N.A. Panov

Advances in intense femtosecond laser filamentation in air

This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with a very brief description of the fundamental physics of single filamentation of powerful femtosecond laser pulses in air. Intensity clamping is emphasized. One consequence is that the peak intensity inside one or more filaments would not increase significantly even if one focuses the pulse at very high peak power even up to the peta-watt level. Another is that the clamped intensity is independent of pressure. One interesting outcome of the high intensity inside a filament is filament fusion which comes from the nonlinear change of index of refraction inside the filament leading to cross beam focusing. Because of the high intensity inside the filament, one can envisage nonlinear phenomena taking place inside a filament such as a new type of Raman red shift and the generation of very broad band supercontinuum into the infrared through four-wave-mixing. This is what we call by filamentation nonlinear optics. It includes also terahertz generation from inside the filament. The latter is discussed separately because of its special importance to those working in the field of safety and security in recent years. When the filamenting pulse is linearly polarized, the isotropic nature of air becomes birefringent both electronically (instantaneous) and through molecular wave packet rotation and revival (delayed). Such birefringence is discussed in detailed. Because, in principle, a filament can be projected to a long distance in air, applications to pollution measurement as well as other atmospheric science could be earned out. We call this filamentation atmospheric science. Thus, the following subjects are discussed briefly, namely, lightning control, rain making, remote measurement of electric field, microwave guidance and remote sensing of pollutants. A discussion on the higher order Kerr effect on the physics of filamentation is also given. This is a new hot subject of current debate. This review ends on giving our view of the prospect of progress of this field of filamentation in the future. We believe it hinges upon the development of the laser technology based upon the physical understanding of filamentation and on the reduction in price of the laser system.

Ultrabroad Terahertz Spectrum Generation from an Air-Based Filament Plasma.

We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.

Arrest of self-focusing collapse in femtosecond air filaments: higher order Kerr or plasma defocusing?

Experimentally measured conical emission rings on the blue side of the filament supercontinuum of a 800 nm 50 fs pulse in air are reproduced in simulations with plasma and the third-order Kerr as the nonlinear terms. This agreement indicates plasma as the dominant mechanism arresting the self-focusing collapse. The higher order Kerr terms with the recently measured coefficients stop the collapse at a lower intensity than the plasma does and lead to the spherical angle-wavelength spectrum without blueshifted rings.

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.

Transformation of terahertz spectra emitted from dual-frequency femtosecond pulse interaction in gases

Experimental and theoretical study of the mechanisms affecting the spectral form of the pulsed THz emission generated due to the interaction between a bichromatic laser field and a gaseous medium is presented. The influence of a photoionization process and Raman rotational molecular transitions on the THz spectrum is discussed. Typical spectral modifications of THz emission are tracked down and linked to the duration of the two-color laser pump pulse.

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.

Evolution and termination of a femtosecond laser filament in air

The evolution of the filamentation of a femtosecond laser pulse in air is measured. The divergence of the filament core is almost constant over a long distance, encompassing a zone with efficient ionization followed by another where ionization is much weaker. At the end, the core diverges out linearly with a low divergence due to self-spatial filtering.

Polarization rotation due to femtosecond filamentation in an atomic gas

The linear-to-elliptical transformation of a 400 nm femtosecond-probe pulse in the birefringent filament in argon of an 800 nm linearly polarized femtosecond-pump pulse is studied numerically and experimentally. The rotation of the probe elliptical polarization is the largest in the high-intensity filament core. With propagation, the rotated radiation diffracts outward by the pump-produced plasma. The transmission of the analyzer crossing the probes polarization is maximum at the pump-probe angle of 45 degrees and gives equal values for each pair of angles symmetrically situated at both sides of the maximum.

Filamentation of arbitrary polarized femtosecond laser pulses in case of high-order Kerr effect

We developed a model of femtosecond filamentation which includes high-order Kerr effect and an arbitrary polarization of a laser pulse. We show that a circularly polarized pulse has maximum filament intensity. Also, we show that, independently of the initial pulse polarization, the value of a maximum filament intensity tends to the maximum intensity of either linearly or circularly polarized pulse.

Few-cycle optical pulse production from collimated femtosecond laser beam filamentation

A scheme for stable shot-to-shot few-cycle pulse production has been realized by launching an initially collimated laser beam into a gas tube. We found that the optimum parameters for the sevenfold compression of 55 fs, 5 mJ, Ti:Sapphire laser pulses are the following: 0.8-0.9 atm argon gas pressure and the registration aperture with a diameter of 300-700 μm. A technique for the efficient extraction of the self-compressed pulse from the desired position along the filament has been provided by moving the registration aperture along the tube. With this technique, pulses as short as 8 fs were detected at a distance of 1 m from the filament starting position, in agreement with numerical simulations performed using a 3D+time axially symmetric code.