Phase effects on group velocity propagation in a V-type system with spontaneously generated coherence

The effects of the relative phase between two laser beams on the propagation of a weak electromagnetic pulse are investigated in a V-type system with spontaneously generated coherence (SGC). Due to the relative phase, the subluminal and superluminal group velocity can be unified. Meanwhile, SGC can be regarded as a knob to manipulate light propagation between subluminal and superluminal.     

Group velocity of probe light pulse in an open lambda system

The group velocity of the probe light pulse (GVPLP) propagating through an open Λ-type atomic system with a spontaneously generated coherence is investigated when the weak probe and strong driving light fields have different frequencies. It is found that adjusting the detuning or Rabi frequency of the probe light field can realize switching of the GVPLP from subluminal to superluminal. Changing the relative phase between the probe and driving light fields or atomic exit and injection rates can lead to GVPLP varying in a wider range, but cannot induce transformation of the property of the GVPLP. The absolute value of the GVPLP always increases with Rabi frequency of the driving light field increasing. For subluminal and superluminal propagation, the system always exhibits the probe absorption, and GVPLP is mainly determined by the slope of the steep dispersion.     

Subluminal and superluminal light propagation via electromagnetically induced transparency in radiatively and inhomogeneously broadened media

Contrary to popular belief, we demonstrate the feasibility of generating superluminal (and subluminal) probe (and signal) light via electromagnetically induced transparency in a medium comprising coupled double-ladder systems. This scheme can be realized in both homogeneously (radiative) as well as in inhomogeneously (Doppler) broadened atomic systems. Unlike more intricate earlier schemes, our scheme is based simply on steady-state propagation dynamics resulting from compensation of the inevitable absorption losses by large nonlinear gain generated through appropriate choice of the pump and coupling fields. We show how easily in this scheme the speed of weak probe (and signal) fields can be switched from subluminal to superluminal by simply varying the strengths of the coherent pump and coupling fields. Furthermore, it is shown that under these conditions both the signal and probe fields are intensity matched and both propagate with the same subluminal (or superluminal) group velocity without suffering loss or gain for long distance in the medium.     

Role of the instantaneous spectrum on pulse propagation in causal linear dielectrics.

A model-independent theorem demonstrates how a causal linear dielectric medium responds to the instantaneous spectrum, that is, the spectrum of the electric field pulse that is truncated at each new instant (as a given locale in the medium experiences the pulse). This process leads the medium to exchange energy with the front of a pulse differently than with the back as the instantaneous spectrum laps onto or off of nearby resonances. So-called superluminal pulse propagation in either absorbing or amplifying media as well as highly subluminal pulse propagation are understood qualitatively and quantitatively within this context.     

Graphical simulation of superluminal acoustic localized wave pulses

This paper investigates, graphically, a simulation of the launching of recently obtained superluminal localized wave solutions of the homogeneous wave equation. These solutions represent focused interference patterns that travel with speed greater than c in the direction of propagation. Graphical simulation indicates that approximations to these solutions may be launched from a finite array of point sources, and the superluminal nature of the launched pulses is maintained in the near field of the array     

Two-colour laser-induced electron dynamics in two-level quantum system

To attempt to control the quantum state of a physical system with a femtosecond two-colour laser field, a model for the two-level system is analysed as a first step. We investigate the coherent control of the two-colour laser pulses propagating in a two-level medium. Based on calculating the influence of the laser field with various laser parameters on the electron dynamics, it is found the electronic state can be changed up and down by choosing the appropriate laser pulses and the coherent control of the two-colour laser pulses can substantially modify the behaviour of the electronic dynamics: a quicker change of two states can be produced even for small pulse duration. Moreover, the oscillatory structures around the resonant frequency and the propagation features of the laser pulses depend sensitively on the relative phase of the two-colour laser pulses. Finally, the influence of a finite lifetime of the upper level is discussed in brief.     

Analytical expressions for the longitudinal evolution of nondiffracting pulses truncated by finite apertures

Starting from some general and plausible assumptions based on geometrical optics and on a common feature of the truncated Bessel beams, a heuristic derivation is presented of very simple analytical expressions capable of describing the longitudinal (on-axis) evolution of axially symmetric nondiffracting pulses truncated by finite apertures. The analytical formulation is applied to several situations involving subluminal, luminal, or superluminal localized pulses, and the results are compared with those obtained by numerical simulations of the Rayleigh-Sommerfeld diffraction integrals. The results are in excellent agreement. The present approach can be rather useful, because it yields, in general, closed-form expressions, avoiding the need for time-consuming numerical simulations, and also because such expressions provide a powerful tool for exploring several important properties of the truncated localized pulses, such as their depth of fields, the longitudinal pulse behavior, and the decaying rates.     

Exhibition of subluminal solitons in a four-level atomic system with two-folded levels

With one weak probe, one strong pumping and one control field, the group velocity dispersion can be balanced by the Kerr effects in a lifetime-broadened four-level atomic medium with two folded levels with proper parameters. This results from the subluminal optical solitons that are formed. Additionally, the conditions for which subluminal solitons occur are given.     

