Q. Luo

Explosive photodissociation of methane induced by ultrafast intense laser

A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2 x 10(14) W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n = 3-->2), CH (A 2Delta, B 2Sigma-, and C 2Sigma+-->X 2Pi), or C2 (d 3Pi g-->a 3Pi u) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A-->X) emission of methane on a log-log scale has a slope of 10 +/- 1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation.

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.

Long-range detection and length estimation of light filaments using extra-attenuation of terawatt femtosecond laser pulses propagating in air

High-energy femtosecond laser pulses propagating in the atmosphere undergo self-focusing resulting in the appearance of the phenomenon of filamentation. We observed an extra-attenuation of such (terawatt) femtosecond laser pulses propagating in the atmosphere when compared with long pulses (200 ps) with the same energy. This is because, in contrast to the linear propagation of the long pulse, the input femtosecond laser pulse is attenuated owing to either absorption through multiphoton ionization/tunnel ionization or to scattering on the laser-induced plasma; self-phase-modulation and self-steepening further convert partially the energy initially contained in the fundamental bandwidth into the broad side bands of the laser, becoming eventually a white-light laser pulse (supercontinuum). The experimental data allow us to extract an effective extra-attenuation coefficient for an exponential decay of the input pulse energy with the propagation distance. Such a coefficient allows us to estimate an upper bound of the filament length under the experimental conditions used. More generally, our observation leads to a new technique to remotely detect light filaments in the atmosphere.

Some fundamental concepts of femtosecond laser filamentation

Powerful femtosecond laser pulses propagate in an apparent form of filamentation in all transparent optical media. This universal nonlinear phenomenon is currently an interesting topic of research at the forefront of applied physics and attracts more and more people to enter this field. This paper attempts to clarify some of the fundamental physics behind filamentation. The basic concepts include the slice-by-slice self-focusing, intensity clamping, white light laser generation and background energy reservoir as well as multiple filamentation competition. Some important potential applications are also discussed.

Controlling the bunch of filaments formed by high-power femtosecond laser pulse in air

We have shown that control of stochastic multiple filamentation may be performed with either large - scale spatial modifications of the beam, such as squeezing the whole beam, or relatively small-scale periodic light field perturbations introduced into the transverse beam distribution. We have found that the average conversion efficiency to the supercontinuum grows according to the similar law in both small beam and large beam cases, starting from the point of the parent filament formation. Stability of the supercontinuum signal grows essentially with decreasing initial beam size. Periodic intensity and phase perturbations are used to control stochastic filamentation arising in atmospheric turbulence. Regular phase fluctuations are introduced into the beam in the form of a lens array. With decreasing array period the spatial arrangement is attained earlier in the propagation distance. In addition, the amplitude of multiple filaments has smaller fluctuations relatively to the propagation in the regular medium. In the case of periodic intensity perturbations, control of stochastic filaments is more pronounced as compared with the phase perturbations of the same period. However, introduction of amplitude perturbations leads to the energy loss from the initial pulse.