Characterisation of carbon nano-onions using Raman spectroscopy

Characteristics of the Raman spectrum from carbon onions have been identified in terms of the position of the G peak and appearance of the transverse optic phonon peaks. Five new peaks were observed in the low wavenumber region, at about 1100, 861, 700, 450 and 250 cm(-1). The origins of these peaks are discussed in terms of the phonon density of states (PDOS) and phonon dispersion curves of graphite. The curvature of the graphene planes is invoked to explain the relaxation of the Raman selection rules and the appearance of the new peaks. The Raman spectrum of carbon onions is compared with that of highly oriented pyrolytic graphite (HOPG). The strain of graphene planes due to curvature has been estimated analytically and is used to account for the downward shift of the G peak. (C) 2003 Elsevier Science B.V. All rights reserved.

Structure-flexoelastic properties of bimesogenic liquid crystals

In this paper, we report on the flexoelastic and viscoelastic ratios for a number of bimesogens compounds with the same generic structure. Values are obtained indirectly by measuring the flexoelectro-optic response in the chiral nematic phase. By varying the molecular structure we alter the bend angle, transverse dipole moment, and length of the molecule. First, to examine the influence of the bend angle we use a homologous series whereby the only alteration in the molecular structure is the number of methylene units in the aliphatic spacer, n. Results show that the flexoelastic ratio, e K, and the effective flexoelectric coefficient, e, both exhibit an odd-even effect with values for n=odd being greater than that for n=even. This is understood in terms of an increase in the bend angle of the molecule and an increase in the transverse dipole moment. Second, in order to investigate the impact of the dipole moment, we have altered the mesogenic units so as to vary the longitudinal dipole moment and used different linkages in the aliphatic spacer in an attempt to alter the transverse dipole moment. Qualitatively, the results demonstrate that the odd-spaced bimesogen with larger transverse dipole moments exhibit larger flexoelastic ratios. © 2007 The American Physical Society.

Surface-waves, stability, and scattering for a lined duct with flow

We consider a straight cylindrical duct with a steady subsonic axial flow and a reacting boundary (e.g. an acoustic lining). The wave modes are separated into ordinary acoustic duct modes, and surface modes confined to a small neighbourhood of the boundary. Many researchers have used a mass-spring-damper boundary model, for which one surface mode has previously been identified as a convective instability; however, we show the stability analysis used in such cases to be questionable. We investigate instead the stability of the surface modes using the Briggs-Bers criterion for a Flügge thin-shell boundary model. For modest frequencies and wavenumbers the thin-shell has an impedance which is effectively that of a mass-spring-damper, although for the large wavenumbers needed for the stability analysis the thin-shell and mass-spring-damper impedances diverge, owing to the thin shell's bending stiffness. The thin shell model may therefore be viewed as a regularization of the mass-spring-damper model which accounts for nonlocally-reacting effects. We find all modes to be stable for realistic thin-shell parameters, while absolute instabilities are demonstrated for extremely thin boundary thicknesses. The limit of vanishing bending stiffness is found to be a singular limit, yielding absolute instabilities of arbitrarily large temporal growth rate. We propose that the problems with previous stability analyses are due to the neglect of something akin to bending stiffness in the boundary model. Our conclusion is that the surface mode previously identified as a convective instability may well be stable in reality. Finally, inspired by Rienstra's recent analysis, we investigate the scattering of an acoustic mode as it encounters a sudden change from a hard-wall to a thin-shell boundary, using a Wiener-Hopf technique. The thin-shell is considered to be clamped to the hard-wall. The acoustic mode is found to scatter into transmitted and reflected acoustic modes, and surface modes strongly linked to the solid waves in the boundary, although no longitudinal or transverse waves within the boundary are excited. Examples are provided that demonstrate total transmission, total reflection, and a combination of the two. This thin-shell scattering problem is preferable to the mass-spring-damper scattering problem presented by Rienstra, since the thin-shell problem is fully determined and does not need to appeal to a Kutta-like condition or the inclusion of an instability in order to avoid a surface-streamline cusp at the boundary change.

