Stimulated generation of superluminal light pulses via four-wave mixing

We report on the four-wave mixing of superluminal pulses, in which both the injected and generated pulses involved in the process propagate with negative group velocities. Generated pulses with negative group velocities of up to v(g)=-1/880c are demonstrated, corresponding to the generated pulse's peak exiting the 1.7 cm long medium ≈50 ns earlier than if it had propagated at the speed of light in vacuum, c. We also show that in some cases the seeded pulse may propagate with a group velocity larger than c, and that the generated conjugate pulse peak may exit the medium even earlier than the amplified seed pulse peak. We can control the group velocities of the two pulses by changing the seed detuning and the input seed power.     

Abnormal behaviour of one-dimensional photonic crystal with defect

The effect of introduction a defect in a conventional one-dimensional photonic band gap (PBG) structure in respect of the dispersion relation, reflectivity, group velocity and effective group index of such a structure has been studied. In particular, the dependence of various properties of such a structure on the angle of incidence of the electromagnetic waves has been given more importance in the present study. The study shows that inside the conventional PBG structure as well as defect PBG structure, the group velocity and effective group index become negative for certain ranges of normalized frequency. In a defect PBG structure, it is possible to achieve desired values (negative or positive) of group velocity and effective group index by choosing appropriate angle of incidence, whereas in conventional PBG structures, the maximum (negative and positive) values of group velocities and effective group index are found to be almost independent of the angle of incidence. This is a unique property of defect PBG structure which is different from the properties of conventional PBG structures. Because of this unique property, defect PBG structure may be widely used for construction super lenses, lasing without inversion and other optical systems in photonics.     

Ultrasonic properties of a suspension of microspheres supporting negative group velocities

The ultrasonic attenuation coefficient, phase velocity, and group velocity spectra are reported for a suspension that supports negative group velocities. The suspension consists of plastic microspheres with an average radius of 80 microm in an aqueous medium at a volume fraction of 3%. The spectra are measured using a broadband method covering a range from 2 to 20 MHz. The suspension exhibits negative group delays over a band near 4.5 MHz, with the group velocity magnitude exceeding 4.3 x 10(8) m/s at one point. The causal consistency of these results is confirmed using Kramers-Kronig relations.     

Atomic recoil effects in slow light propagation

We theoretically investigate the effect of atomic recoil on the propagation of ultraslow light pulses through a coherently driven Bose-Einstein condensed gas. For a sample at rest, the group velocity of the light pulse is the sum of the group velocity that one would observe in the absence of mechanical effects (infinite mass limit) and the velocity of the recoiling atoms (light-dragging effect). We predict that atomic recoil may give rise to a lower bound for the observable group velocities, as well as to pulse propagation at negative group velocities without appreciable absorption.     

Optical peculiarities in quasi-sandwiching periodic one-dimensional photonic crystals

Within the Maxwell framework and using the transfer-matrix method, we have determined an exact expression that governs the photonic band structure and the density of states (the group velocity) of one-dimensional superlattices composed of three alternate layers (quasi-sandwiching periodic structure) characterized by three different refractive indexes. We begin by giving the band gap of the quasi-sandwiching period structure. Owing to the advantage of the quasi-sandwiching periodic structure, we develop a specific formula that gives the N-period mode density in terms of the complex transmission coefficient of a unit cell. The specific example of a quarter-wave stack is analyzed.     

Hartman effect in presence of Aharanov-Bohm flux

The Hartman effect for a tunnelling particle implies the independence of group delay time on the opaque barrier width with superluminal velocities as a consequence. This effect is further examined on a quantum ring geometry in the presence of Aharonov-Bohm flux. We show that while tunnelling through an opaque barrier, the group delay time for given incident energy becomes independent of the barrier thickness as well as the magnitude of the flux. The Hartman effect is thereby extended beyond one dimension in the presence of Aharonov-Bohm flux.     

Radiated power of oscillators traveling in a moving medium

We consider the radiation from oscillating electric and magnetic dipoles moving with constant velocity directed parallel or antiparallel to the velocity of the surrounding medium. It is assumed that the medium in its rest frame is isotropic and has no spatial dispersion. We obtain expressions for the spectral density of the radiated power. In the case of a nondispersive medium, algebraic expressions for the total radiated power in the regime of “subluminal relative motion” are also obtained. In particular, it is shown that the energy loss of a source is negative if it moves in the direction of the superluminal motion of the medium and the source velocity is somewhat smaller than the medium velocity. It is noted that this phenomenon takes place for a smaller difference between the velocities of the source and the medium compared with a similar phenomenon for nonoscillating sources. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 4, pp. 316–328, May 2007.     

Dispersive Properties of tunnelling—induced transparency in an asymmetric double quantum well

We have investigated the dispersive properties of tunnelling-induced transparency in asymmetric double quantum well structures where two excited states are coupled by resonant tunnelling through a thin barrier in a three-level system of electronic subbands. The intersubband transitions exhibit high dispersion at zero absorption, which leads to the slow light velocity in this medium as compared with that in vacuum (c=3×10⁸). The group velocity in a specific GaAs/AlGaAs sample is calculated to be v_g=c/4.30. This structure can be used to compensate for the dispersion and energy loss in fibre optical communications.     

