Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration and peak powers exceeding several terawatt (TW). When such pulses propagate through optically transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid on the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. The dynamics of single filaments created over laboratory scales in various materials such as noble gases, liquids and dielectrics reveal new perspectives in pulse shortening techniques. Far-field spectra provide promising diagnostics. Attention is also paid to the multifilamentation instability of broad beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy and the possibility of guiding electric discharges in air are finally addressed on the basis of experimental results.

Most long-path remote spectroscopic studies of the atmosphere rely on ambient light or narrow-band lasers. High-power femtosecond laser pulses have been found to propagate in the atmosphere as dynamically self-guided filaments that emit in a continuum from the ultraviolet to the infrared. This white light exhibits a directional behavior with enhanced backward scattering and was detected from an altitude of more than 20 kilometers. This light source opens the way to white-light and nonlinear light detection and ranging applications for atmospheric trace-gas remote sensing or remote identification of aerosols. Air ionization inside the filaments also opens promising perspectives for laser-induced condensation and lightning control. The mobile femtosecond-terawatt laser system, Teramobile, has been constructed to study these applications.

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration, high energies beyond the Joule level and peak powers exceeding several terawatt (TW). When such pulses propagate through optically-transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium, and the defocusing action of the electron plasma. Discovered one decade ago, this selfchanneling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid to the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. The dynamics of single filaments created over laboratory scales in various materials such as noble gases, liquids and dielectrics reveal new perspectives in pulse shortening techniques. Differences between femtosecond pulses propagating in gaseous or condensed materials are underlined. Attention is also paid to the multifilamentation instability of broad, powerful beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy, and the possibility to guide electric discharges in air are finally addressed on the basis of experimental results.

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We show that higher-order nonlinear indices (n(4), n(6), n(8), n(10)) provide the main defocusing contribution to self-channeling of ultrashort laser pulses in air and argon at 800 nm, in contrast with the previously accepted mechanism of filamentation where plasma was considered as the dominant defocusing process. Their consideration allows us to reproduce experimentally observed intensities and plasma densities in self-guided filaments.

Abstract:We review the properties and applications of ultrashort laser pulses in the atmosphere, with a particular focus on filamentation. Filamentation is a non-linear propagation regime specific of ultrashort and ultraintense laser pulses in the atmosphere. Typical applications include remote sensing of atmospheric gases and aerosols, lightning control, laserinduced spectroscopy, coherent anti-stokes Raman scattering, and the generation of sub-THz radiation. References and Links 1. P. L. Kelley, "Self-focusing of optical beams," Phys. Rev. Lett. 15, 1005-1008 (1965); Erratum in Phys.Rev. Lett. 16, 384 (1965) 2. G. A. Askar'yan, "The self-focusing effect," Sov. Phys. J. 16 680 (1974) 3. A. Braun, G. Korn, X. Liu, D. Du, J. Squier, G. Mourou, "Self-channeling of high-peak-power femtosecond laser pulses in air," Opt. Lett. 20, 73-75 (1995) 4. L. Roso-Franco, "Self-reflected wave insid a very dense saturable absorber," Phys. Rev. Lett. 55, 2149-2151 (1985) 5. R. R. Alfano, S. L. Shapiro, "Emission in the region 4000 to 7000 Å," Phys. Rev. Lett. 24, 584-587 (1970) 6. R. R. Alfano, S. L. Shapiro, "Observation of self-phase modulation and small-scale filaments in crystals and glasses," Phys. Rev. Lett. 24, 592-595 (1970) 7.R. R. Alfano, S. L. Shapiro, "Direct distortion of electric clouds of rare-gas atoms in intense electric fields," Phys. Rev. Lett. 24, 1217Lett. 24, -1220Lett. 24, (1970. A. Brodeur, S. L. Chin, "Ultrafast white-light continuum generation and self-focusing in transparent condensed media," J. Opt. Soc. Am. B 16, 637-650 (1999) 9. Y. Shen, "The principles of nonlinear optics," John Wiley & Sons (1984) 10. G. Yang, Y. Shen, "Spectral broadening of ultrashort pulses in a nonlinear medium," Opt. Lett. 9, 510-512 (1984) 11. A. L. Gaeta, "Catastrophic collapse of ultrashort pulses, Phys. Rev. Lett." 84, 3582-3585 (2000) 12. K. Ranka, R. W. Schirmer, A. L. Gaeta, "Observation of pulse splitting in nonlinear dispersive media," Phys. Rev. Lett. 77, 3783-3786 (1996) 13. A. Proulx, A. Talebpour, S. Petit, S. L. Chin, "Fast pulsed electric field created from the self-generated filament of a femtosecond Ti:Sapphire laser pulse in air," Opt. Commun. 174, 305-309 (2000) 14. S. Tzortzakis, M. A. Franco, Y.-B. André, A. Chiron, B. Lamouroux, B. S. Prade, A. Mysyrowicz, "Formation of a conducting channel in air by self-guided femtosecond laser pulses," Phys. Rev. E 60, R3505-R3507 (1999) 15. S. Tzortzakis, B. Prade, M. Franco, A. Mysyrowicz, "Time evolution of the plasma channel at the trail of a self-guided IR femtosecond laser pulse in air," Optics Commun. 181, 123-127 (2000) 16. H. Schillinger, R. Sauerbrey, "Electrical conductivity of long plasma channels in air generated by selfguided femtosecond laser pulses," Appl. Phys. B 68, 753-756 (1999) 17. D. Strickland, G. Mourou, "Compression of amplified chirped optical pulses," Opt. Commun. 56, 219-221 (1985) #89041 556-558 (1968). Note that the radius considered in the classical writing of Marburger's formula is the half-width at ...

Ultrashort, high-power laser pulses propagating vertically in the atmosphere have been observed over more than 20 km using an imaging 2-m astronomical telescope. This direct observation in several wavelength bands shows indications for filament formation at distances as far as 2 km in the atmosphere. Moreover, the beam divergence at 5 km altitude is smaller than expected, bearing evidence for whole-beam parallelization about the nonlinear focus. We discuss implications for white-light Lidar applications.

Triggering rain on demand is an old dream of mankind, with a huge potential socio-economical benefit. To date, efforts have mainly focused on cloud-seeding using silver salt particles. We demonstrate that self-guided ionized filaments generated by ultrashort laser pulses are also able to induce water-cloud condensation in the free, sub-saturated atmosphere. Potential contributing mechanisms include photo-oxidative chemistry and electrostatic effects. As well as revealing the potential for influencing or triggering water precipitation, laser-induced water condensation provides a new tool for the remote sensing of nucleation processes in clouds