Torsional Behaviour of Cello Strings

The behaviour of a bowed string depends, among other things, on the frequency, impedance and internal damping of torsional waves on the string. Very little published information is available about these quantities, especially the torsional damping. Measurements of all relevant torsional properties have been made on cello strings of three different constructions. These show that the torsional modes are harmonically spaced to reasonable accuracy, and that the Q factors are approximately equal for all modes of a given string. These torsional Q factors are roughly an order of magnitude smaller than those of the transverse modes of the same string. The torsional wave speed varies somewhat with the tension in the string, decreasing with higher tension. The damping factors are not significantly influenced by tension. These results have been expressed in terms of a novel "reflection function" [1] suitable for direct incorporation into simulations of the bowing process.

Polarisation dependent all-optical nonlinearity far from the bandedge in active GaAs/AlGaAs quantum well waveguides

This paper describes a measurement on a GaAs quantum well waveguide with a high built in field across the quantum wells at a wavelength far from the bandedge. The device structure used for the measurement has been fabricated at STC Technology Ltd and is that of a standard laser ridge structure. In fabrication double heterostructure layers are grown on a [001] n + GaAs substrate, with the active region containing two intrinsic GaAs quantum wells of 10nm thickness separated by 10nm. A 4μm wide ridge is etched to provide transverse optical guiding. The experimental work has involved the use of 1.06μm wavelength light from a Q-switched Nd:YAG laser. Any induced change in refractive index is determined by measuring the change in transmission of the quantum well waveguide Fabry-Perot cavity. The waveguide is placed on a Peltier temperature controller to allow thermal tuning.

BISTABLE PERFORMANCE OF CLOSELY COUPLED TWIN STRIPE LASERS.

Results are given for bistable effects in closely coupled twin stripe lasers. These devices use controlled adjustment of asymmetric transverse optical gain to obtain bistability. Various bistable effects have been observed. Initially the authors reported a large light/current hysteresis loop obtained as the drive current to the laser was raised and lowered. Information concerning the bistable mechanisms was then obtained by applying small current pulses into each stripe. It was thus found that bistability was involved with the switching from one stable laser waveguiding mechanism to another. More recently the experimental measurement system has been much improved. Through the use of computer control of motorised micromovements and computer controlled data management, time resolved near and far field, and charge carrier concentration distribution measurements have been more accurately carried out. The paper will outline briefly this system, and report on how it has helped to reveal new mechanisms of bistability in twin stripe lasers.

Structure-flexoelastic properties of bimesogenic liquid crystals.

In this paper, we report on the flexoelastic and viscoelastic ratios for a number of bimesogens compounds with the same generic structure. Values are obtained indirectly by measuring the flexoelectro-optic response in the chiral nematic phase. By varying the molecular structure we alter the bend angle, transverse dipole moment, and length of the molecule. First, to examine the influence of the bend angle we use a homologous series whereby the only alteration in the molecular structure is the number of methylene units in the aliphatic spacer, n . Results show that the flexoelastic ratio, e/K , and the effective flexoelectric coefficient, e , both exhibit an odd-even effect with values for n=odd being greater than that for n=even . This is understood in terms of an increase in the bend angle of the molecule and an increase in the transverse dipole moment. Second, in order to investigate the impact of the dipole moment, we have altered the mesogenic units so as to vary the longitudinal dipole moment and used different linkages in the aliphatic spacer in an attempt to alter the transverse dipole moment. Qualitatively, the results demonstrate that the odd-spaced bimesogen with larger transverse dipole moments exhibit larger flexoelastic ratios.

A zero-stiffness elastic shell structure

A remarkable shell structure is described that, due to a particular combination of geometry and initial stress, has zero stiffness for any finite deformation along a twisting path; the shell is in a neutrally stable state of equilibrium. Initially the shell is straight in a longitudinal direction, but has a constant, nonzero curvature in the transverse direction. If residual stresses are induced in the shell by, for example, plastic deformation, to leave a particular resultant bending moment, then an analytical inextensional model of the shell shows it to have no change in energy along a path of twisted configurations. Real shells become closer to the inextensional idealization as their thickness is decreased; experimental thin-shell models have confirmed the neutrally stable configurations predicted by the inextensional theory. A simple model is described that shows that the resultant bending moment that leads to zero stiffness gives the shell a hidden symmetry, which explains this remarkable property.

EFFECT OF GRAIN-BOUNDARY MICROSTRUCTURE ON CAVITY NUCLEATION IN A NICKEL-BASED SUPERALLOY.

