Z箍缩等离子体振荡引起的电磁脉冲辐射

从理论上研究了Z箍缩过程中单模的等离子体振荡引起的电磁脉冲辐射。当模式扰动的相速度大于真空中的光速时,电磁脉冲辐射具有类似于切伦科夫辐射的角分布特征,并且随着相速度的增大这种定向辐射特征越显著;不同波长的电磁辐射也具有不同的角分布特征,短波长的电磁脉冲具有更好的定向性。电磁辐射功率强烈依赖于电子的等离子体振荡频率和等离子体柱的半径。     

Superluminal motion (review)

Prior to the development of Special Relativity, no restrictions were imposed on the velocity of the motion of particles and material bodies, as well as on energy transfer and signal propagation. At the end of the 19th century and the beginning of the 20th century, it was shown that a charge that moves at a velocity faster than the speed of light in an optical medium, in particular, in vacuum, gives rise to impact radiation, which later was termed the Vavilov-Cherenkov radiation. Shortly after the development of Special Relativity, some researchers considered the possibility of superluminal motion. In 1923, the Soviet physicist L.Ya. Strum suggested the existence of tachyons, which, however, have not been discovered yet. Superluminal motions can occur only for images, e.g., for so-called ??light spots,?? which were considered in 1972 by V.L. Ginzburg and B.M. Bolotovskii. These spots can move with a superluminal phase velocity but are incapable of transferring energy and information. Nevertheless, these light spots may induce quite real generation of microwave radiation in closed waveguides and create the Vavilov-Cherenkov radiation in vacuum. In this work, we consider various paradoxes, illusions, and artifacts associated with superluminal motion.     

Bandwidth constraints for passive superluminal propagation through metamaterials

Superluminal transmission of electromagnetic waves is usually observed in a narrow bandwidth range and the velocity outside this range is subluminal. In this paper, it is shown that the transmission coefficient for superluminal propagation through a periodic metamaterial structure satisfies a sum rule. The sum rule and its corresponding physical bound relate frequency regions with a phase velocity above an arbitrary threshold with the thickness of the slab. The theoretical results are illustrated with numerical examples.     

Phase-Controlled Transparent Superluminal Light Propagation in a Doppler-Broadened Four-Level N-Type System

The propagation of a weak probe field in a four-level N-type quantum system in the presence of spontaneously generated coherence(SGC) is theoretically investigated. The optical properties of the system are studied and it is shown that the group velocity of light pulse can be controlled by relative phase of applied fields. By changing the relative phase of applied fields, the group velocity of light pulse changes from transparent subluminal to the transparent superluminal light propagation. Thus, the phase-controlled absorption-free superluminal light propagation is obtained without applying an incoherent laser fields to the system. The propagation of a weak probe light pulse is studied by solving the Maxwell's wave equation on numerical grid in space and time. Moreover, we study the third order self- and cross-Kerr susceptibility of probe field and calculate the nonlinear cross-phase shift for different values of intensity of applied fields. In addition, we take into account the effect of Doppler broadening on the light pulse propagation and it is found that a suitable choice of laser propagation directions allows us to preserve our results even in the presence of Doppler effect. It is demonstrated that by increasing the Doppler width of distribution to the room temperature,the dispersion changes from transparent subluminal to transparent superluminal light propagation which is our major motivation for this work.     

Subluminal and superluminal parametric doppler effects in the case of light reflection from a moving smooth medium inhomogeneity

The reflection of test radiation from a smooth inhomogeneity of medium characteristics propagating with a subluminal or superluminal velocity is analyzed. The equations describing the propagation of the forward- and counter-propagating waves in such an inhomogeneous medium are derived. Quasi-phase conjugation is demonstrated in the case of superluminal inhomogeneities. The Bragg resonance conditions are formulated and the conditions for increasing the reflection coefficient of radiation from an inhomogeneity are discussed.     

