Valery P. Kandidov

Self and forced periodic arrangement of multiple filaments in glass

The formation of permanent periodic structural changes in fused silica induced by the multifilamentation process was investigated. A cylindrical lens was used to focus 800 nm 50 fs pulses with 0.5 - 3 mJ energy down to a line, resulting in a quasi-periodic linear self-arrangement of multiple filaments (MF). The quasi-period of multiple filaments is shown to be uniquely defined by the critical power of the material and the peak intensity on the sample entrance surface. A novel technique to control this spatial self-arrangement of MF is demonstrated based on the use of a binary phase mask. This technique allowed us to decrease the relative variation of spacing between the adjacent tracks of refractive index modifications by a factor of 4 as compared with the case without the phase mask. 3D + time numerical simulations qualitatively reproduce the main features of multiple filament formation obtained in the experiment.

Interference effects in the conical emission of a femtosecond filament in fused silica

It is shown both experimentally and theoretically that interference effects play the key role in the formation of frequency-angular spectrum of the filament conical emission. For the first time, we investigated experimentally the transformation of the conical emission frequency-angular spectrum with an increase in the filament length inside fused silica. We discovered the appearance of fine structure of the conical emission rings produced by lengthy filament. It is shown that the conical emission frequency-angular spectrum is produced by interference of coherent radiation from one or several moving point sources in the filament. The shape of the conical emission spectrum depends on the medium material dispersion, the spectrum structure is determined by length and relative location of filament emitting regions.

Some fundamental concepts of femtosecond laser filamentation

Powerful femtosecond laser pulses propagate in an apparent form of filamentation in all transparent optical media. This universal nonlinear phenomenon is currently an interesting topic of research at the forefront of applied physics and attracts more and more people to enter this field. This paper attempts to clarify some of the fundamental physics behind filamentation. The basic concepts include the slice-by-slice self-focusing, intensity clamping, white light laser generation and background energy reservoir as well as multiple filamentation competition. Some important potential applications are also discussed.

Effect of nonlinearity in the pass-through optics on femtosecond laser filament in air

An influence of pass-through optics on femtosecond laser pulse filamentation in ambient air is analyzed for the first time both experimentally and numerically. Propagation of high-power femtosecond laser pulse through solid optical elements introduces spatiotemporal phase modulation due to the Kerr effect. This modulation may have a strong ef-fect on the pulse filamentation in air. We demonstrated that the phase modulation obtained in the thin pass-through dielectric plate reduces the distance to the filament onset and increases the plasma channel length

Supercontinuum spectrum in IR Bessel-Gauss and Gauss pulsed beam filament under anomalous group velocity dispersion in fused silica

Paper concerns experimental and numerical investigation on supercontinuum emission in the process of femtosecond beam filamentation under different geometrical focusing in presence of anomalous group velocity dispersion in fused silica. It was shown that energy of supercontinuum visible part increases discretely with increasing of input laser pulse energy. It is connected with light bullets formation. Also it was found that energy of supercontinuum visible part connected with each bullet doesn’t depend on focusing geometry.

Self-focusing and Filamentation of Powerful Femtosecond Laser Pulses

The physical picture of the phenomenon of filamentation of a powerful femtosecond laser pulse in the bulk of a transparent dielectric is considered on the basis of the experimental results at Laval University (Quebec, Canada), and the computer simulations performed at M.V. Lomonosov Moscow State University (Russia).

Femtosecond laser filament and plasma channels in focused beam in air

The filamentation of focused beams at wavelength of 800 and 248 nm in air is studied experimentally and numerically. The results indicate that relatively tight focusing can lead to the coalescence of individual regions of high fluence and high plasma density that result from multiple refocusing, whereas in the case of weak focusing such regions are separated in the pulse propagation direction. The lower multiphoton ionization order in the case of UV radiation leads to a stronger effect of geometric focusing on filament formation. We show the possibility to control the parameters of femtosecond laser plasma filaments by introducing astigmatism in laser beam wavefront. Strong astigmatism can lead to the splitting of the channel into two separate regions. We demonstrate that the self-phase modulation in the thin passthrough dielectric plate decreases the distance to the filament start in air and increases the length of plasma channel.

Improved reproducibility of conical emission from glass under axicon focusing of femtosecond laser pulse

The supercontinuum conical emission of a 50-fs laser pulse focused into a Κ 108 glass is studied experimentally and numerically. It is found that, as the pulse energy was increased from 2 to 30 uj, the continuous picture of conical emission decomposed into speckles upon focusing with a lens and split into narrow rings upon focusing with an axicon. Preserving of distinct interference picture under more than 1000 shots exposure evidences in favor of much more stable positioning of microfilament in the case of axicon.