2,113 research outputs found
Mitigation of dynamical instabilities in laser arrays via non-Hermitian coupling
Arrays of coupled semiconductor lasers are systems possessing complex
dynamical behavior that are of major interest in photonics and laser science.
Dynamical instabilities, arising from supermode competition and slow carrier
dynamics, are known to prevent stable phase locking in a wide range of
parameter space, requiring special methods to realize stable laser operation.
Inspired by recent concepts of parity-time () and non-Hermitian
photonics, in this work we consider non-Hermitian coupling engineering in laser
arrays in a ring geometry and show, both analytically and numerically, that
non-Hermitian coupling can help to mitigate the onset of dynamical laser
instabilities. In particular, we consider in details two kinds of
nearest-neighbor non-Hermitian couplings: symmetric but complex mode coupling
(type-I non-Hermitian coupling) and asymmetric mode coupling (type-II
non-Hermitian coupling). Suppression of dynamical instabilities can be realized
in both coupling schemes, resulting in stable phase-locking laser emission with
the lasers emitting in phase (for type-I coupling) or with phase
gradient (for type-II coupling), resulting in a vortex far-field beam. In
type-II non-Hermitian coupling, chirality induced by asymmetric mode coupling
enables laser phase locking even in presence of moderate disorder in the
resonance frequencies of the lasers.Comment: revised version, changed title, added one figure and some reference
Non-exponential decay via tunneling in tight-binding lattices and the optical Zeno effect
An exactly-solvable model for the decay of a metastable state coupled to a
semi-infinite tight-binding lattice, showing large deviations from exponential
decay in the strong coupling regime, is presented. An optical realization of
the lattice model, based on discrete diffraction in a semi-infinite array of
tunneling-coupled optical waveguides, is proposed to test non-exponential decay
and for the observation of an optical analog of the quantum Zeno effect
A study of the interests of high school boys in their physical activity program
Thesis (M.A.)--Boston University, 1949. This item was digitized by the Internet Archive
Nonlinear directional coupler for polychromatic light
We demonstrate that nonlinear directional coupler with special bending of
waveguide axes can be used for all-optical switching of polychromatic light
with very broad spectrum covering all visible region. The bandwidth of
suggested device is enhanced five times compared to conventional couplers. Our
results suggest novel opportunities for creation of all-optical logical gates
and switches for polychromatic light with white-light and super-continuum
spectrum.Comment: 3 pages, 3 figure
Getting back into work after job loss: The role of partner effects
We investigate the reaction of couples to a job loss during periods of growth and recession in the UK focussing on re-employment of the spouse who lost their job. Re-employment was faster for those with a partner in work, but was not generally affected by other measures of the partner's labour market attachment or resources. For men, the strongest partner effects were for entry into high quality jobs; and having a working partner substantially mitigated the negative impact of the recession on entry into these jobs. For women, an employed partner was associated with a greater likelihood of re-entry into any type of job. Hence, while dual earner families may be able to restore the pre-job loss income level, single earner families are more likely to be trapped in cycles of low-quality jobs and no jobs leading to a decrease in household income over time. The difference in outcomes between single and dual earner couples is likely to increase during recessions
Couples' Labour Supply Responses to Job Loss: Boom and Recession Compared
We examine how couples' labour supply behaviour in the UK responds to a job loss by one partner, using the Labour Force Survey to compare the period of growth of 1995-2007 to the Great Recession and its aftermath of 2008-11. In single earner couples during the recession, both men and women substantially increased their job search activity following a partner's job loss, while the increase in search during the boom was smaller (and non-existent for men). However, the increase in job search during recession did not appear to translate into more success in finding work for either men or women. Among dual earner couples, we find little evidence that individuals searched for alternative jobs or tried to increase their hours if their partner lost their job, except that women working part-time were more likely to start looking for another job. Both men and women were more likely to quit their job voluntarily if their partner lost their job, but the recession seems to have made people more cautious about voluntarily quitting their job. We find little evidence that people react in advance of job losses, suggesting that unemployment typically comes as a surprise
Zitterbewegung of optical pulses in nonlinear frequency conversion
Pulse walk-off in the process of sum frequency generation in a nonlinear
crystal is shown to be responsible for pulse jittering which is
reminiscent to the Zitterbewegung (trembling motion) of a relativistic freely
moving Dirac particle. An analytical expression for the pulse center of mass
trajectory is derived in the no-pump-depletion limit, and numerical examples of
Zitterbewegung are presented for sum frequency generation in periodically-poled
lithium niobate. The proposed quantum-optical analogy indicates that frequency
conversion in nonlinear optics could provide an experimentally accessible
simulator of the Dirac equation.Comment: to be published in Journal of Physics B: Atomic, Molecular & Optical
Physic
Spectral singularities and Bragg scattering in complex crystals
Spectral singularities that spoil the completeness of Bloch-Floquet states
may occur in non-Hermitian Hamiltonians with complex periodic potentials. Here
an equivalence is established between spectral singularities in complex
crystals and secularities that arise in Bragg diffraction patterns. Signatures
of spectral singularities in a scattering process with wave packets are
elucidated for a PT-symmetric complex crystal.Comment: 6 pages, 5 figures, to be published in Phys. Rev.
Nonlinearity-induced broadening of resonances in dynamically modulated couplers
We report the observation of nonlinearity-induced broadening of resonances in
dynamically modulated directional couplers. When the refractive index of the
guiding channels in the coupler is harmonically modulated along the propagation
direction and out-of-phase in two channels, coupling can be completely
inhibited at resonant modulation frequencies. We observe that nonlinearity
broadens such resonances and that localization can be achieved even in detuned
systems at power levels well below those required in unmodulated couplers.Comment: 14 pages, 4 figures, to appear in Optics Letter
Quantization of a generally covariant gauge system with two super Hamiltonian constraints
The Becci-Rouet-Stora-Tyutin (BRST) operator quantization of a
finite-dimensional gauge system featuring two quadratic super Hamiltonian and m
linear supermomentum constraints is studied as a model for quantizing generally
covariant gauge theories. The proposed model ``completely'' mimics the
constraint algebra of General Relativity. The Dirac constraint operators are
identified by realizing the BRST generator of the system as a Hermitian
nilpotent operator, and a physical inner product is introduced to complete a
consistent quantization procedure.Comment: 17 pages. Latex file. Minor changes, two references adde
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