Organizing multiple femtosecond filaments in air

We show that it is possible to organize regular filamentation patterns in air by imposing either strong field gradients or phase distortions in the input-beam profile of an intense femtosecond laser pulse. A comparison between experiments and 3+1 dimensional numerical simulations confirms this concept and shows for the first time that a control of the transport of high intensities over long distances may be achieved by forcing this well ordered propagation regime. In this case, deterministic effects prevail in multiple femtosecond filamentation, and no transition to the optical turbulence regime is obtained [Phys. Rev. Lett. 83, 2938 (1999)]].     

Self-focusing of profiled ultrashort-wavelength laser beams in air

We report on the results of laboratory experiments of filamentation of sharply focused gigawatt femtosecond laser radiation passed through various aperture diaphragms in air. For the multiple filamentation regime, the dependences of the length and spatial structure of the filamentation region on the initial beam profile are established. It is found that light beam profiling by a diaphragm leads in some cases to a displacement of the filamentation region and to repeated self-focusing of radiation behind the linear focal waist. In the beam of the same power in the absence of a diaphragm and in the regime of the formation of a single filament, this effect terminates in front of the geometrical focus. The experimental results are illustrated by numerical simulation data.     

Efficient generation of third harmonic radiation in air filaments: A revisit

In this paper we revisit the third harmonic generation from a femtosecond laser filament in air and its significant enhancement (~ 100 times) with a intercepting pump pulse, which has been reported very recently. The enhanced third harmonic is studied as a function of the pulse polarization, time delay between the pump and signal pulses, laser pulse energy, etc. We provide an explanation for the enhancement of third harmonic based on a quenching of interference effects present near filamentation threshold. Simulations based on a two-color propagation code reproduce well the experimental observations and confirm our interpretation.     

Multiple filamentation of circularly polarized beams

We derive a new system of equations that describes the propagation of circularly polarized laser beams in a Kerr medium. Analysis and simulations of this system show that multiple filamentation is suppressed for circularly polarized beams.     

Space-angle distribution of the reflected charged particles adjusted for spin calculations

The efficiency of the proposed method of solution for the spin-considered highly anisotropic multiple scattering problem is based, following optics, upon the admission that the complete solution of the vectorial transport equation boundary problem is the superposition of the anisotropic and smooth parts, calculated separately. We use a vectorial small angle modification of the spherical harmonics method to evaluate the anisotropic part and the vectorial discrete ordinates method to obtain a smooth one. Some calculation examples for reflected radiation are given.     

Multifilamentation of high-power femtosecond laser pulse in turbulent atmosphere with aerosol

The filamentation of a femtosecond laser pulse in atmosphere under the joint influence of turbulence and aerosol has been numerically studied for the first time. A result of these studies is that the presence of aerosol in a turbulent atmosphere increases the distance to the onset of multifilamentation. We have studied the competition between two factors of aerosol coherent scattering in the process of multiple filamentation of a femtosecond pulse in the atmosphere: generation of light field perturbations and energy losses. Similarity parameters have been determined for different conditions in the problem of multifilamentation in aerosol.     

Filamentation of an annular laser beam in a short wavelength laser produced plasma

We report on the first observations of periodic structures caused by the filamentation instability. These structures were observed on 0.35 μm laser irradiated planar target experiments. Comparison of these observations with self-focusing theory indicates that even though thermal filamentation may initiate the instability, ponderomotive force effects eventually dominate the actual filamentation process.     

Femtosecond laser-induced color change and filamentation in Ag+-doped silicate glass

We investigate the influence of multiple filamentation （MF） on the micromachining in Ag^＋-doped silicate glass irradiated by a l-kHz femtosecond laser. The thresholds of MF and color change （CC） are measured for both linearly and circularly polarized laser beams. The results demonstrate that the thresholds of MF and CC are very close. The thresholds of CC and MF for circular polarization increase by ~1.4 times compared with linear polarization. Circular polarization can suppress the number of filaments to some extent compared with linear polarization. However, it is difficult to obtain CC without any filamentation if circular polarization technique is used alone.     

Control of femtosecond filamentation by field-free revivals of molecular alignment

We show that the filamentation dynamics of a femtosecond laser probe pulse can be readily controlled by properly matching it to the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation with an advancing ultrashort pump pulse. Several features of the filamentation process including supercontinuum generation, the length of the plasma channel generated in the wake of the filament, the associated secondary radiations and the multiple filamentation pattern are all easily modified by tuning the cross phase modulation induced by the field-free revivals of molecular alignment, through the delay between the pump and the probe pulses. We show that molecular alignment can also be used to generate conical waves with extremely short intensity spike called shocked X-waves and to further tune the frequency of a few-cycle laser pulse in the wake of a self-guided intense filament.     

