185 research outputs found
From Superluminal Velocity To Time Machines?
Various experiments have shown superluminal group and signal velocities
recently. Experiments were essentials carried out with microwave tunnelling,
with frustrated total internal reflection, and with gain-assisted anomalous
dispersion. According to text books a superluminal signal velocity violates
Einstein causality implying that cause and effect can be changed and time
machines known from science fiction could be constructed. This naive analysis,
however, assumes a signal to be a point in the time dimension neglecting its
finite duration. A signal is not presented by a point nor by its front, but by
its total length. On the other hand a signal energy is finite thus its
frequency band is limited, the latter is a fundamental physical property in
consequence of field quantization with quantum . All superluminal
experiments have been carried out with rather narrow frequency bands. The
narrow band width is a condition sine qua non to avoid pulse reshaping of the
signal due to the dispersion relation of the tunnelling barrier or of the
excited gas, respectively. In consequence of the narrow frequency band width
the time duration of the signal is long so that causality is preserved.
However, superluminal signal velocity shortens the otherwise luminal time span
between cause and effect.Comment: 5 pages, 3 figure
Synchrotron Tomography for the Study of Void Formation in Internal Tin NbSn Superconductors
Synchrotron absorption tomography has been applied for the study of voids formed during the thermal treatment of internal tin NbSn strands. Possible void formation mechanisms and in particular the effect of Sn phase transformations and melting are discussed based on a quantitative void description. Sn melting changes mainly the shape and volume of the individual voids but does not increase the total void volume in the strand
On the formation of voids in internal tin NbSn superconductors
In this article we describe three void growth mechanisms in NbSn strands of the internal tin design on the basis of combined synchrotron micro-tomography and x-ray diffraction measurements during in-situ heating cycles. Initially void growth is driven by a reduction of void surface area by void agglomeration. The main void volume increase is caused by density changes during the formation of Cu3Sn in the strand. Subsequent transformation of Cu-Sn intermetallics into the lower density a-bronze reduces the void volume again. Long lasting temperature ramps and isothermal holding steps can neither reduce the void volume nor improve the chemical strand homogeneity prior to the superconducting A15 phase nucleation and growth
Nonlocal reflection by photonic barriers
The time behaviour of microwaves undergoing partial reflection by photonic
barriers was measured in the time and in the frequency domain. It was observed
that unlike the duration of partial reflection by dielectric layers, the
measured reflection duration of barriers is independent of their length. The
experimental results point to a nonlocal behaviour of evanescent modes at least
over a distance of some ten wavelengths. Evanescent modes correspond to
photonic tunnelling in quantum mechanics.Comment: 8 pages, 5 figure
Negative phase time for Scattering at Quantum Wells: A Microwave Analogy Experiment
If a quantum mechanical particle is scattered by a potential well, the wave
function of the particle can propagate with negative phase time. Due to the
analogy of the Schr\"odinger and the Helmholtz equation this phenomenon is
expected to be observable for electromagnetic wave propagation. Experimental
data of electromagnetic wells realized by wave guides filled with different
dielectrics confirm this conjecture now.Comment: 10 pages, 6 figure
Tunneling Violates Special Relativity
Experiments with evanescent modes and tunneling particles have shown that i)
their signal velocity may be faster than light, ii) they are described by
virtual particles, iii) they are nonlocal and act at a distance, iv)
experimental tunneling data of phonons, photons, and electrons display a
universal scattering time at the tunneling barrier front, and v) the properties
of evanescent, i.e. tunneling modes is not compatible with the special theory
of relativity
Multibarrier tunneling
We study the tunneling through an arbitrary number of finite rectangular
opaque barriers and generalize earlier results by showing that the total
tunneling phase time depends neither on the barrier thickness nor on the
inter-barrier separation. We also predict two novel peculiar features of the
system considered, namely the independence of the transit time (for non
resonant tunneling) and the resonant frequency on the number of barriers
crossed, which can be directly tested in photonic experiments. A thorough
analysis of the role played by inter-barrier multiple reflections and a
physical interpretation of the results obtained is reported, showing that
multibarrier tunneling is a highly non-local phenomenon.Comment: RevTex, 7 pages, 1 eps figur
Fresnel laws at curved dielectric interfaces of microresonators
We discuss curvature corrections to Fresnel's laws for the reflection and
transmission of light at a non-planar refractive-index boundary. The reflection
coefficients are obtained from the resonances of a dielectric disk within a
sequential-reflection model. The Goos-H\"anchen effect for curved light fronts
at a planar interface can be adapted to provide a qualitative and quantitative
extension of the ray model which explains the observed deviations from
Fresnel's laws.Comment: submitted to Phys. Rev.
Resonant laser tunnelling
We propose an experiment involving a gaussian laser tunneling through a twin
barrier dielectric structure. Of particular interest are the conditions upon
the incident angle for resonance to occur. We provide some numerical
calculations for a particular choice of laser wave length and dielectric
refractive index which confirm our expectations.Comment: 15 pages, 6 figure
Concave and Convex photonic Barriers in Gradient Optics
Propagation and tunneling of light through photonic barriers formed by thin
dielectric films with continuous curvilinear distributions of dielectric
susceptibility across the film, are considered. Giant heterogeneity-induced
dispersion of these films, both convex and concave, and its influence on their
reflectivity and transmittivity are visualized by means of exact analytical
solutions of Maxwell equations. Depending on the cut-off frequency of the film,
governed by the spatial profile of its refractive index, propagation or
tunneling of light through such barriers are examined. Subject to the shape of
refractive index profile the group velocities of EM waves in these films are
shown to be either increased or deccreased as compared with the homogeneous
layers; however, these velocities for both propagation and tunneling regimes
remain subluminal. The decisive influence of gradient and curvature of photonic
barriers on the efficiency of tunneling is examined by means of generalized
Fresnel formulae. Saturation of the phase of the wave tunneling through a stack
of such films (Hartman effect), is demonstrated. The evanescent modes in lossy
barriers and violation of Hartman effect in this case is discussed
- …