Measurement of net group and reshaping delays for optical pulses in dispersive media

We experimentally demonstrate the direct measurement of net group and reshaping delays for arbitrary optical pulses in dispersive media, verifying the earlier prediction of Peatross et al. [Phys. Rev. Lett. 84, 2370 (2000)]. Incoherent pulse propagation in an absorptionless system is well described by net group delay; even the medium causes a great deal of deformation in the transmitted pulse. However, in the case of phase modulated chirping pulses in a resonant absorber, the so-called superluminal or subluminal propagation velocity is strongly influenced by the reshaping delay.     

Superluminal parametric Doppler effect in insulators and in an electron-positron vacuum

It has been shown that counterpropagating electromagnetic waves with different frequencies generate two lattices in a cubically nonlinear medium, one of which moves at a superluminal velocity. When weak radiation reflects from the superluminal lattice, the wavefront is quasi-conjugated with distortions owing to the Doppler frequency shift. These effects occur both in insulators with fast nonlinearity and in an electron-positron vacuum.     

Emission of a spatially modulated resonant medium excited with superluminal velocity

Specific characteristics of the radiation of a resonant medium excited by an ultrashort light pulse propagating through the medium with a superluminal velocity are considered. The medium is assumed to consist of identical linear harmonic oscillators with a spatial density periodically modulated along the direction of propagation of the superluminal excitation. The field of radiation of the resonant medium under these conditions is calculated. It is shown that, under the superluminal excitation, the radiation spectrum of the medium shows, along with the fundamental frequency of the oscillators, new frequencies that depend on the spatial frequency of the distribution of oscillators and on the angle of observation. Possible application of the effect is discussed.     

Controlling the Radiation Parameters of a Resonant Medium Excited by a Sequence of Ultrashort Superluminal Pulses

We investigate the possibility of controlling the radiation parameters of a spatially periodic one-dimensional medium consisting of classical harmonic oscillators by means of a sequence of ultrashort pulses that propagate through the medium with a superluminal velocity. We show that, in the spectrum of the transient process, in addition to the radiation at a resonant frequency of oscillators, new frequencies arise that depend on the period of the spatial distribution of the oscillator density, the excitation velocity, and the angle of observation. We have examined in detail the case of excitation of the medium by a periodic sequence of ultrashort pulses that travel with a superluminal velocity. We show that it is possible to excite oscillations of complex shapes and to control the radiation parameters of the resonant medium by changing the relationship between the pulse repetition rate, the medium resonant frequency, and the new frequency.     

光波位相耦合色散效应与固态介质中室温下的光速调控

文章介绍了作者利用光波位相耦合的色散效应调控光速的新机制，并利用光折变位相耦合的色散效应在室温下在Bi12SO20晶体中实现极慢光速和超光速现象．进一步的研究表明，在Bi12SO20晶体中传播的、由位相耦合色散效应引起的慢光和快光由于光折变光强耦合效应而得到了相干放大．     

Optical generation of terahertz plasmons on comb-shaped surface of metal

The possibilities of a recently proposed (Opt. Express 17, 9323 (2009)) method for generating terahertz surface plasmons on a microstructured (comb-shaped) metal surface using a nonlinear polarization pulse that moves with a superluminal velocity and is induced by an ultrashort laser pulse in a strip of electrooptic material deposited on the surface are theoretically studied. For an arbitrary direction of motion of the nonlinear source along the comb-shaped surface, fields of excited terahertz plasmons and the angular spectral distribution of the radiated energy are calculated. It is shown that the spectral and energy characteristics of plasmons can be efficiently controlled by varying the direction of motion of the source. Conditions (parameters of the comb-shaped structure, direction and velocity of motion of the source) that ensure the maximal efficiency of the optical-to-terahertz conversion are found. The developed method of terahertz generation is promising for surface terahertz spectroscopy.     

Superluminal optical soliton via resonant tunneling in coupled quantum dots

We demonstrate the formation of superluminal optical soliton in an symmetry semiconductor double quantum dot (QD) driven coherently by a weak pulsed laser using the tunnel coupling. It is shown that the group velocity of the soliton can be larger than the vacuum light speed c, i.e., superluminal soliton can be produced. The results obtained can be used for the development of new types of nanoelectronic devices for realizing high-speed optical modulation and rapidly responding quantum switching.     