Bragg空芯光纤中的群速度超光速的理论探讨

运用渐近法计算了多模B ragg光纤的场分布、有效折射率、各模式的色散参量、群速度及TE01模的能量速度。结果表明:各模式色散参量有正有负,但群速度都大于光速,而能量速度不大于光速。     

Anomalous behavior of group velocity and index of refraction in a defect photonic band gap structure

In the present paper, we have made an analysis to observe the effect of introduction of defect on dispersion relation, group velocity, and effective group index in a conventional photonic band gap (PBG) structure. The study shows that inside the PBG materials group velocity and effective group index becomes negative in both types (conventional as well as defect PBG structure) of structure at a certain range of frequencies. Also, near the edges of the bands it attains very high values of index of refraction. A defect PBG structure gives a very unique feature that group velocity becomes exactly zero at a particular value of frequency and also becomes several hundred times greater than the velocity of light which is not attainable with the conventional PBG structure. Defect PBG structures with such peculiar characteristics are seen in lasing without inversion, in construction of perfect lens, in trapping of photon and other optical devices.     

Phase velocities of plane waves in a pipe with a flow whose velocity varies along the radius

The phase velocities of plane waves in a pipe filled with a moving acoustic medium are studied for different laws of flow velocity variation along the pipe radius. The wave equation is solved by the discretization method, which breaks the entire pipe volume into individual cylinders under the assumption that, within each of the cylinders, the flow velocity of the medium is constant. This approach makes it possible to reduce the solution to the wave problem to solving Helmholtz equations for individual cylinders. Based on boundary conditions satisfied at the boundaries between neighboring cylinders, a homogeneous system of linear algebraic equations is obtained. From this system, with the use of the scattering matrices, a simple dispersion equation is derived for determining the phase velocities of plane waves. The stability of the numerical solution to the dispersion equation with respect to the number of cylinders is investigated. The phase velocities of quasi-homogeneous and inhomogeneous waves in a pipe are numerically calculated and analyzed for different velocities of a moving medium and different laws of flow velocity variation along the radius. It is shown that the variation that occurs in the phase velocity of a homogeneous plane wave in a pipe due to the motion of the medium is identical to the mean flow velocity for different laws of flow velocity variation along the radius. For inhomogeneous plane waves, the phase velocity increment exceeds the mean flow velocity several times and depends on both the law of wave amplitude distribution along the radius and the law of the flow velocity variation along the radius.     

Full Quantum Theory of Transient-State Electromagnetically Induced Transparency

We develop a full quantum theory of transient-state electromagnetically induced transparency (EIT) in thevapor of three-level A-type atoms interacting with probe and coupling lasers. As applications of the full quantum theory,we show that transient-state EIT medium exhibits normal dispersion and find that group velocities of both coupling andprobe lasers are greatly reduced. It is shown that the group velocity of the probe laser in the transient-state EIT case isequal to that in the adiabatic EIT case and that the coupling laser group velocity in the transient-state EIT is generallyless than that in the adiabatic EIT.     

Pulse-echo measurement of longitudinal sound velocity in nanometer thin films

We propose a method to measure the longitudinal sound velocity in thin films of a few nanometer thickness using laser-based picosecond ultrasonics. In periodic multilayer structures, picosecond pulse-echo techniques were used to measure the effective sound velocity, which is related to the velocities of individual constituents through the superlattice phonon dispersion relation. The individual sound velocities can then be extracted, provided two or more effective velocities are obtained from multilayers of different thickness ratios. Longitudinal sound velocities in ion-beam sputtered Mo and amorphous Si films of 2 to 5 nm thickness have been determined to be 98 and 94% of the bulk speed, respectively. We believe this technique has general applicability to sound velocity measurement in ultra-thin films. PACS 68.60.Bs; 68.65.Ac; 78.47.+p     

Waves in an active medium guided by a reactive surface with reactance of gain modulation

The characteristics of surface waves (i) in a passive medium supported by a passive surface with reactance modulation and an active surface with the modulation of either the reactance or the negative resistance, and (ii) in an active medium supported by a passive surface with a reactance modulation are investigated with emphasis on the band regions. The periodic variation is sinusoidal and is in the propagation direction. For a passive medium terminated by a passive surface with a reactance modulation, there are stop bands in frequency for the surface waves and the structure of the first two stop bands is analyzed. For a passive medium terminated by an active surface and for an active medium terminated by a reactance-modulated passive surface, the characteristics of the absolute instabilities occurring in the first-order band are examined. The nature of the convective instability taking place in the second-order band for an active medium terminated by a reactance-modulated surface is discussed. Analytical expressions for the frequency shift of the second-order band are deduced. The Floquet theory is used to obtain the exact dispersion relation in the form of a continued fraction which is analyzed by singular expansions.     

