V.P. Kandidov

Moving focus in the propagation of ultrashort laser pulses in air

The long light filaments generated in air by powerful ultrashort laser pulses, previously attributed to self-channeling, were investigated by use of gigawatt pulses from a Ti:sapphire chirped-pulse-amplification laser system. A filament contained only a small fraction of the pulse energy and always ended at the diffraction length of the beam (~100 m), independently of the pulse energy. These features are explained by the moving-focus model, which is presented as an alternative to the self-channeling model. Computer simulations involving ionization of the air also support the moving-focus model.

Conical emission from laser–plasma interactions in the filamentation of powerful ultrashort laser pulses in air

We performed detailed experimental and numerical investigations of the conical emission (CE) accompanying the filamentation of powerful ultrashort laser pulses in air. It was found that the CE originates from self-phase modulation in the plasma produced by the pulse during propagation. The experiment and the simulation agree on the essential features of the CE: The CE angle decreases with increasing wavelength and is independent of the position along the filament, and there is no CE at Stokes-shifted wavelengths.

Filamentation and supercontinuum generation during the propagation of powerful ultrashort laser pulses in optical media (white light laser)

The fundamental physical mechanism responsible for the self-focussing, filamentation, supercontinuum generation and conical emission of a powerful ultrashort laser pulse in a transparent optical medium is reviewed. The propagation can be described by the model of moving focus modified by the defocussing effect of the self-induced plasma through multiphoton interaction with the medium. Spatial and temporal self-phase modulation in both the neutral Kerr medium and the plasma transform the pulse into a chirped (elongated) and strongly deformed pulse both temporally and spatially. The manifestation of the deformation is supercontinuum generation and conical emission. A new phenomenon of refocussing was observed. It is due to the diffraction of the trailing part of the pulse by the plasma that results in a ring structure of positive index changes surrounding the plasma column. This ring structure refocuses the pulse partially. The measured coherence lengths of the various frequencies components of the supercontinuum are independent of the optical media used and are essentially equal to that of the pump laser pulse when compared to an incoherent white light source. We thus justify that such a deformed pulse with a very broad spectrum could be called a chirped white light laser pulse.

Interference of transverse rings in multifilamentation of powerful femtosecond laser pulses in air

We observe multiple filaments and interference of their ring structures in the propagation of 14 mJ, 45 fs infrared laser pulse in air. We suggest a simple physical model describing the formation and the interference of rings as the result of superposition of the background field of the whole beam and the fields that diverge from the filaments due to the defocusing in the laser-produced plasma. The size and the number of maxima in the interference pattern depend on the position of the filament formation along the direction of propagation. The simulated picture of the ring structure interference is in qualitative agreement with the one obtained from the experiment.

Can we reach very high intensity in air with femtosecond PW laser pulses

In the course of femtosecond pulse filamentation in atmospheric density gases, the peak intensity is always limited by optical-field-induced ionization. This intensity clamping phenomenon is universal in all the cases we studied, namely, single and multiple filament regimes with and without external focusing using pulses of up to subpetawatt level. Even in the tight focusing cases, the clamped intensity along the propagation direction does not exceed 30% of the global intensity maximum. The remarkable shot-to-shot stability of the clamped intensity (better than 1% of the maximum value) is revealed both experimentally and numerically in a single filament regime in air.

Experimental observation and simulations of the self-action of white light laser pulse propagating in air

We present here a recent experiment on long-distance free propagation of powerful ultrafast laser pulses. A large divergence of the beam pattern at the anti-Stokes side was experimentally observed, which contrasts the tiny spots at the Stokes side at long distances, while the pattern at the central laser wavelength was practically unchanged (self-guiding). White light laser self-interference patterns were also recorded and discussed.

From Filamentation in Condensed Media to Filamentation in Gases

We demonstrated how the moving focus model developed for condensed media should be modified for the explanation of experiments on filamentation of high-power femtosecond laser pulses in air. This modification is based on the consideration of gas ionization in high-intensity laser field. Numerical simulations based on this modified model allow for the explanation not only of filamentation itself but also the accompanying phenomena such as refocusing and conical emission.

Laser filament induced microwave waveguide in air

The feasibility of a microwave waveguide made of plasma channels under laser pulse filamentation is discussed. The necessary channel bunch configuration is achieved by initial intensity modulation of a femtosecond pulse. Estimations are given for wavelength of the microwave radiation and for parameters of the plasma channels configuration.

Light bullets from near-IR filament in fused silica

The formation of light bullets during femtosecond laser pulse filamentation in the presence of anomalous group velocity dispersion has been recorded for the first time. The minimum experimentally detected width of the light bullet autocorrelation function is 27 fs, which corresponds to a duration of about 13.5 fs. The duration of the light bullet at a wavelength of 1800 nm is about two periods of the light field oscillation. The numerically calculated width of the autocorrelation function for such a light bullet is 23 fs, which is in good agreement with the experimental value.

Anti-Stokes wing of femtosecond laser filament supercontinuum in fused silica.

We have demonstrated that in the IR pulse filament the anomalous dispersion of fused silica leads to the formation of an isolated anti-Stokes wing (ASW), which is located in the visible region of the supercontinuum (SC). It is shown that the isolated ASW is formed by the interference of the light field of a SC undergoing anomalous group velocity dispersion.