Sub- and superluminal velocity of supercontinuum pulses propagating in scattering media

The XFROG technique has been applied to measuring temporal shifts of supercontinuum pulses propagating in scattering media consisting of spherical beads. Negative (subluminal) and positive (superluminal) changes in the group velocity have been measured. This behaviour can be explained in terms of spectral scattering losses and is directly related to the complex time formulation used in other phenomena where tunnelling times are used to describe the velocity changes. A numerical simulation of the XFROG measurement is consistent with the experimental results. PACS 42.25.Dd; 42.25.Fx; 42.65.Re     

Ultralocalized superluminal light pulses

We consider a sophisticated localized light wave—the so-called focused X wave which possesses a strong exponential localization in its waist region and propagates faster than the speed of light in a vacuum or in a linear medium. We show how this wave—until now considered in the literature as a mathematical object only—could be generated in reality by making use of cylindrical diffraction gratings.     

Sub- and superluminal propagation of intense pulses in media with saturated and reverse absorption

We develop models for the propagation of intense pulses in solid state media which can have either saturated absorption or reverse absorption. We model subluminal propagation in ruby and superluminal propagation in alexandrite as three and four level systems, respectively, coupled to Maxwell's equations. We present results well beyond the traditional pump-probe approach and explain the experiments of Bigelow et al. [Phys. Rev. Lett. 90, 113903 (2003)]Science 301, 200 (2003)]] on solid state materials.     

What causes the superluminal propagation of light pulses

In this paper, we discuss what causes the superluminal propagation of a pulse through dispersion bysolving Maxwell's equations without any approximation. The coherence of the pulse plays an importantrole for superluminal propagation. When the pulse becomes partially coherent,the propagation changesfrom superluminal to subluminal. The energy velocity is always less than the vacuum velocity.The shapeof the pulse is changed during the propagation.     

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.     

Focusing of heat pulses along nonequilibrium nanowires

As a consequence of the temperature dependence of the speed of heat pulses, rectangular heat pulses will shrink (or extend) spatially, and will increase (or decrease) their temperature when propagating along a temperature gradient. Here, we consider heat pulse propagation along silicon nanowires, because of their interest in nanotechnology. The relative rates of variation per meter may be very high, and variations along relatively short lengths could be experimentally observable.     

Acceleration of femtosecond pulses to superluminal velocities by Gouy phase shift

The phase and the group velocities are calculated in a three-dimensional neighbourhood of the focus of an aberration-free lens illuminated by a spatially Gaussian beam. The Gouy phase shift caused by the diffraction results in superluminal pulse propagation on the optical axis within the Rayleigh range.     

Probing nonequilibrium dynamics with white-light femtosecond pulses

Femtosecond pulsed lasers have become an invaluable tool for examining ultrafast nonequilibrium dynamics. With pulsewidths of a few hundred femtoseconds (fs) to less than 10 fs, these lasers can clearly provide unprecedented temporal resolution. By amplifying ultrashort laser pulses to sufficient levels of energy per pulse, it is possible to exploit the nonlinear optical properties of certain materials to generate extremely broadband pulses. These pulses retain the time structure of the incident pulse, but contain a spectral bandwidth extending from the infrared to as far as the ultraviolet. By generating white-light pulses, it becomes possible to probe ultrafast nonlinear processes over a large range of energies. In this paper, the process of generating white-light ultrashort pulses will be presented, along with a discussion of different probing techniques and procedures necessary for modeling the transient optical data. Finally, results from pump-probe measurements using a white-light probe on indium phosphide (InP) films will be presented as a demonstration of this technique.     

Weak shock waves in negative-dispersion nonequilibrium media

The “frozen” and equilibrium shock adiabats for a gas with sustained steady-state nonequilibrium are constructed accurate to the second order of smallness. With these adiabats, the pattern of and stability conditions for weak shock waves in negative-dispersion nonequilibrium media, where the speed of low-frequency (equilibrium) sound exceeds that of high-frequency (frozen) sound, are considered. On the basis of a model nonlinear equation describing the evolution of gasdynamic perturbations in low-dispersion media, the nonstationary evolution of shock waves at a negative low-frequency nonlinearity coefficient is analyzed. This situation corresponds to a low-frequency adiabat convex upwards. It is shown that a step autowave may arise in this case whose amplitude is entirely specified by the nonequilibrium parameters of the medium and correlates with the point where the low-frequency and high-frequency adiabats intersect. In addition, it is found that the initial unsteady shock wave may split into two steady ones: a step autowave followed by a steady smooth-front expansion shock.     

Space-time development of chaos in nonequilibrium media

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 33, No. 4, pp. 425–430, April, 1990.     

On nonlinear propagation of extremely short pulses in optically uniaxial media

Nonlinear wave equations describing the propagation of optical pulses of duration up to a period of electromagnetic oscillations in transparent media with uniaxial optical anisotropy are derived on the basis of a quantum-mechanical model of material response. The electron and electron-vibrational nonlinearities, electron and ion dispersion, and diffraction are taken into account. It is shown that the inclusion of the electron response alone leads to a system of two constitutive equations for the ordinary and extraordinary polarization components. When a pulse propagates across the optical axis, this system is reduced to an inhomogeneous model of the Henon-Heiles type and, hence, generalizes the Lorentz classical electron model. In order to take into account stimulated Raman scattering (SRS) processes, an anisotropic analog of the Bloembergen-Shen quantum-mechanical model taking into account the population dynamics of SRS sublevels is obtained. The generation of an extraordinary wave video pulse with the help of the high-frequency ordinary component in the Zakharov-Benney resonance mode is investigated. Some analytic soliton-like solutions in the form of propagating bound states of ordinary and extraordinary video pulses corresponding to different birefringence modes are considered and their stability to self-focusing is analyzed.     

On diagnostics of media using extremely short terahertz radiation pulses

The possibility of diagnosing the linear and nonlinear electrodynamic susceptibilities of media by examining the time profiles of extremely short terahertz radiation pulses (using pulsed terahertz spectroscopy methods) that are incident on a thin layer of a medium under study, are reflected from the layer, and are transmitted through it is shown theoretically. In the general case, the linear and nonlinear susceptibilities of different orders can be found by solving linear integral equations. Diagnostics is considerably simplified in the case of an isolated resonance of a medium with homogeneous spectral broadening, which is modeled by the response of an anharmonic oscillator.     

On Newton-Wigner localization and superluminal propagation speeds

We study the acausal behavior of free massive relativistic particles, as predicted by the Newton-Wigner position operator. The main result is an upper bound for the probability of superluminal propagation speeds. This bound implies the detection probability of acausal events is vanishingly small under present laboratory conditions.     

Generation of unipolar pulses in nonlinear media

Methods recently proposed for generating unipolar pulses in nonlinear media in terahertz and optical electromagnetic ranges are reviewed. Such pulses have nonzero “electric area” (time integral of the field strength over the entire duration of a pulse) and, correspondingly, a significant component of the field with zero frequency, thus exhibiting quasistatic properties. Effective generation of unipolar pulses would allow, e.g., transferring mechanical momentum to charged particles and, thereby, controlling the motion of wave packets of matter, which can be useful for compact accelerators of charged particles and for other applications.     

Localized optical subcycle pulses in dispersive media

The conditions for creation of broadband localized optical fields (Bessel X pulses and focus wave modes) in dispersive media are analyzed. It is shown that transmission of focus wave modes with subcycle pulse durations through fused silica is possible.