On the velocity of propagation of a frequency-modulated signal in a strongly dispersive medium

The velocity of propagation of a narrowband, frequency-modulated signal in a medium characterized by a strong dispersion of absorption or amplification may significantly differ from the group velocity, with which an analogous narrowband signal without frequency modulation propagates in the same medium. In particular, the velocity of the frequency-modulated signal can be supraluminal or negative, even when the group velocity is subluminal.     

Combination of electromagnetically induced transparency and a mollow absorption spectrum in a degenerate two-level atomic ensemble: F e = 1 ↔ F g = 0

Abstract

We investigate the quantum coherence effects of a closed transition F _e = 1 ? F _g = 0 driven by transverse linearly polarized light field (with [sgrave] ± components) and probed by another transverse, linearly polarized light field. Owing to the competition of two-photon gain and single-photon absorption, the absorption spectrum is shown as a combination of electromagnetically induced transparency (EIT) and Mollow absorption spectrum (MAS). In the degenerate case, the MAS covers the EIT window at the pump-probe detuning centre. In the magnetic field, two single-photon absorption peaks inside the EIT windows appear when the drive Rabi frequency is smaller than the Zeeman splitting while, when the Rabi frequency is larger than the Zeeman splitting, we find four emission peaks due to the two-photon gain. A transition from positive to negative dispersion is obtained, which means a shift between the subluminal and superluminal group velocities.     

简单链形悬挂接触网整体吊弦瞬态动力学性能研究

以我国京津线高速铁路接触网整体吊弦为研究对象,建立弓网系统耦合模型。建模时将吊弦等效为仅可承受拉力的非线性弹簧,并利用罚函数法实现弓网系统的耦合。对建立的弓网耦合模型进行验证后,研究了弓网相互作用下整体吊弦的瞬态动力学响应。研究结果表明:弓网相对滑动速度为300 km/h及350 km/h时,同一跨内不同吊弦的动态接触力振动主频相同,均为7.8 Hz;弓网相对滑动速度300 km/h下,受电弓滑过后不同吊弦上下节点横向振动振幅很小,均小于3 mm;同一吊弦上下节点垂向振动相位基本一致,且不同吊弦上下节点垂向振动的主频均为1.42 Hz;弓网相对滑动速度250～350 km/h内,同一跨内3#吊弦的松弛时间最长。     

Dynamic control of the photonic smectic order of membranes

Self-assembled soft matter is often used in photonics because its characteristic length scale can be of the order of the wavelength of light. For example, a hyperswollen lamellar phase composed of bilayer membranes reflects visible light by the Bragg diffraction and acts as a photonic smectic crystal. The softness of such a structure allows us to dynamically control its photonic characteristics using an external field, as reported here. The smectic order of membranes is destabilized by doping charged colloidal particles into intermembrane water regions. However, we found that anisotropic coherent motion of particles along membranes induced by an alternating electric field enhances the degree of the photonic smectic order significantly. This demonstrates that entropic interactions can be controlled by modulating the membrane fluctuations through their dynamic coupling to an external field.     

Active Galactic Nuclei and Gamma Ray Bursts: Relativistic shocks and the origin of ultra high energy cosmic rays

The importance of explaining and achieving the highest observed cosmic ray energies by the first order Fermi acceleration mechanism in a given astrophysical environment is a recurring theme in astroparticle physics. In this contribution, we discuss acceleration of cosmic rays in relativistic shock formations, focusing on numerical studies concerning proton acceleration efficiency by subluminal and superluminal shocks, emphasising the dependence of the scattering model, shock Lorentz factor and the angle between the magnetic field and the shock front. We present a diffuse cosmic ray prediction model, where results are compared with the measured flux of cosmic rays at the highest energies. We show that steeper Active Galactic Nuclei spectra provide an excellent fit. Our model explains well the first evidence of a correlation between the cosmic ray flux above 5.7 ×10¹⁰ GeV and the distribution of Active Galactic Nuclei provided by AUGER. On the other hand, Gamma Ray Burst spectra being flatter can hardly explain the highest energy observed flux. We note though that recent Fermi observations of GRB090816c indicate very flat spectra which are expected within our model predictions and support evidence that GRB particle spectra can be flat, when the shock Lorentz factor is of order 1000.     

Electro-magneto-thermo-visco-elastic Plane Waves in Rotating Media with Thermal Relaxation

A study is made of the propagation of plane electro-magneto-thermo-visco-elastic harmonic waves in an unbounded isotropic conducting visco-elastic medium of Kelvin–Voigt type permeated by a primary uniform magnetic field when the entire medium rotates with a uniform angular velocity. The thermal relaxation time of heat conduction, the electric displacement current, the coupling between heat flow density and current density, and that between the temperature gradient and the electric current are included in the analysis. A more general dispersion relation is obtained to determine the effects of rotation, thermal relaxation time, visco-elastic parameters, and the external magnetic field on the phase velocity of the waves. Perturbation techniques are used to study the influence of small magneto-elastic and thermo-elastic couplings on the phase velocity of the waves. Cases of low and high frequencies are also analyzed to determine the effect of rotation, visco-elastic parameters, thermo elastic and magneto-elastic coupling, as well as thermal relaxation time of heat conduction on the waves.     

