S.A. Hosseini

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.

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.

Long-range third-harmonic generation in air using ultrashort intense laser pulses

The generation of a powerful third-harmonic pulse in atmospheric air has been controlled over long-distance propagation using divergent and negatively chirped fundamental pulses. The cofilamentation of the high-intensity fundamental and third-harmonic pulses was observed over long propagation distance using the Lidar technique. The high peak intensity and the extremely broad spectral content generated by both fundamental and third-harmonic pulses imply promising applications for atmospheric remote sensing of pollutants and bioaerosols.

Population trapping and rotational revival of N2 molecules during filamentation of a femtosecond laser pulse in air

We study the fluorescence emitted from filaments in air using a pump–probe scheme with a femtosecond Ti–sapphire laser. The fluorescence intensities from the first negative band (B 2 � + → X 2 � +) and the second positive band (C 3 � u→B 3 � g) show enhancement and change periodically as a function of the pump–probe time delay. We attribute this phenomenon to the universal yet probably forgotten phenomenon of population trapping of nitrogen molecules in highly excited states together with field-induced alignment of nitrogen molecules followed by revivals of the rotational wavepackets. Theoretical calculation of the alignment dynamics of nitrogen molecules is consistent with the experimental data. (Some figures in this article are in colour only in the electronic version)

Dynamic behavior of postfilamentation Raman pulses

We report on the postfilamentation behavior of a Stokes pulse created from intense and collimated ultrashort pulses propagating in air. A systematic analysis of the pulse propagation revealed that the redshifted Raman pulse produced during filamentation had a larger divergence than the postfilamentation intense pump pulse. Also, the analysis of the far-field Stokes transverse ring revealed that the intensity in this ionization-free light channel is still sufficiently high to induce stimulated Raman scattering after ionization had ended. This behavior further extends the potential of filamentation to remotely induce third-order nonlinearities.

Neutral dissociation of hydrogen molecules in a strong laser field through superexcited states

Fragmentation of hydrogen molecules in a strong laser field is studied experimentally and theoretically. Balmer lines (n→2) from hydrogen atoms are observed up to n = 14. The emission intensity of the two strongest Balmer lines versus laser intensity in a log–log plot gives a slope of more than 11. This indicates that some high-lying states in the continuum are excited resulting in neutral dissociation. Adapting Rydberg state approximation, the potential energy curves of superexcited states of hydrogen are plotted. Semi-empirical calculation confirms dissociation through these superexcited states. The upper limit of the lifetime of the superexcited states is obtained by an ultrafast pump and probe experiment by monitoring one of the fluorescence lines.

Population trapping in Xe atoms

We study the interference stabilization (population trapping) of Xe atoms using a fs Ti – Sapphire laser both experimentally and theoretically. The investigation is performed for two pulses of different duration. The signature of population trapping arising from the dynamic multiphoton resonance of the initial state and a group of Rydberg states of the atom is found to exist. The results obtained can be considered as the manifestation that population trapping is indeed a universal phenomena.

Neutral dissociation of simple molecules in strong laser field

We report neutral dissociation of simple molecules in strong laser field, experimentally. Moreover, theoretical calculations of potential energy curves justify neutral dissociation through super excited states in all of the investigated gases.

Competition of multiple filaments during the propagation of intense femtosecond laser pulses

We observed the universal phenomenon of competition among multiple filaments generated during the propagation of intense femtosecond laser pulses in air. The backscattered fluorescence signal of nitrogen molecules excited inside multiple filaments yielded irregular changes from shot to shot without any correlation to the laser energy fluctuations. Numerical simulations reveal that the irregular fluorescence signal is due to a complex dynamics of interacting multi‐filaments. In particular, we show that interference effects between two hot spots, that give rise two filaments, may either fuse together or further branch out creating new filaments. This process is determined by the separation of hot spots in the beam and thus is very sensitive to the initial laser pulse input conditions. Steep