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Search Results: 1 - 10 of 308388 matches for " William J. Firth "
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Interaction of cavity solitons in degenerate optical parametric oscillators
Dmitry V. Skryabin,William J. Firth
Physics , 1999, DOI: 10.1364/OL.24.001056
Abstract: Numerical studies together with asymptotic and spectral analysis establish regimes where soliton pairs in degenerate optical parametric oscillators fuse, repel, or form bound states. A novel bound state stabilized by coupled internal oscillations is predicted.
Instabilities of Higher-Order Parametric Solitons. Filamentation versus Coalescence
Dmitry V. Skryabin,William J. Firth
Physics , 1999,
Abstract: We investigate stability and dynamics of higher-order solitary waves in quadratic media, which have a central peak and one or more surrounding rings. We show existence of two qualitatively different behaviours. For positive phase mismatch the rings break up into filaments which move radially to initial ring. For sufficient negative mismatches rings are found to coalesce with central peak, forming a single oscillating filament.
Modulational instability of solitary waves in non-degenerate three-wave mixing: The role of phase symmetries
Dmitry V. Skryabin,William J. Firth
Physics , 1999, DOI: 10.1103/PhysRevLett.81.3379
Abstract: We show how the analytical approach of Zakharov and Rubenchik [Sov. Phys. JETP {\bf 38}, 494 (1974)] to modulational instability (MI) of solitary waves in the nonlinear Schr\"oedinger equation (NLS) can be generalised for models with two phase symmetries. MI of three-wave parametric spatial solitons due to group velocity dispersion (GVD) is investigated as a typical example of such models. We reveal a new branch of neck instability, which dominates the usual snake type MI found for normal GVD. The resultant nonlinear evolution is thereby qualitatively different from cases with only a single phase symmetry.
Modulational instability of bright solitary waves in incoherently coupled nonlinear Schr?dinger equations
Dmitry V. Skryabin,William J. Firth
Physics , 1999, DOI: 10.1103/PhysRevE.60.6242.2
Abstract: We present a detailed analysis of the modulational instability (MI) of ground-state bright solitary solutions of two incoherently coupled nonlinear Schr\"odinger equations. Varying the relative strength of cross-phase and self-phase effects we show existence and origin of four branches of MI of the two-wave solitary solutions. We give a physical interpretation of our results in terms of the group velocity dispersion (GVD) induced polarization dynamics of spatial solitary waves. In particular, we show that in media with normal GVD spatial symmetry breaking changes to polarization symmetry breaking when the relative strength of the cross-phase modulation exceeds a certain threshold value. The analytical and numerical stability analyses are fully supported by an extensive series of numerical simulations of the full model.
Frequency selection by soliton excitation in nondegenerate intracavity downconversion
Dmitry V. Skryabin,Alan R. Champneys,William J. Firth
Physics , 1999, DOI: 10.1103/PhysRevLett.84.463
Abstract: We show that soliton excitation in intracavity downconversion naturally selects a strictly defined frequency difference between the signal and idler fields. In particular, this phenomenon implies that if the signal has smaller losses than the idler then its frequency is pulled away from the cavity resonance and the idler frequency is pulled towards the resonance and {\em vice versa}. The frequency selection is shown to be closely linked with the relative energy balance between the idler and signal fields.
Stability of multi-hump optical solitons
Elena A. Ostrovskaya,Yuri S. Kivshar,Dmitry V. Skryabin,William J. Firth
Physics , 1999, DOI: 10.1103/PhysRevLett.83.296
Abstract: We demonstrate that, in contrast with what was previously believed, multi-hump solitary waves can be stable. By means of linear stability analysis and numerical simulations, we investigate the stability of two- and three-hump solitary waves governed by incoherent beam interaction in a saturable medium, providing a theoretical background for the experimental results reported by M. Mitchell, M. Segev, and D. Christodoulides [Phys. Rev. Lett. v. 80, p. 4657 (1998)].
