatics in the original heterogeneous multiferroic. The paper formulates a variational principle proving that the effective fields are minimizers of appropriate close-form energy functional. The proposed phase field approach produces the energy minimizing effective fields (and thus solving the general multiferroic problem) as a result of artificial relaxation process described by the Ginzburg–Landau–Khalatnikov kinetic equations.
108(2010); http://dx.doi.org/10.1063/1.3457221View Description Hide Description
Compression-compression fatigue testing of metallic-glass foam is performed. A stress-life curve is constructed, which reveals an endurance limit at a fatigue ratio of about 0.1. The origin of fatigue resistance of this foam is identified to be the tendency of intracellular struts to undergo elastic and reversible buckling, while the fatigue process is understood to advance by anelastic strut buckling leading to localized plasticity (shear banding) and ultimate strut fracture. Curves of peak and valley strain versus number of cycles coupled with plots of hysteresis loops and estimates of energy dissipation at various loading cycles confirm the four stages of foam-fatigue.
108(2010); http://dx.doi.org/10.1063/1.3436592View Description Hide Description
Results of x-ray absorption studies of the structure of amorphousInSb are reported. We demonstrate that approximately 1% bond elongation in the amorphous phase (as compared to the crystalline phase) is accompanied by a counterintuitive increase (approximately 5%) in density. We argue that this controversy is due to the formation of wrong bonds in the amorphous phase with both Sb and In atoms effectively preserving their tetrahedral coordination. Our results additionally offer an alternative interpretation of the semiconductor-metal transition observed upon melting of InSb.
108(2010); http://dx.doi.org/10.1063/1.3452374View Description Hide Description
The crystal structures of the synthesized superhard diamondlike have been extensively explored through ab initio evolutionary algorithm. We uncovered seven intriguing low-energy structures all possessing hybridizations. After examining the dynamical stability, it is found that two structures ( and ) are energetically more preferable. The simulated x-ray diffraction pattern, Raman modes, and Vickers hardness for and structures show remarkable agreement with the experimental data. Electronic and electron–phonon coupling calculations reveal that the two structures are hole conducting and superconducting with critical temperature .
Complex phase compositions in nanostructured coatings as evidenced by photoelectron spectroscopy: The case of Al–Si–N hard coatings108(2010); http://dx.doi.org/10.1063/1.3460099View Description Hide Description
The chemical state evolution of the Al–Si–N thin films at various Si contents is investigated by x-ray photoelectron spectroscopy(XPS). The detailed evolution of the , , and photoelectrons line positions and widths are used to identify different chemical environments as the Si content is changed. The results are compared to x-ray diffraction(XRD) data that indicate the formation of a two-phase composite when the solubility limit of 6 at. % of Si in AlN is exceeded. In contrast to XRD data, no particular effect is observed in the XPS data at the solubility limit of Si. Instead, two compositional regions can be identified that are separated by a distinct change in the evolution of core level binding energy differences and chemical shifts at about 10–15 at. % of Si. This silicon concentration is identified as the onset of the formation of a intergranular phase that is a few monolayers thick, having a chemical bonding similar to that in bulk silicon nitride. The observed changes in the XPS data coincide well with the structural changes in the material at different silicon contents. The unambiguous identification of phases, especially of minority phases from XPS data, is, however, not possible.