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We have studied multiple refocusing in fused silica and measured the critical power for self-focusing using third harmonic generation at an air/glass interface. The third harmonic energy as a function of geometrical focal position obtained at different input energies provides much information, such as the critical power for self-focusing, intensity clamping during filamentation, and evolution of multiple refocusing filaments. [ABSTRACT FROM AUTHOR]

All matters in the path of filaments induced by an intense femtosecond laser pulse propagating in air could be fragmented and result in the emission of characteristic fluorescence spectra from the excited fragments. The fluorescence spectra exhibit specific signatures (fingerprints) that can be used for the identification of various substances including chemical and biological species. In this paper, we present an overview of the recent progress in our laboratory concerning the “remote” sensing of chemical and biological agents/pollutants in air using filamentation-induced nonlinear fluorescence techniques. [ABSTRACT FROM AUTHOR]

We observed a new filamentation nonlinear process: continuous self-frequency down shift inside the filament zone during the propagation of a femtosecond laser pulse in air. The frequency shift depends strongly on the length of the self-guided column (filament). [ABSTRACT FROM AUTHOR]

Because of the dynamic equilibrium between Kerr self-focussing and plasma induced defocusing and the inexistence of collisional ionization the critical intensity during femtosecond laser filamentation in air is independent of pressure. An analytical analysis is given which is justified by a direct experimental verification. [ABSTRACT FROM AUTHOR]

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance. [ABSTRACT FROM AUTHOR]

Remote filament induced breakdown spectroscopy (R-FIBS) is used for probing a cloud of microdroplets where table salt has been dissolved. These microdroplets are a good simulant for aerosols. We demonstrated experimentally that R-FIBS can efficiently be used as a ppm-level sensing technique to remotely retrieve the composition of microdroplets in clouds located at a distance. The technique has been successfully tested up to 70 m and, as revealed by extrapolation, showed great potential for kilometer-range application. The proposed technique is sensitive to the solvent as well. For the first time, to the best of our knowledge, four hydrogen bands from the Balmer series were observed in an aqueous microdroplet cloud after H2O molecules were broken by the light filaments. [ABSTRACT FROM AUTHOR]

We have performed statistical simulations of stochastic filament control in a femtosecond laser pulse. Suppression of random multiple filaments is performed by means of a mesh introduced into the transverse beam section at the laser system output. The optimum size of a single square mesh unit contains (3.1-3.2)Pcr for self-focusing in the medium. Controlled filaments are formed in several groups along the laser pulse propagation direction. Almost simultaneous formation of multiple filaments can be attained by means of spatial modulation of the mesh transmission coefficient. [ABSTRACT FROM AUTHOR]

We demonstrated the feasibility of remote detection and differentiation of some very similar agricultural-activity related bioaerosols, namely barley, corn, and wheat grain dusts, through nonlinear fluorescence of fragments induced by the high-intensity inside filaments of femtosecond laser pulses in air. The signals were detected in Lidar configuration with targets located at 4.7 m away from the detection system. All the species showed identical spectra, namely those from molecular C2 and CN bands, as well as atomic Si, C, Mg, Al, Na, Ca, Mn, Fe, Sr and K lines. These identical spectral bands and lines reveal similar chemical compositions; however, the relative intensities of the spectra are different showing different element abundances from these three bio-targets. The intensity ratios of different elemental lines were used to distinguish these three samples. Good reproducibility was obtained. We expect that this technique could be used at long distance and thus played as a sensor of similar biological hazards for public and defense security. [ABSTRACT FROM AUTHOR]

A filamenting femtosecond laser pulse self-stabilizes the intensity fluctuation inside the filament core due to intensity clamping and generates an excellent spatial beam quality inside the core due to self-spatial filtering. The high quality of the core can be sampled by nonlinear processes. A few experimental examples are shown: self-phase modulation, four-wave mixing, third-harmonic generation and waveguide writing in glass. [ABSTRACT FROM AUTHOR]

Writing conditions for the fabrication of optical waveguides in bulk fused silica glass by use of 1 kHz focused femtosecond laser pulses at 800 nm were systematically determined for different focusing geometries. The results demonstrate that waveguides can be formed based on optical breakdown, filamentation (single or multiple), or a combination of both processes, when using pulse energies lower than the threshold of structural damage. The mechanisms of laser-induced index change are also discussed. [ABSTRACT FROM AUTHOR]

The unavoidable hot spots in a practical terawatt level laser pulse will self-focus in air at a short distance. The short distance cannot be changed significantly by only controlling the chirp or divergence. We overcome such early self-focusing by using a telescope, which enlarges the diameter of the beam, thus that of the hot spots. The telescope’s effective focal length is much shorter than the self-focusing distance of both the enlarged beam and the hot spots. Then, the resulting filaments merge into the geometrical focus whose position is controllable by the telescope. This technique also minimizes the generation of white light. [ABSTRACT FROM AUTHOR]

Femtosecond laser-induced nonlinear molecular spectroscopy is applied for sensing atmospheric greenhouse gas methane (CH4). The high intensity inside the Ti-sapphire femtosecond laser filaments can dissociate the CH4 molecules into small fragments which emit characteristic fluorescence. Backward CH radical fluorescence is used to quantitatively analyze the pollutant concentration and its remote detection limit. [ABSTRACT FROM AUTHOR]

We have questioned the conventional criterion, which is the critical power for self-focusing, to predict the stopping position of a filament. Our experimental result suggests that during the filamentation process the plasma induces an additional diffraction of the laser beam. This is another crucial factor that may lead to the termination of the filament even if the power is still much higher than the critical power. This should be taken into account in order to estimate the end of the filament as well as its length. [ABSTRACT FROM AUTHOR]

We demonstrate a three orders of magnitude increase and stability in the backscattered fluorescence signal from nitrogen molecules by terawatt femtosecond laser pulse induced air filaments using a new method. The method is based on squeezing the initial beam diameter using a telescope. The effect of laser shot-to-shot fluctuations was included in numerical simulations by a random distribution of the initial intensity in both squeezed and non-squeezed beams. Statistical processing of the simulation results shows that the average diameter of plasma channels as well as the total amount of free electrons generated in a bunch of multiple filaments in air is larger in the squeezed beam. Shot-to-shot stability of the simulated plasma density increases in the squeezed beam. The change of this plasma density with propagation distance is in good qualitative agreement with the change of the range-corrected nitrogen fluorescence signal with distance. [ABSTRACT FROM AUTHOR]

This paper describes the effects observed during the propagation of intense femtosecond laser pulses in air following the modification of the laser beam diameter with a pair of convex-concave lenses placed/mounted in a telescopic configuration. We observed that by reducing the diameter of the beam the detected back-scattered nitrogen fluorescence from the filaments becomes more stable on a shot-to-shot basis while, for a larger beam size, greater fluctuations are observed that are not correlated to shot-to-shot fluctuations in the laser pulse energy. This result leads to a new method to control the fluorescence signal which can be very important in remote-sensing applications. [ABSTRACT FROM AUTHOR]

The mass spectra of two isomers of butane, 1-butene and cis-2-butene, interacting with linearly polarized Ti:sapphire laser pulses, were compared. It was demonstrated that the difference between the two spectra are much more pronounced than the corresponding spectra prepared by 100 eV electron ionization ion source. In addition, the difference could be varied by changing the peak laser intensity. The observation suggests potential application of dissociative multiphoton ionization by ultrafast laser pulses as a source for high performance mass spectroscopy. [ABSTRACT FROM AUTHOR]