Optomechanical self-structuring in cold atomic gases
Guillaume Labeyrie,Enrico Tesio,Pedro M. Gomes,Gian-Luca Oppo,William J. Firth,Gordon R. M. Robb,Aidan S. Arnold,Robin Kaiser,Thorsten Ackemann
Physics , 2013,
Abstract: The rapidly developing field of optomechanics aims at the combined control of optical and mechanical (solid-state or atomic) modes. In particular, laser cooled atoms have been used to exploit optomechanical coupling for self-organization in a variety of schemes where the accessible length scales are constrained by a combination of pump modes and those associated to a second imposed axis, typically a cavity axis. Here, we consider a system with many spatial degrees of freedom around a single distinguished axis, in which two symmetries - rotations and translations in the plane orthogonal to the pump axis - are spontaneously broken. We observe the simultaneous spatial structuring of the density of a cold atomic cloud and an optical pump beam. The resulting patterns have hexagonal symmetry. The experiment demonstrates the manipulation of matter by opto-mechanical self-assembly with adjustable length scales and can be potentially extended to quantum degenerate gases.
Familial Hypercholesterolaemia: The Cape Town Experience
J C Firth, A D Marais
South African Medical Journal , 2008,
Abstract: Familial hypercholesterolaemia (FH), an autosomal dominantly inherited disorder characterised by elevated plasma low-density lipoprotein (LDL) cholesterol levels, tendon xanthomata and premature ischaemic heart disease, is amenable to treatment with modern medication. The clinical and biochemical details of 1 031 patients with FH were analysed. FH is the most common monogenic disorder of lipoprotein metabolism presenting to the Lipid Clinic at Groote Schuur Hospital, accounting for about 20% of consultations. The hospital classified 55% of the FH patients as white, 43% as coloured, 1.5% as Asian and 0.5% as black. In the FH cohort (whose mean age at presentation was 44 years), 80% had tendon xanthomata, 36% had arcus cornealis, and 14% had xanthelasma. Tendon xanthomata was present in almost 90% of patients by the age of 50 years. Arcus cornealis was present in about 45% by the age of 40 years, further increasing in frequency with age. Cardiovascular complications included ischaemic heart disease (43%), stroke (1.5%), transient ischaemic attacks (1.3%), and peripheral vascular disease (3.7%). The mean age of death was 55 (±13) years; 51 (±10) years in men and 61 (±12) years in women. In 46% of the cohort, a defective gene was identified by testing for locally prevalent mutations. South African Medical Journal Vol. 98 (2) 2008: pp. 99-104
Interprofessional Relationships and Collaborative Working: Encouraging Reflective Practice
Ross, A., King, N., Firth, J
Online Journal of Issues in Nursing , 2005,
Abstract: A challenge for those involved in the education and professional development of health and social care practitioners is to find ways of encouraging and enabling them to critically reflect upon complex collaborative situations and consider how they might improve interprofessional relationships. To meet this challenge, we piloted and developed a reflective exercise derived from methods used in personal construct psychology (Hargreaves,1979; Salmon,1994), which has proved to be useful in three overlapping areas; research, professional development, and classroom teaching. To illustrate the technique, this paper presents a case study of one district nurse who used the method to help her examine complex interprofessional relationships when providing long-term community care. The reflective technique (which uses arrow-shaped cards displayed on large visual layouts) was found to provide a rich description of the individual’s relationships. By employing the visual displays the district nurse was able to explore the meanings of professional identity and roles in terms of professional relationships, and to consider her intentions and actions within a complex multidisciplinary situation.
Realization of a semiconductor-based cavity soliton laser
Y. Tanguy,T. Ackemann,W. J. Firth,R. Jaeger
Physics , 2007, DOI: 10.1103/PhysRevLett.100.013907
Abstract: The realization of a cavity soliton laser using a vertical-cavity surface-emitting semiconductor gain structure coupled to an external cavity with a frequency-selective element is reported. All-optical control of bistable solitonic emission states representing small microlasers is demonstrated by injection of an external beam. The control scheme is phase-insensitive and hence expected to be robust for all-optical processing applications. The motility of these structures is also demonstrated.
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