Biomedical subjects

Herbert G Winful

Publications and source records attributed to Herbert G Winful.

11 recordsLinked to original sources

PubMed2006

Incoherent self-similarities of the coupled amplified nonlinear Schrödinger equations.

Self-similar propagation in a system of coupled amplified nonlinear Schrödinger equations is studied. We find that each individual amplified nonlinear Schrödinger equation can sustain a component similariton with a quadratic phase, which is the asymptotic self-similar solution of the corresponding equation. Under a width-matching condition, the incoherent summation of all the component similaritons leads to another similariton with parabolic profile. Numerical simulations show that this incoherent parabolic similariton maintains all the characteristics of its coherent counterpart.

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PubMed2006

Generalized eikonal treatment of the Gouy phase shift.

We use a generalized refractive index that includes diffraction effects to show that the Gouy phase shift can be seen as an intensity averaged optical path difference between the generalized eikonal and the geometrical eikonal. This approach generalizes previous treatments to include the effects of phase distortion and confirms the role of transverse spatial confinement in the Gouy shift.

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PubMed2005

Apparent superluminality and the generalized Hartman effect in double-barrier tunneling.

Recent papers suggest that tunneling wave packets traverse the region of allowed propagation between two potential barriers with superluminal group velocity and in a time independent of barrier separation. This phenomenon has been termed the "generalized Hartman effect" and extended to multiple barriers. Here we show that this delay time is not a transit time but a cavity lifetime. It does not imply superluminal velocity. Reported experimental verifications of this effect are reinterpreted.

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PubMed2005

Zero-n gap soliton.

Periodic structures consisting of alternating layers of positive-index and negative-index materials have a novel bandgap at the frequency at which the average refractive index is zero. We show that, in the presence of a Kerr nonlinearity, this zero-n gap can switch from low transmission to a perfectly transmitting state, forming a nonlinear resonance or gap soliton in the process. This zero-n gap soliton is omnidirectional, in contrast to the usual Bragg gap soliton of positive-index periodic structures.

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PubMed2005

Self-similar parabolic beam generation and propagation.

The (1+1) -dimensional and (2+1) -dimensional amplified nonlinear Schrödinger equations incorporating diffraction, Kerr nonlinearity, and gain are solved analytically and numerically. An asymptotic solution is found corresponding to self-similar propagation of a beam with parabolic amplitude and phase profiles. While the (1+1) -dimensional solution is directly analogous to parabolic pulse propagation in nonlinear dispersive media, the existence of self-similar propagation in (2+1) dimensions is a nontrivial question, given that spatial solitons are unstable in bulk media with nonsaturating nonlinearities. We show that self-similar parabolic beams are possible in such media with gain and a negative nonlinear index.

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PubMed2004

Dependence of parabolic pulse amplification on stimulated Raman scattering and gain bandwidth.

An analytical model has been developed and verified by numerical simulations to determine limits induced by stimulated Raman scattering (SRS) on parabolic pulse evolution in high-power, high-energy Yb-fiber amplifiers. Our results show that the maximum achievable parabolic pulse energies are limited by SRS at low amplifier gains and by the finite gain bandwidth at high gains. Therefore, an optimum gain value exists that maximizes the achievable output pulse energy.

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PubMed2003

Delay time and the hartman effect in quantum tunneling.

A general relation between the group delay and the dwell time is derived for quantum tunneling. It is shown that the group delay is equal to the dwell time plus a self-interference delay. The Hartman effect in quantum tunneling is explained on the basis of saturation of the integrated probability density (or number of particles) under the barrier.

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PubMed2003

Optics (communication arising): mechanism for 'superluminal' tunnelling.

In their discussion of a mechanism that I proposed to explain the apparent superluminal (that is, faster than light) tunnelling of light pulses observed in photonic barrier experiments, Büttiker and Washburn used an old 'reshaping' argument that is at variance with my model and is not supported by the bulk of the experimental tunnelling evidence. The mechanism I proposed agrees with experiment and resolves a long-standing paradox - namely, the lack of dependence of tunnelling time on barrier length for thick barriers (the Hartman effect).

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PubMed2003

Group delay, stored energy, and the tunneling of evanescent electromagnetic waves.

A general relation between group delay and stored electric and magnetic energies is presented for two-port networks. It generalizes the results of Dicke to situations where electric and magnetic stored energies differ. The general result is applied to tunneling evanescent waves in cutoff waveguides. It is shown explicitly that the group delay is equal to the dwell time plus a self-interference delay which is proportional to the net reactive stored energy. The Hartman effect, the saturation of group delay with length in cutoff waveguides, is explained on the basis of saturation of stored energy with guide length. It is pointed out that the anomalously short delays observed in tunneling experiments are not propagation delays and should not be associated with superluminal velocities. A strictly luminal energy velocity is derived and a method is suggested for the measurement of dwell time and energy velocity.

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PubMed2003

Optimization of supercontinuum generation in photonic crystal fibers for pulse compression.

A theoretical study of super generation in photonic crystal fiber and its application to pulse compression is presented. The evolution of the spectrum can be divided into three stages: initial broadening below a certain threshold propagation distance, dramatic broadening to a supercontinuum at a threshold distance, and, finally, saturation of the spectral width on propagation. It is found that the group delay and group-delay dispersion of the supercontinum are sensitive to the input pulse peak power after further propagation at the third stage. Fluctuations from the input pulse are amplified and translated into fluctuations and time shift of the compressed pulses. There exists an optimum compressed distance at which compressed pulses with negligible fluctuation and time shift can be obtained.

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PubMed2003

Nature of "superluminal" barrier tunneling.

We show that the distortionless tunneling of electromagnetic pulses through a barrier is a quasistatic process in which the slowly varying envelope of the incident pulse modulates the amplitude of a standing wave. For pulses longer than the barrier width, the barrier acts as a lumped element with respect to the pulse envelope. The envelopes of the transmitted and reflected fields can adiabatically follow the incident pulse with only a small delay that originates from energy storage. The theory presented here provides a physical explanation of the tunneling process and resolves the mystery of apparent superluminality.

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