Acoustooptic method for measuring sound velocity in solids

An acoustooptic interferometric method for measuring sound velocity in solids is described. This method allows one to measure sound velocity in arbitrary materials by using a composite buffer-sample resonant system. Materials which are non-transparent to the laser radiation or such which are acoustooptically inactive are also subject of this investigation.     

Anomalies of Radiation Absorption and Superluminal Propagation of light: II. A Layer of a Periodic Medium with a Helical Structure

The features of superluminal propagation of light through a layer of a helical periodic medium (HPM) are investigated and the group velocity is calculated. Multilayer systems with a layer of an HPM that provide superluminal propagation of light over large distances with compensation of losses upon light transmission through the system are considered. The features of radiation absorption in a layer of an HPM are considered. Effects of anomalously high and anomalously low absorption are found. It is shown that these effects are caused by an increase (decrease) in the density of light energy in the layer and by changes in the group velocity. The possibility of experimental observation of the effects discovered is discussed.     

Subluminal and Superluminal Phenomena in a Four-Level Atom

In a four-level atomic system, we investigate the light pulse propagation properties interacting with only one laser field. It is shown that in the steady state, the group velocity of the light pulse can be changed from subluminal to superluminal by varying the field detuning. Meanwhile, the effects of the field intensity on the group velocity are also shown. At last, with special parameters, the analytical solution for the group index is also obtained.     

Group velocity of the light pulse in an open V-type system

We investigate the group velocity of the probe light pulse in an open V-type system with spontaneously generated coherence. We find that, not only varying the relative phase between the probe and driving pulses can but varying the atomic exit rate or incoherent pumping rate also can manipulate dramatically the group velocity, even make the pulse propagation switching from subluminal to superluminal; the subliminal propagation can be companied with gain or absorption, but the superluminal propagation is always companied with absorption.     

Electron dynamics and harmonics radiation due to electron oscillation driven by laser standing wave

The electron dynamics and harmonics radiation spectra due to electron oscillation driven by laser standing wave (SW) have been investigated considering a single electron mode. It is found that electron can pass through or be reflected from laser SW as electron is shot into laser SW from antinodes plane with initial velocity parallel to electric field of SW, and for electron incidence at arbitrary plane the attraction plane of electron trajectories will transfer from nodes plane to antinodes plane if initial electron kinetic energy exceeds a critical value. Further, the harmonics emission due to electron oscillation during passing through laser SW is considered in two cases of electron incidence plane at nodes and antinodes. In case of electron incidence at nodes, the spectrum of radiation is redshifted for backward radiation and blueshifted for forward radiation, and broadening of spectrum is noticed. These effects are much more pronounced at higher laser intensities. However, for case with electron incidence at antinodes, these phenomena vanish.     

SGC Switching Between Subluminal to Superluminal Propagation in V-Type Atom

For a V-type three-level atomic system with two closely spaced upper levels, we investigate the light pulse propagation properties with only one laser field. Due to spontaneously generated coherence, the group velocity of the light pulse can be changed from subluminal to superluminal. The effects of the field intensity and the two-upper level splitting on the group velocity are also shown. At last, an analytical expression for the group velocity is given in the case of a weak field.     

SGC Switching Between Subluminal to Superluminal Propagation in V-Type Atom

For a V-type three-level atomic system with two closely spaced upper levels, we investigate the light pulse propagation properties with only one laser field. Due to spontaneously generated coherence, the group velocity of the light pulse can be changed from subluminal to superluminal. The effects of the field intensity and the two-upper level splitting on the group velocity are also shown. At last, an analytical expression for the group velocity is given in the case of a weak field.     

Anomalies of radiation absorption and superluminal propagation of light: I. An isotropic layer

The features of superluminal propagation of light through an isotropic layer are investigated and the group velocity is calculated. Multilayer systems providing superluminal propagation of light over large distances with compensation of losses upon light transmission through the system are considered. The situations in which the propagation speed of a light pulse decreases or in which it is equal to zero are also investigated. The features of radiation absorption in a thin isotropic layer are considered. The effects of anomalously high and anomalously low absorption are found. It is shown that these effects are caused by an increase (decrease) in the density of light energy in the layer and by changes in the group velocity. The possibility of experimental observation of the effects discovered is discussed.