Full Quantum Theory of Transient-State Electromagnetically Induced Transparency

We develop a full quantum theory of transient-state electromagnetically induced transparency (EIT) in the vapor of three-level A-type atoms interacting with probe and coupling lasers. As applications of the full quantum theory, we show that transient-state EIT medium exhibits normal dispersion and find that group velocities of both coupling and probe lasers are greatly reduced. It is shown that the group velocity of the probe laser in the transient-state EIT case is equal to that in the adiabatic EIT case and that the coupling laser group velocity in the transient-state EIT is generally less than that in the adiabatic EIT.     

DEM simulation of small particles clogging in the packing of large beads

The percolation of small particles through a periodic random loose packing of large beads is studied numerically with the Distinct Element Method. The representativity of periodic mono-sized sphere packing of varying system size was first studied by comparing their pore size distributions and tortuosities with those of a larger system, considered as an infinite medium. The results show that a periodic packing of size as low as 4-grain diameters gives a reasonable representation of the porous medium and allows reducing considerably the number of particles that has to be used in the simulations. The flow and clogging of small particles of varying concentrations and friction coefficients flowing through the former packing are then studied numerically. Results show that a steady state is rapidly reached where the mean velocity and mean vertical velocity of small particles are both constant. These mean velocities decrease with an increase in friction coefficient and in small particle concentration. The influence of the friction coefficient μ is much less marked for values of μ larger than or equal to 0.5. The distribution of small particles throughout the crossed packing becomes rapidly heterogeneous. Small particles concentrate in some pores where their velocity vanishes and where the density can reach values larger than the density of the random loose packing. The proportion of particles blocked in these pores varies linearly with concentration. Finally, the narrow throats of the porous medium responsible for blocking are identified and characterized for different values of the friction coefficient.     

Symmetry properties of the large-deviation function of the velocity of a self-propelled polar particle

A geometrically polar granular rod confined in 2D geometry, subjected to a sinusoidal vertical oscillation, undergoes noisy self-propulsion in a direction determined by its polarity. When surrounded by a medium of crystalline spherical beads, it displays substantial negative fluctuations in its velocity. We find that the large-deviation function (LDF) for the normalized velocity is strongly non-Gaussian with a kink at zero velocity, and that the antisymmetric part of the LDF is linear, resembling the fluctuation relation known for entropy production, even when the velocity distribution is clearly non-Gaussian. We extract an analogue of the phase-space contraction rate and find that it compares well with an independent estimate based on the persistence of forward and reverse velocities.     

Adiabatic propagation of quantized light pulses in an atomic medium with the tripod level configuration

We consider adiabatic propagation of a pair of quantized light pulses in a coherently prepared atomic medium with the tripod level configuration. We find that under conditions of electromagnetically induced transparency, two distinct polariton modes are simultaneously formed in the medium. These polaritons, represented by certain coherent superpositions of the quantized fields, have different group velocities; the fast one propagates at essentially the speed of light, while the group velocity of the slow polariton can be dynamically reduced to zero. The state mapping between the electromagnetic field and atomic ensemble is also demonstrated. The text was submitted by the author in English.     

Modal propagation characteristics of EM waves in ternary one-dimensional plasma photonic crystals

We have theoretically studied the modal dispersion characteristics, group velocity, and effective group as well as phase index of refraction of ternary one-dimensional (1D) plasma photonic band gap (PBG) structure having periodic multilayers of three different materials in one unit cell. The dispersion characteristics related for such structure is derived by solving Maxwell wave equation based on principle of Kronig-Penny model. From the computed results we observe that the dispersion characteristics of such structure also show the frequency gap and cutoffs as found in (binary) one-dimensional plasma photonic crystal. The frequency gap is shown to become larger with the increase of plasma frequency as well as plasma width. It is seen that such structure provide additional degree of freedom to control dispersion characteristic, group velocity and effective index of refraction compared to conventional one-dimensional plasma photonic crystal.     

Anomalous scaling in a non-Gaussian random shell model for passive scalars

In this paper, we have introduced a shell-model of Kraichnan's passive scalar problem. Different from the original problem, the prescribed random velocity field is non-Gaussian and $\delta$ correlated in time, and its introduction is inspired by She and L\'{e}v\^{e}que (Phys. Rev. Lett. {\bf 72}, 336 (1994)). For comparison, we also give the passive scalar advected by the Gaussian random velocity field. The anomalous scaling exponents $H(p)$ of passive scalar advected by these two kinds of random velocities above are determined for structure function with values of $p$ up to 15 by Monte Carlo simulations of the random shell model, with Gear methods used to solve the stochastic differential equations. We find that the $H(p)$ advected by the non-Gaussian random velocity is not more anomalous than that advected by the Gaussian random velocity. Whether the advecting velocity is non-Gaussian or Gaussian, similar scaling exponents of passive scalar are obtained with the same molecular diffusivity.