A study is presented of grain-boundary cavitation produced in Nimonic 80A by cold-deformation and stress-free annealing. The cavities were found to originate either from transverse cracking of carbide particles, or from decohesion of the particle-grain boundary interfaces. This decohesion could occur either during deformation, or during annealing. The cavities were invariably located at or close to the point of impingement of a matrix slip band on the grain boundary, but not all slip bands at a particular boundary were associated with cavitation. Quantitative evidence is presented showing that the mean number of dislocations associated with each slip band increases with macroscopic strain, but there is considerable variation between slip bands. This accounts for the differential ability of slip bands to result in cavity nucleation.

Gain-switched dynamics of tapered waveguide bow-tie lasers

The use of tapered waveguide lasers and amplifiers for enhanced picosecond pulse generation has led to order-of-magnitude peak power and pulse energy improvements. Monolithic pulse generation schemes have so far relied on a double-tapered bow-tie structure. The modeling of tapered lasers has so far been limited to steady-state operation or has lacked experimental comparison. This paper considers both experimentally and theoretically the gain-switched performance of bow-tie lasers of various taper angles. The role of transverse-mode spatial hole burning in tapered waveguide lasers is thereby investigated.

Mode control in vertical-cavity surface-emitting lasers by post-processing using focused ion-beaih etching

Single-mode emission is achieved in previously multimode gain-guided vertical-cavity surface-emitting lasers (VCSEL's) by localized modification of the mirror reflectivity using focused ion-beam etching. Reflectivity engineering is also demonstrated to suppress transverse mode emission in an oxide-confined device, reducing the spectral width from 1.2 nm to less than 0.5 nm.

Performance of single mode laser components using 2D photonic lattice reflectors

This paper will report studies of the placement of 2D photonic gratings on either side of the ridge in a Fabry Perot laser device in order to cause single mode emission. Using this approach, side mode suppression ratios of up to 30 dB are achieved, the emission remaining single mode even under 10 Gb/s large signal modulation. It is found that the use of the grating not only causes spectrally dependent reflection but in addition can lead to transverse mode fluctuations. The action of the grating has been studied not just in terms of its edge emission where conversion of the transverse modes is achieved, but also through measurement of the vertical emission from the structure where strong filtering action is observed.

Behavior of bulk melt-textured YBCO single domains subjected to crossed magnetic fields

We have experimentally investigated the crossed magnetic field effects on bulk melt-processed YBCO single domains. The samples were first permanently magnetized along their c-axis and then subjected to several cycles of a transverse magnetic field parallel to the ab planes. The magnetic properties along the c and ab directions were simultaneously measured using a couple of orthogonal pick-up coils as well as a Hall probe placed against the sample surface. The effects of both sweep amplitude and polarity were investigated. Field sweeps of alternate polarities are shown to affect the decay of the c-axis magnetization much more strongly than field sweeps of unique polarity do. However, the c-axis magnetization does not show any saturation even after a large number of field sweeps. Next, a micro-Hall probe scanning system was used to measure the distribution of magnetic induction over the top surface of the single domain subjected to the same combination of magnetic fields. The results are shown to be consistent with those determined with the sensing coils and bring out the role played by geometric effects.

Waveguide lasers in Er:Yb-doped phosphate glass fabricated by femtosecond laser writing

Advanced waveguide lasers, operating both in continuous wave and pulsed regimes, have been realized in an active phosphate glass by direct writing with femtosecond laser pulses. Stable single mode operation was obtained; the laser provided more than 50 m W in single longitudinal and transverse mode operation with 21% slope efficiency. Furthermore, by combining a high gain waveguide and an innovated fiber-pigtailed saturable absorber based on carbon nanotubes, a mode-locked ring laser providing transform limited 1.6 ps pulses was demonstrated. © 2007 IEEE.

Analysis of basic processes inside the keyhole during deep penetration ND-YAG CW laser welding

During laser welding, the keyhole is generated by the recoil pressure induced by the evaporation processes occurring mainly on the front keyhole wall (KW). In order to characterize the evaporation process, we have measured this recoil pressure by using a plume deflection technique, where the plume generated for static conditions (i. e. with no sample displacement) is deflected by a transverse side gas jet. From the measurement of the plume deflection angle, the recoil pressure can be determined as a function of incident intensity and sample material. From these data one can estimate the pressure generated on the front KW, during laser welding. Therefore, the corresponding dynamic pressure exerted by the vapor plume expansion on the rear KW, in contact with the melt pool, can be also estimated. These pressures appear to be in close agreement with those generated by an additional side jet that has been used in previous experiments, for stabilizing the observed melt pool oscillations or fluctuations.