M. Sharifi

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.

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.

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.

Intensity clamping during laser filamentation by TW level femtosecond laser in air and argon

A systematic research on intensity clamping phenomenon was conducted both in air and argon by using a TW level femtosecond laser. Though the laser peak power was increased from 0.1 up to 1.5 TW in the experiment, highly stabilized peak intensity inside the filament was observed in both gases. The peak intensities inside filaments were experimentally determined to be about 6.4 × 1013 W/cm2 (f = 20 cm) in air and 1.2, 1.3, and 1.7 × 1014 W/cm2 when different focal lenses (f = 100, 60, and 20 cm) were used in argon, respectively.

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.

The (critical) clamped intensity of a plasma filament induced by a femtosecond laser pulse in helium

The clamped intensity of a plasma-filament induced by a femtosecond laser pulse in one-atmospheric-pressure helium was measured. The measured electron density of ~7times10¹⁷cm^-3 corresponds to a clamped intensity of about 7.8times10¹⁴W/cm².