A. Azarm

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.

Direct observation of super-excited states in methane created by a femtosecond intense laser field

We report, for the first time, a direct observation of the super-excited states of CH4 in femtosecond intense laser fields using a pump (800 nm)–probe (1338 nm) technique. An unambiguous depletion of the CH (A 2 � → X 2 �) fluorescence signal as a function of the delay time is attributed to the de-excitation of the super-excited states by the probe laser pulse. The lifetime of the super-excited state is measured to be about 160 fs. (Some figures in this article are in colour only in the electronic version)

Controlling fluorescence from N2 inside femtosecond laser filaments in air by two-color laser pulses

We experimentally demonstrate the feasibility of controlling the fluorescence emission of nitrogen molecules in air induced by femtosecond laser filamentation by using a pump-probe method. An obvious enhancement or reduction in the filament-induced fluorescence signals of nitrogen molecules can be realized when a blue (400 nm) or an infrared (1338 nm) laser pulse is used as the probe. The completely opposite effect is ascribed to the excitation enhancement of ionization and population trapping of some highly excited states including Rydberg states of nitrogen molecules.

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)

Experiments in Population Trapping in Atoms and Molecules by an Intense Short Laser Pulse

This chapter discusses some experimental manifestations of interference stabilization or population trapping in atoms and molecules during the filamentation of strong 800-nm femtosecond laser pulses propagating in air and other gases. Particular emphasis is given to nitrogen molecules whose fluorescence induced by the 800-nm pump is probed respectively by 400 nm, 1,338 nm and THz radiations. Fluorescence enhancement and reduction were observed under different probe conditions and at the revival times of the rotational wave packet of nitrogen. Population trapping in the Rydberg states of the molecule is central to the explanation of these observations. We conclude that population trapping through interference stabilization in the multiphoton regime is a universal phenomenon in atoms and molecules in intense laser fields. This includes the excitation of super-excited states of molecules.

Neutral Dissociation of Superexcited Oxygen Molecules in Intense Laser Fields

Superexcited states (SESs) of oxygen molecules and their neutral dissociation processes have been studied both experimentally and theoretically using intense femtosecond laser. We find that at the laser intensity of approximately 2 x 10(14) W/cm(2), ultrashort laser pulse causes neutral dissociation of oxygen molecule by way of SESs. The dissociation products are the excited neutral oxygen atoms, which are observed through fluorescence spectroscopy. Laser power dependence of the fluorescence intensity shows that each molecule effectively absorbs an average of ten laser photons. The total energy absorbed is sufficient to stimulate the molecule to many of the SESs. The effect is equivalent to single photon excitation in the extreme-ultraviolet (XUV) region by synchrotron radiation (SR). Morse potential energy curves (PECs) are constructed for the SESs of O(2) molecules. In light of the PECs, predissociation mechanism is proposed for the neutral dissociation. Quasi-classical trajectory (QCT) calculations show that the predissociation time is as short as 100 fs, which is consistent with our experimental measurement using ultrafast pump-probe technique.

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 Superexcited Nitric Oxide Induced by Intense Laser Fields

Superexcited states of NO molecule and their neutral dissociation processes have been studied both experimentally and theoretically. Neutral excited N* and O* atoms are detected by fluorescence spectroscopy for the NO molecule upon interaction with 800 nm intense laser radiation of duration 60 fs and intensity 0.2 PW/cm2. Intense laser pulse causes neutral dissociation of superexcited NO molecule by way of multiphoton excitation, which is equivalent to single photon excitation in the extreme-ultraviolet region by synchrotron radiation. Potential energy curves (PECs) are also built using the calculated superexcited state of NO+. In light of the PECs, direct dissociation and pre-dissociation mechanisms are proposed respectively for the neutral dissociation leading to excited fragments N* and O*.

Long Range Trace Detection in Aqueous Aerosol using Remote Filament-Induced Breakdown Spectroscopy (R-FIBS)

R-FIBS is used for probing salt water aerosol. We demonstrate experimentally that it can be used to sense ppm level concentrations up to 70 m away and shows potential for kilometer range applications.