Y. Kamali

Simultaneous detection and identification of multigas pollutants using filament-induced nonlinear spectroscopy

The authors report on an approach for simultaneous monitoring of multigas pollutants based on fluorescence emission of trace gases, induced by the filamentation of intense femtosecond laser pulses in air. The high intensity inside a filament can dissociate the gas molecules into small fragments which emit characteristic fluorescence. This method is illustrated for simultaneously sensing atmospheric trace gases, methane and acetylene. The spectra of an “unknown” mixture were analyzed by using a genetic algorithm, showing good concentration agreement with the experimental results within an error of 25%.

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)

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)

Filament-induced breakdown remote spectroscopy in a polar environment

We demonstrate the feasibility of filament-induced breakdown spectroscopy (FIBS) for remote sensing of solid samples in a polar environment. FIBS spectra from an aluminum target induced by 800-nm laser pulses propagating in air were probed. The air visibility in an open winter field was as low as 3.2 km fluctuating with precipitation, pressure and relative humidity. Under such polar condition, clean spectral Al I lines from an aluminum target located at a distance of 60 m were obtained. This shows the technique FIBS could be potentially useful for sensing remote targets in a variety of polar environments.

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.

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.

Filamentation nonlinear optics: a new frontier

The filament core of a femtosecond laser pulse propagating in an optical medium has extra-ordinary quality for exploitation that includes high quality tunable pulse generation from the UV to the THz. The peak intensity inside the filament is also high enough to dissociate/ionize any molecules resulting in remarkably distinct spectra which can be use for remote sensing of Chem-bio agent.

Remote detection of aluminum and trace methane using mobile femtosecond laser system of T&T Lab

We report two remote sensing experiments of aluminum in the winter time and trace methane in the summer time using the mobile femtosecond laser facility T&T (Terawatt & Terahertz) designed by the Defence R&D Canada-Valcartier (DRDC-Valcartier) [1]. This system is similar in many respects to Teramobile, the Franco-German laser system [2].

Ultrafast laser filamentation control techniques for remote applications

Two methods of filamentation control for remote applications were studied. The first one consisted in an adaptive optic system, implemented in a specially designed focusing beam expander, which corrected for wavefront aberrations. Using this setup we are able to generate extraordinarily strong nitrogen signals at a distance as far as 90 m using 40 mJ laser pulses. Moreover, the filaments produced were used in a remote sensing scheme to detect and identify multiple targets such as trace hydrocarbon gases, solid metallic targets and aqueous aerosol clouds. The other method consists in a filament regularization method where a circular aperture of variable diameter is centered on the laser propagation axis, prior to filamentation. Longer filaments with higher ionization densities were produced in air and studied via the backscattered N2 fluorescence. 3D + time stochastic numerical simulations has shown that the optimum aperture size corresponds to the case of multiple filament ‘squeezing’ around the propagation axis forming the regularized elongated structure with higher overall amount of plasma.