Sub- and superluminance with gain in a three-level ladder-type atomic system with spontaneously generated coherence

The effects of spontaneously generated coherence (SGC) on the propagation of a weak probe pulse are investigated in a ladder-type atomic system. Due to SGC, the properties of the dispersion and the absorption of a three-level ladder-type system are changed greatly. With proper parameters, the system can produce normal and anomalous dispersion regions in the gain region. The effects of the incoherent pumping and the relative phase on the group velocity are also studied. They all can be used as switches to manipulate the group velocity of the weak probe field from gain-assisted sub- to superluminance.     

First experience with the new Coupling Loss Induced Quench system

New-generation high-field superconducting magnets pose a challenge relating to the protection of the coil winding pack in the case of a quench. The high stored energy per unit volume calls for a very efficient quench detection and fast quench propagation in order to avoid damage due to overheating.A new protection system called Coupling-Loss Induced Quench (CLIQ) was recently developed and tested at CERN. This method provokes a fast change in the magnet transport current by means of a capacitive discharge. The resulting change in the local magnetic field induces inter-filament and inter-strand coupling losses which heat up the superconductor and eventually initiate a quench in a large fraction of the coil winding pack.The method is extensively tested on a Nb–Ti single-wire test solenoid magnet in the CERN Cryogenic Laboratory in order to assess its performance, optimize its operating parameters, and study new electrical configurations. Each parameter is thoroughly analyzed and its impact on the quench efficiency highlighted.Furthermore, an alternative method is also considered, based on a CLIQ discharge through a resistive coil magnetically coupled with the solenoid but external to it. Due to the strong coupling between the external coil and the magnet, the oscillating current in the external coil changes the magnetic field in the solenoid strands and thus generates coupling losses in the strands. Although for a given charging voltage this configuration usually yields poorer quench performance than a standard CLIQ discharge, it has the advantage of being electrically insulated from the solenoid coil, and thus it can work with much higher voltage.     

Effects of an external magnetic field on thermo-acoustical waves in a linear isotropic thermo-elastic dielectric material

Summary Thermo-mechanico-electromagnetic coupled waves propagating in a linear isotropic thermo-elastic dielectric material are theoretically investigated, in case an external magnetic field is applied to the material. Here the constitutive equations derived from the Clausius-Duhem inequality and Vernotte's heat conduction law are adopted. There are three types of coupled waves: the predominantly electromagnetic wave, the predominantly mechanical transverse wave and the predominantly thermo-mechanical longitudinal wave. The first and second waves have no thermal coupling. The third wave has thermal coupling and its propagation velocity and attenuation constant are perturbed by the external magnetic field.     

Electro-magneto-thermo-elastic plane waves in rotating media with thermal relaxation

A study is made of the propagation of plane electro-magneto-thermo-elastic harmonic waves in an unbounded isotropic conducting medium permeated by a primary uniform magnetic field when the entire medium rotates with a uniform angular velocity. The thermal relaxation time of heat conduction, the electric displacement current, the coupling between heat flow density and current density and that between the temperature gradient and the electric current are included in the analysis. A more general dispersion relation is obtained to determine the effects of rotation, relaxation time and the external magnetic field on the phase velocity of the waves. Perturbation techniques are used to study the influence of small magneto-elastic and thermo-elastic couplings on the phase velocity of the waves. Cases of low and high frequencies are also studied to determine the effect of rotation, thermoelastic and magneto-elastic couplings on the waves.     

Spectral and polarization characteristics of the nonspherically decaying radiation generated by polarization currents with superluminally rotating distribution patterns: comment

I repeat my (very short and easy) disproof of the recurring main claim of Ardavan et al. [J. Opt. Soc. Am. A 21, 858 (2004)] that a smooth source of electromagnetic fields moving in a confined region can generate an intensity decaying more slowly than the inverse square of the distance away ("nonspherical" decay). The field is not isotropic, so energy conservation is not enough to dismiss the claim. Instead my disproof follows directly from Maxwell's equations, supplying an upper bound with inverse square decay on the intensity. It therefore applies under all circumstances, quite irrespective of any fast or slow motion of the source. Despite the falsity of the main claim, the derivation of the uniform approximation to the Green function for superluminal circulation, which was needed for the claim and is based on the previous work of the first author, is valid. Its validity, importantly, extends significantly beyond the regime envisaged by the authors, and it stands as a basic result of superluminal circulation.     

Green's-function method for the analysis of propagation in holey fibers

A Green's-function method is employed to provide a rigorous analysis to the propagation and coupling phenomena in holey fibers. The analysis is carried out for an arbitrary grid of circular air holes of the fiber guide, while the electromagnetic field is taken to be a vector quantity. Application of the Green's-function concept leads to a coupled system of equations incorporating as unknowns the field expansion coefficients to cylindrical wave functions within the air holes. The propagation constants of the guided waves are computed accurately by determining the singular points of the corresponding system's matrix. Field distribution and dispersion properties of guided modes as well as coupling phenomena between parallel-running holey fibers are investigated, and numerical results are presented.