Direct measurement of coherency limits for strain relaxation in heteroepitaxial core/shell nanowires

The growth of heteroepitaxially strained semiconductors at the nanoscale enables tailoring of material properties for enhanced device performance. For core/shell nanowires (NWs), theoretical predictions of the coherency limits and the implications they carry remain uncertain without proper identification of the mechanisms by which strains relax. We present here for the Ge/Si core/shell NW system the first experimental measurement of critical shell thickness for strain relaxation in a semiconductor NW heterostructure and the identification of the relaxation mechanisms. Axial and tangential strain relief is initiated by the formation of periodic a/2 〈110〉 perfect dislocations via nucleation and glide on {111} slip-planes. Glide of dislocation segments is directly confirmed by real-time in situ transmission electron microscope observations and by dislocation dynamics simulations. Further shell growth leads to roughening and grain formation which provides additional strain relief. As a consequence of core/shell strain sharing in NWs, a 16 nm radius Ge NW with a 3 nm Si shell is shown to accommodate 3% coherent strain at equilibrium, a factor of 3 increase over the 1 nm equilibrium critical thickness for planar Si/Ge heteroepitaxial growth. © 2012 American Chemical Society.

A study of the properties and applications of electrostatic charged particle oscillators

This investigation originated from work by Dr. A.H. McIlraith of the National Physical Laboratory who, in 1966, described a new type of charged particle oscillator. This makes use of two equal cylindrical electrodes to constrain the particles in such a way that they follow extremely long oscillatory paths between the electrodes under the influence of an electrostatic field alone. The object of this work has been to study the principle of the oscillator in detail and to investigate its properties and applications. Any device which is capable of creating long electron trajectories has potential application in the field of ultra high vacuum technology. It was therefore considered that a critical review of the problems associated with the production and measurement of ultra high vacuum was relevant in the initial stages of the work. The oscillator has been applied with a considerable degree of success as a high energy electrostatic ion source. This offers several advantages over existing ion sources. It can be operated at much lower pressures without the need of a magnetic field. The oscillator principle has also been applied as a thermionic ionization gauge and has been compared with other ionization gauges to pressures as low as 5 x 10- 11 torr.. This new gauge exhibited a number of advantages over most of the existing gauges. Finally the oscillator has been used in an evaporation ion pump and has exhibited fairly high pumping speeds for argon gas relative to those for nitrogen. This investigation supports the original work of Dr. A.H. McIlraith and shows that his proposed oscillator has considerable potential in the fields of vacuum technology and electron physics.

Dielectric studies of molecules and intramolecular relaxation processes

A new bridge technique for the measurement of the dielectric absorption of liquids and solutions at microwave frequencies has been described and its accuracy assessed. 'l'he dielectric data of the systems studied is discussed in terms of the relaxation processes contributing to the dielectric absorption and the apparent dipole moments. Pyridine, thiophen and furan in solution have a distribution of relaxation times which may be attributed to the small size of the solute molecules relative to the solvent. Larger rigid molecules in solution were characterized by a single relaxation time as would be anticipated from theory. The dielectric data of toluene, ethyl-, isopropyl- and t-butylbenzene as pure liquids and in solution were described by two relaxation times, one identified with molecular re-orientation and a shorter relaxation time.· The subsequent work was investigation of the possible explanations of this short relaxation process. Comparable short relaxation times were obtained from the analysis of the dielectric data of solutions of p-chloro- and p-bromotoluene below 40°C, o- and m-xylene at 25°C and 1-methyl- and 2 methylnaphthalene at 50 C. Rigid molecules of similar shapes and sizes were characterized by a single relaxation time identified with molecular re-orientation. Contributions from a long relaxation process attributed to dipolar origins were reported for solutions of nitrobenzene, benzonitrile and p-nitrotoluene. A short relaxation process of possible dipolar origins contributed to the dielectric absorption of 4-methyl- and 4-t-butylpyridine in cyclohexane at 25°C. It was concluded that the most plausible explanation of the short relaxation process of the alkyl-aryl hydrocarbons studied appears to be intramolecular relaxation about the alkyl-aryl bond. Finally the mean relaxation times of some phenylsubstituted compounds were investigated to evaluate any shortening due to contributions from the process of relaxation about the phenyl-central atom bond. The relaxation times of triphenylsilane and phenyltrimethylsilane were significantly short.

Study of synchronisation in chains of coupled non-linear oscillators and resonators

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Emergent behaviour in arrays of coupled nonlinear oscillators

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Statistics of noise-driven coupled nonlinear oscillators:applications to systems with Kerr nonlinearity

We present exact analytical results for the statistics of nonlinear coupled oscillators under the influence of additive white noise. We suggest a perturbative approach for analysing the statistics of such systems under the action of a deterministic perturbation, based on the exact expressions for probability density functions for noise-driven oscillators. Using our perturbation technique we show that our results can be applied to studying the optical signal propagation in noisy fibres at (nearly) zero dispersion as well as to weakly nonlinear lattice models with additive noise. The approach proposed can account for a wide spectrum of physically meaningful perturbations and is applicable to the case of large noise strength. © 2005 Elsevier B.V. All rights reserved.

