Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air.

Supercontinuum (SC) light source has advanced ultrafast laser spectroscopy in condensed matter science, biology, physics, and chemistry. Compared to the frequently used photonic crystal fibers and bulk materials, femtosecond laser filamentation in gases is damage-immune for supercontinuum generation. A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level.

Effect of Molecular Orbital Angular Momentum on the Spatial Distribution of Fluorescence during Femtosecond Laser Filamentation in Air.

Nonuniform azimuthal distribution of N fluorescence emitted from the femtosecond laser filament in air was discovered. The fluorescence is stronger when the detector is placed perpendicular or parallel to the laser polarization. The experimental results have been confirmed by the theoretical calculation that the azimuthal distribution of fluorescence is reproduced by the convolution of the transition of the dipole and the molecular alignment in the strong laser field.

Femtosecond laser filament-assisted AgI-type pyrotechnic nucleant-induced water condensation in cloud chamber.

AgI-type pyrotechnics are widely used in the field of weather modification, as a kind of artificial ice nuclei. However, their precipitation yield remains an intensively studied area. In this paper, we present a study of AgI-type pyrotechnic nucleant-induced water condensation promoted by femtosecond laser filaments in a cloud chamber.

Laser guided ionic wind.

We report on a method to experimentally generate ionic wind by coupling an external large electric field with an intense femtosecond laser induced air plasma channel. The measured ionic wind velocity could be as strong as >4 m/s. It could be optimized by increasing the strength of the applied electric field and the volume of the laser induced plasma channel.

Temporal evolution of condensation and precipitation induced by a 22-TW laser.

Water condensation and precipitation induced by 22-TW 800-nm laser pulses at 1 Hz in an open cloud chamber were investigated in a time-resolved manner. Two parts of precipitation in two independent periods of time were observed directly following each laser shot. One part started around the filament zone at t < 500 μs and ended at t ≅ 1.

Probing the effective length of plasma inside a filament.

We present a novel method based on plasma-guided corona discharges to probe the plasma density longitudinal distribution, which is particularly good for the weakly ionized plasmas (~10 cm). With this method, plasma density longitudinal distribution inside both a weakly ionized plasma and a filament were characterized. When a high voltage electric field was applied onto a plasma channel, the original ionization created by a laser pulse would be enhanced and streamer coronas formed along the channel.

Picosecond laser-induced water condensation in a cloud chamber.

We investigated water condensation in a laboratory cloud chamber induced by picosecond (ps) laser pulses at ~350 ps (800 nm/1-1000 Hz) with a maximum peak power of ~25 MW. The peak power was much lower than the critical power for self-focusing in air (~3-10 GW depending on the pulse duration). Sparks, airflow and snow formation were observed under different laser energies or repetition rates.

Simultaneous identification of multi-combustion-intermediates of alkanol-air flames by femtosecond filament excitation for combustion sensing.

Laser filamentation produced by the propagation of intense laser pulses in flames is opening up new possibility in application to combustion diagnostics that can provide useful information on understanding combustion processes, enhancing combustion efficiency and reducing pollutant products. Here we present simultaneous identification of multiple combustion intermediates by femtosecond filament excitation for five alkanol-air flames fueled by methanol, ethanol, n-propanol, n-butanol, and n-pentanol. We experimentally demonstrate that the intensities of filament-induced photoemission signals from the combustion intermediates C, C2, CH, CN increase with the increasing number of carbons in the fuel molecules, and the signal ratios between the intermediates (CH/C, CH/C2, CN/C, CH/C2, CN/CH) are different for different alkanol combustion flames.

Femtosecond laser filament induced condensation and precipitation in a cloud chamber.

A unified picture of femtosecond laser induced precipitation in a cloud chamber is proposed. Among the three principal consequences of filamentation from the point of view of thermodynamics, namely, generation of chemicals, shock waves and thermal air flow motion (due to convection), the last one turns out to be the principal cause. Much of the filament induced chemicals would stick onto the existing background CCN's (Cloud Condensation Nuclei) through collision making the latter more active.

Laser-filamentation-induced water condensation and snow formation in a cloud chamber filled with different ambient gases.

We investigated femtosecond laser-filamentation-induced airflow, water condensation and snow formation in a cloud chamber filled respectively with air, argon and helium. The mass of snow induced by laser filaments was found being the maximum when the chamber was filled with argon, followed by air and being the minimum with helium. We also discussed the mechanisms of water condensation in different gases.

Critical power and clamping intensity inside a filament in a flame.

We report on measurements of both the critical power for self-focusing of a Ti: Sapphire 800 nm femtosecond laser and the peak intensity clamped inside a single filament in an ethanol-air flame on an alcohol burner array. By observing the shift of focal position of femtosecond laser pulses, we determine the critical power in the flame to be 2.2 ± 0.

Direct observation of laser guided corona discharges.

Laser based lightning control holds a promising way to solve the problem of the long standing disaster of lightning strikes. But it is a challenging project due to insufficient understanding of the interaction between laser plasma channel and high voltage electric filed. In this work, a direct observation of laser guided corona discharge is reported.

