1,479 research outputs found
Effective potential for composite operators and for an auxiliary scalar field in a Nambu-Jona-Lasinio model
We derive the effective potentials for composite operators in a
Nambu-Jona-Lasinio (NJL) model at zero and finite temperature and show that in
each case they are equivalent to the corresponding effective potentials based
on an auxiliary scalar field. The both effective potentials could lead to the
same possible spontaneous breaking and restoration of symmetries including
chiral symmetry if the momentum cutoff in the loop integrals is large enough,
and can be transformed to each other when the Schwinger-Dyson (SD) equation of
the dynamical fermion mass from the fermion-antifermion vacuum (or thermal)
condensates is used. The results also generally indicate that two effective
potentials with the same single order parameter but rather different
mathematical expressions can still be considered physically equivalent if the
SD equation corresponding to the extreme value conditions of the two potentials
have the same form.Comment: 7 pages, no figur
Interaction-induced localization of anomalously-diffracting nonlinear waves
We study experimentally the interactions between normal solitons and tilted
beams in glass waveguide arrays. We find that as a tilted beam, traversing away
from a normally propagating soliton, coincides with the self-defocusing regime
of the array, it can be refocused and routed back into any of the intermediate
sites due to the interaction, as a function of the initial phase difference.
Numerically, distinct parameter regimes exhibiting this behavior of the
interaction are identified.Comment: Physical Review Letters, in pres
Collapse arrest and soliton stabilization in nonlocal nonlinear media
We investigate the properties of localized waves in systems governed by
nonlocal nonlinear Schrodinger type equations. We prove rigorously by bounding
the Hamiltonian that nonlocality of the nonlinearity prevents collapse in,
e.g., Bose-Einstein condensates and optical Kerr media in all physical
dimensions. The nonlocal nonlinear response must be symmetric, but can be of
completely arbitrary shape. We use variational techniques to find the soliton
solutions and illustrate the stabilizing effect of nonlocality.Comment: 4 pages with 3 figure
Identifying cell class specific losses from serially generated electroretinogram components
Purpose. Processing of information through the cellular layers of the retina occurs in a serial manner. In the electroretinogram (ERG), this complicates interpretation of inner retinal changes as dysfunction may arise from “upstream” neurons or may indicate a direct loss to that neural generator. We propose an approach that addresses this issue by defining ERG gain relationships. Methods. Regression analyses between two serial ERG parameters in a control cohort of rats are used to define gain relationships. These gains are then applied to two models of retinal disease. Results. The to gain is unity whereas the to and to gains are greater than unity, indicating “amplification” (). Timing relationships show amplification between to and compression for to and to , (). Application of these gains to -3-deficiency indicates that all timing changes are downstream of photoreceptor changes, but a direct pSTR amplitude loss occurs (). Application to diabetes indicates widespread inner retinal dysfunction which cannot be attributed to outer retinal changes (). Conclusions. This simple approach aids in the interpretation of inner retinal ERG changes by taking into account gain characteristics found between successive ERG components of normal animals
Modulational instability in periodic quadratic nonlinear materials
We investigate the modulational instability of plane waves in quadratic
nonlinear materials with linear and nonlinear quasi-phase-matching gratings.
Exact Floquet calculations, confirmed by numerical simulations, show that the
periodicity can drastically alter the gain spectrum but never completely
removes the instability. The low-frequency part of the gain spectrum is
accurately predicted by an averaged theory and disappears for certain gratings.
The high-frequency part is related to the inherent gain of the homogeneous
non-phase-matched material and is a consistent spectral feature.Comment: 4 pages, 7 figures corrected minor misprint
Using the electroretinogram to understand how intraocular pressure elevation affects the rat retina
Intraocular pressure (IOP) elevation is a key risk factor for glaucoma. Our understanding of the effect that IOP elevation has on the eye has been greatly enhanced by the application of the electroretinogram (ERG). In this paper, we describe how the ERG in the rodent eye is affected by changes in IOP magnitude, duration, and number of spikes. We consider how the variables of blood pressure and age can modify the effect of IOP elevation on the ERG. Finally, we contrast the effects that acute and chronic IOP elevation can have on the rodent ERG
Polychromatic solitons in a quadratic medium
We introduce the simplest model to describe parametric interactions in a
quadratically nonlinear optical medium with the fundamental harmonic containing
two components with (slightly) different carrier frequencies [which is a direct
analog of wavelength-division multiplexed (WDM) models, well known in media
with cubic nonlinearity]. The model takes a closed form with three different
second-harmonic components, and it is formulated in the spatial domain. We
demonstrate that the model supports both polychromatic solitons (PCSs), with
all the components present in them, and two types of mutually orthogonal simple
solitons, both types being stable in a broad parametric region. An essential
peculiarity of PCS is that its power is much smaller than that of a simple
(usual) soliton (taken at the same values of control parameters), which may be
an advantage for experimental generation of PCSs. Collisions between the
orthogonal simple solitons are simulated in detail, leading to the conclusion
that the collisions are strongly inelastic, converting the simple solitons into
polychromatic ones, and generating one or two additional PCSs. A collision
velocity at which the inelastic effects are strongest is identified, and it is
demonstrated that the collision may be used as a basis to design a simple
all-optical XOR logic gate.Comment: 9 pages, 8 figures, accepted to Phys. Rev.
Modulational instability, solitons and beam propagation in spatially nonlocal nonlinear media
We present an overview of recent advances in the understanding of optical
beams in nonlinear media with a spatially nonlocal nonlinear response. We
discuss the impact of nonlocality on the modulational instability of plane
waves, the collapse of finite-size beams, and the formation and interaction of
spatial solitons.Comment: Review article, will be published in Journal of Optics B, special
issue on Optical Solitons, 6 figure
Dispersive properties of quasi-phase-matched optical parametric amplifiers
The dispersive properties of non-degenerate optical parametric amplification
in quasi-phase-matched (QPM) nonlinear quadratic crystals with an arbitrary
grating profile are theoretically investigated in the no-pump-depletion limit.
The spectral group delay curve of the amplifier is shown to be univocally
determined by its spectral power gain curve through a Hilbert transform. Such a
constraint has important implications on the propagation of spectrally-narrow
optical pulses through the amplifier. In particular, it is shown that anomalous
transit times, corresponding to superluminal or even negative group velocities,
are possible near local minima of the spectral gain curve. A possible
experimental observation of such effects using a QPM Lithium-Niobate crystal is
suggested.Comment: submitted for publicatio
Design of a five-axis ultra-precision micro-milling machine—UltraMill. Part 1: Holistic design approach, design considerations and specifications
High-accuracy three-dimensional miniature components and microstructures are increasingly in demand in the sector of electro-optics, automotive, biotechnology, aerospace and information-technology industries. A rational approach to mechanical micro machining is to develop ultra-precision machines with small footprints. In part 1 of this two-part paper, the-state-of-the-art of ultra-precision machines with micro-machining capability is critically reviewed. The design considerations and specifications of a five-axis ultra-precision micro-milling machine—UltraMill—are discussed. Three prioritised design issues: motion accuracy, dynamic stiffness and thermal stability, formulate the holistic design approach for UltraMill. This approach has been applied to the development of key machine components and their integration so as to achieve high accuracy and nanometer surface finish
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