Real and imaginary parts of the vibrational relaxation of acetonitrile in its electrolyte solutions:new results for the dynamics of solvent molecules

Isotropic scattering Raman spectra of liquid acetonitrile (AN) solutions of LiBF4 and NaI at various temperatures and concentrations have been investigated. For the first time imaginary as well as real parts of the solvent vibrational correlation functions have been extracted from the spectra. Such imaginary parts are currently an important component of modern theories of vibrational relaxation in liquids. This investigation thus provides the first experimental data on imaginary parts of a correlation function in AN solutions. Using the fitting algorithm we recently developed, statistically confident models for the Raman spectra were deduced. The parameters of the band shapes, with an additional correction, of the ν2 AN vibration (CN stretching), together with their confidence intervals are also reported for the first time. It is shown that three distinct species, with lifetimes greater than ∼10−13 s, of the AN molecules can be detected in solutions containing Li+ and Na+. These species are attributed to AN molecules directly solvating cations; the single oriented and polarised molecules interleaving the cation and anion of a Solvent Shared Ion Pair (SShIP); and molecules solvating anions. These last are considered to be equivalent to the next layer of solvent molecules, because the CN end of the molecule is distant from the anion and thus less affected by the ionic charge compared with the anion situation. Calculations showed that at the concentrations employed, 1 and 0.3 M, there were essentially no other solvent molecules remaining that could be considered as bulk solvent. Calculations also showed that the internuclear distance in these solutions supported the proposal that the ionic entity dominating in solution was the SShIP, and other evidence was adduced that confirmed the absence of Contact Ion Pairs at these concentrations. The parameters of the shape of the vibrational correlation functions of all three species are reported. The parameters of intramolecular anharmonic coupling between the potential surfaces in AN and the dynamics of the intermolecular environment fluctuations and intermolecular energy transfer are presented. These results will assist investigations made at higher and lower concentrations, when additional species and interactions with AN molecules will be present.

Transient response in high-resolution Brillouin-based distributed sensing using probe pulses shorter than the acoustic relaxation time

We perform numerical simulations on a model describing a Brillouin-based temperature and strain sensor, testing its response when it is probed with relatively short pulses. Experimental results were recently published [e.g., Opt. Lett. 24, 510 (1999)] that showed a broadening of the Brillouin loss curve when the probe pulse duration is reduced, followed by a sudden and rather surprising reduction of the linewidth when the pulse duration gets shorter than the acoustic relaxation time. Our study reveals the processes responsible for this behavior. We give a clear physical insight into the problem, allowing us to define the best experimental conditions required for one to take the advantage of this effect.

Impact of the carrier relaxation paths on two-state operation in quantum dot lasers

We study InGaAs QD laser operating simultaneously at ground (GS) and excited (ES) states under 30ns pulsed-pumping and distinguish three regimes of operation depending on the pump current and the carrier relaxation pathways. An increased current leads to an increase in ES intensity and to a decrease in GS intensity (or saturation) for low pump range, as typical for the cascade-like pathway. Both the GS and ES intensities are steadily increased for high current ranges, which prove the dominance of the direct capture pathway. The relaxation oscillations are not pronounced for these ranges. For the mediate currents, the interplay between the both pathways leads to the damped large amplitude relaxation oscillations with significant deviation of the relaxation oscillation frequency from the initial value during the pulse.

Molecular alignment relaxation in polymer optical fibers for sensing applications

A systematic study of annealing behavior of drawn PMMA fibers was performed. Annealing dynamics were investigated under different environmental conditions by fiber longitudinal shrinkage monitoring. The shrinkage process was found to follow a stretched exponential decay function revealing the heterogeneous nature of the underlying molecular dynamics. The complex dependence of the fiber shrinkage on initial degree of molecular alignment in the fiber, annealing time and temperature was investigated and interpreted. Moreover, humidity was shown to have a profound effect on the annealing process, which was not recognized previously. Annealing was also shown to have considerable effect on the fiber mechanical properties associated with the relaxation of molecular alignment in the fiber. The consequences of fiber annealing for the climatic stability of certain polymer optical fiber-based sensors are discussed, emphasizing the importance of fiber controlled pre-annealing with respect to the foreseeable operating conditions.

Energy scaling of Kerr-lens mode-locked thin-disk oscillators

Geometric scaling of a Kerr-lens mode-locked Yb:YAG thin-disk oscillator yields femtosecond pulses with an average output power of 270 W. The scaled system delivers femtosecond (210-330 fs) pulses with a peak power of 38 MW. These values of average and peak power surpass the performance of any previously reported femtosecond laser oscillator operated in atmospheric air.