Pulse polarization evolution and control in the wake of molecular alignment inside a filament.

The polarization evolution and control of a femtosecond laser pulse in the wake of molecular alignment inside a laser filament was investigated. A weak probe pulse was delayed with respect to the field-free revivals of the pre-excited rotational wave-packets created by an infrared filamenting pulse in nitrogen gas. 30° was set between the pump and probe's initial linear polarization directions in order to control the output probe's polarization ellipse.

Laser-filament-induced snow formation in a subsaturated zone in a cloud chamber: experimental and theoretical study.

1 kHz, 2 mJ, 45 fs, 800 nm laser pulses were fired into a laboratory diffusion cloud chamber through a subsaturated zone (relative humidity ∼73%, T ∼ 4.3 °C). After 60 min of laser irradiation, an oval-shaped snow pile was observed right below the filament center and weighed ∼12.

Laser filamentation induced air-flow motion in a diffusion cloud chamber.

We numerically simulated the air-flow motion in a diffusion cloud chamber induced by femtosecond laser filaments for different chopping rates. A two dimensional model was employed, where the laser filaments were treated as a heat flux source. The simulated patterns of flow fields and maximum velocity of updraft compare well with the experimental results for the chopping rates of 1, 5, 15 and 150 Hz.

Identification of the physical mechanism of generation of coherent N2(+) emissions in air by femtosecond laser excitation.

Recently, amplification of harmonic-seeded radiation generated through femtosecond laser filamentation in air has been observed, giving rise to coherent emissions at wavelengths corresponding to transitions between different vibrational levels of the electronic B(2)Σ(u)(+) and X(2)Σ(g)(+) states of molecular nitrogen ions [Phys. Rev. A.

Harmonic-seeded remote laser emissions in N₂-Ar, N₂-Xe and N₂-Ne mixtures: a comparative study.

We report on the investigation on harmonic-seeded remote laser emissions at 391 nm wavelength from strong-field ionized nitrogen molecules in three different gas mixtures, i.e., N₂-Ar, N₂-Xe and N₂-Ne.

Water vapor concentration measurement in air using filament-induced fluorescence spectroscopy.

Water vapor fluorescence in air induced by femtosecond laser filaments was systematically investigated. The fluorescence signal intensity was found to be linearly proportional to the water vapor concentration, which opens up the possibility of absolute humidity measurements, even remotely.

Laser-filamentation-induced condensation and snow formation in a cloud chamber.

Using 1 kHz, 9 mJ femtosecond laser pulses, we demonstrate laser-filamentation-induced spectacular snow formation in a cloud chamber. An intense updraft of warm moist air is generated owing to the continuous heating by the high-repetition filamentation. As it encounters the cold air above, water condensation and large-sized particles spread unevenly across the whole cloud chamber via convection and cyclone like action on a macroscopic scale.

Impressive laser intensity increase at the trailing stage of femtosecond laser filamentation in air.

The longitudinal distribution of the laser peak intensity inside a half meter long femtosecond laser filament in air is studied by measuring the signal ratio of two nitrogen fluorescence lines, 391 nm and 337 nm. The experimental results reveal that laser peak intensity initially remains almost constant (~4.3 × 10(13) W/cm2) inside the filament.

Mechanism of nanograting formation on the surface of fused silica.

Nanograting inscription with a tightly focused femtosecond beam on the surface of fused silica was studied. The width and spacing of grooves are shown to decrease with the increase of the number of overlapped shots in both stationary and scanning cases. We propose a model to explain this behavior, based on both the so-called nanoplasmonic model and the incubation effect.

Femtosecond laser filamentation for atmospheric sensing.

Powerful femtosecond laser pulses propagating in transparent materials result in the formation of self-guided structures called filaments. Such filamentation in air can be controlled to occur at a distance as far as a few kilometers, making it ideally suited for remote sensing of pollutants in the atmosphere. On the one hand, the high intensity inside the filaments can induce the fragmentation of all matters in the path of filaments, resulting in the emission of characteristic fluorescence spectra (fingerprints) from the excited fragments, which can be used for the identification of various substances including chemical and biological species.

Simple method of measuring laser peak intensity inside femtosecond laser filament in air.

Measurement of laser intensity inside a femtosecond laser filament is a challenging task. In this work, we suggest a simple way to characterize laser peak intensity inside the filament in air. It is based on the signal ratio measurement of two nitrogen fluorescence lines, namely, 391 nm and 337 nm.

Dynamic behavior of postfilamentation Raman pulses.

We report on the postfilamentation behavior of a Stokes pulse created from intense and collimated ultrashort pulses propagating in air. A systematic analysis of the pulse propagation revealed that the redshifted Raman pulse produced during filamentation had a larger divergence than the postfilamentation intense pump pulse. Also, the analysis of the far-field Stokes transverse ring revealed that the intensity in this ionization-free light channel is still sufficiently high to induce stimulated Raman scattering after ionization had ended.