Diffraction-Ray Model of Single Filamentation of Femtosecond Laser Pulses for Estimating the Characteristics of the Multiple Filamentation Domain in Air

Russian Physics Journal - The characteristics of the multiple filamentation domain of femtosecond laser pulses in air have been estimated based on the single filamentation model. The...     

Characterization of the initial filamentation of a relativistic electron beam passing through a plasma

The linear instability that induces a relativistic electron beam passing through a plasma with return current to filament transversely is often related to some filamentation mode with the wave vector normal to the beam or confused with Weibel modes. We show that these modes may not be relevant in this matter and identify the most unstable mode on the two-stream or filamentation branch as the main trigger for filamentation. This sets both the characteristic transverse and longitudinal filamentation scales in the nonresistive initial stage.     

Vectorial coupled—mode solitons in one—dimensional photonic crystals

We study the dynamics of vectorial coupled-mode solitons in one-dimensional photonic crystals with quadratic and cubic nonlinearities. Starting from Maxwell's equations, the vectorial coupled-mode equations for the envelopes of two fundamental-frequency optical mode and one low-frequency mode components due to optical rectification are derived by means of the method of multiple scales. A set of coupled soliton solutions of the vectorial coupled-mode equations is provided. The results show that a modulation of the fundamental-frequency optical modes occurs due to the optical rectification field resulting from the quadratic nonlinearity. The optical rectification field disappears when the frequency of the fundamental-frequency optical fields approaches the edge of the photonic bands.     

Angular and chromatic dispersion in Kerr-driven conical emission

We report measurements of ultrashort optical pulse filamentation in Kerr media with normal dispersion. We measure the angular dispersion of spontaneously generated colored conical emission and show that for large frequency shifts from the input pump frequency, it is well described by a relation between angle and frequency with slope determined solely by material chromatic dispersion. This measurement is in agreement with recent models that interpret filamentation dynamics in terms of the spontaneous formation of nonlinear X-waves. Finally, experimental results of filamentation in air show, for the first time, that the interpretation of CE in terms of X-waves may be also extended to gaseous media.     

Temperature Controlled Filamentation in Argon Gas

Temperature controlled filamentation is experimentally demonstrated in a temperature gradient gas-filled tube. The proper position of the tube is heated by a furnace and two ends of the tube are cooled by air. The experimental results show that multiple filaments are shrunken into a single filament or no filament only by increasing the temperature at the beginning of the filament. This technique offers another degree of freedom of controlling the filamentation and opens a new way for intense monocycle pulse generation through gradient temperature in a noble gas.     

Multiple refocusing characterization and critical power measurement using third harmonic generation at interface

We have studied multiple refocusing in fused silica and measured the critical power for self-focusing using third harmonic generation at an air/glass interface. The third harmonic energy as a function of geometrical focal position obtained at different input energies provides much information, such as the critical power for self-focusing, intensity clamping during filamentation, and evolution of multiple refocusing filaments.     

Transverse evolution of a plasma channel in air induced by a femtosecond laser

We investigate the evolution of filamentation in air by using a longitudinal diffraction method and a plasma fluorescence imaging technique. The diameter of a single filament in which the intensity is clamped increases as the energy of the pump light pulse increases, until multiple filaments appear.     

Multifilamentation of femtosecond laser pulses induced by?small-scale air turbulence

We report on experimental and numerical investigations of femtosecond pulse propagation locally disturbed by the turbulent flow field of a hot-air blower. The experiments show that turbulence may shorten the collapse/filamentation distance and induce the onset of multiple filaments. This is supported by numerical simulations indicating that the high spatial frequency part of the turbulence spectrum plays a significant role.     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

Two-dimensional organization of a large number of stationary optical filaments by adaptive wave front control

We present an adaptive technique for the formation of multiple co-propagating and stationary filaments in a gaseous medium. Wavefront shaping of the initial beam is performed using a deformable mirror to achieve a complete two-dimensional control of the multi-spot intensity pattern in the laser focus. The spatial organization of these intensity spots yields reliable formation of up to five stable and stationary filaments providing a test bed for fundamental studies on multiple filamentation. PACS 42.65.Jx; 52.38.Hb; 42.65.Sf     

Vectorial representation of single- and multi-domain protein folds

We discuss a vectorial representation applicable to both single- and multi-domain protein folds. This generalized vectorial representation is essentially identical to the previously described vectorial representation for single-domain proteins folds when applied to these, but allows for the additional consistent representation of multi-domain structures. We show that the generalized vectorial representation enables the accurate analytical prediction of site-specific amino acid distributions for both single- and multi-domain protein folds, similarly as the previously described vectorial representation does for single-domain folds.