Postfilament supercontinuum on 100 m path in air.

Pulses at 744 nm with 90 fs duration, 6 mJ energy, and a weakly divergent wavefront propagate for more than 100 m and generate a filament followed by an unprecedently long high intensity (≥1TW/cm2) light channel. Over a 20 m long sub-section of this channel, the pulse energy is transferred continuously to the infrared wing, forming spectral humps that extend up to 850 nm. From 3D+time carrier-resolved simulations of 100 m pulse propagation, we show that spectral humps indicate the formation of a train of femtosecond pulses appearing at a predictable position in the propagation path.

Fifteen meter long uninterrupted filaments from sub-terawatt ultraviolet pulse in air.

A technique is presented to create uninterrupted long ultraviolet filaments in air using appropriately structured transmission mesh. The mesh with different cell sizes was inserted into 10-cm parallel beam of 0.2-J, 248-nm, and 870-fs pulse propagating along ~100-m corridor. Transverse positions of multiple filaments formed by the optimum size cells were reproducible within at least 15 m along the propagation path. 3D+time simulations confirmed uninterrupted plasma channels with fixed positions in the transverse space similar to the experiment. Unoptimized cell size resulted in filaments shifting towards the cell center and destruction of uninterrupted filaments.

Corona discharge induced snow formation in a cloud chamber.

Artificial rainmaking is in strong demand especially in arid regions. Traditional methods of seeding various Cloud Condensation Nuclei (CCN) into the clouds are costly and not environment friendly. Possible solutions based on ionization were proposed more than 100 years ago but there is still a lack of convincing verification or evidence. In this report, we demonstrated for the first time the condensation and precipitation (or snowfall) induced by a corona discharge inside a cloud chamber. Ionic wind was found to have played a more significant role than ions as extra CCN. In comparison with another newly emerging femtosecond laser filamentation ionization method, the snow precipitation induced by the corona discharge has about 4 orders of magnitude higher wall-plug efficiency under similar conditions.

Ultrabroad Terahertz Spectrum Generation from an Air-Based Filament Plasma.

We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.

Transformation of terahertz spectra emitted from dual-frequency femtosecond pulse interaction in gases.

We demonstrate that the two basic physical mechanisms of terahertz (THz) generation in a femtosecond filament, namely, the free electron photocurrent and the nonlinear polarization of neutrals, can be identified through the spectral analysis of THz radiation. The contribution from the photocurrent peaks at the units of THz, while the neutrals yield the peak at the tens of THz. We suggest the practical implementation of such spectral analysis by varying the initial transform-limited laser pulse duration.

Remote THz generation from two-color filamentation: long distance dependence.

Remote terahertz (THz) generation from two-color filamentation is investigated as a function of the onset position of filaments. THz signals emitted by filaments produced at distances up to 55 m from the laser source were measured. However, from 9 m to 55 m, the THz signal decayed monotonically for increasing onset positions. With a simple calculation, the dominant factors associated to this decay were identified as group velocity mismatch of the two-color pulses and linear diffraction induced by focusing and propagating the second harmonic pulse.

Rationale and design of INTERSTROKE: a global case-control study of risk factors for stroke.

UNLABELLED: Stroke is a major global health problem. It is the third leading cause of death and the leading cause of adult disability. INTERHEART, a global case-control study of acute myocardial infarction in 52 countries (29,972 participants), identified nine modifiable risk factors that accounted for >90% of population-attributable risk. However, traditional risk factors (e.g. hypertension, cholesterol) appear to exert contrasting risks for stroke compared with coronary heart disease, and the etiology of stroke is far more heterogeneous. In addition, our knowledge of risk factors for stroke in low-income countries is inadequate, where a very large burden of stroke occurs. Accordingly, a similar epidemiological study is required for stroke, to inform effective population-based strategies to reduce the risk of stroke. METHODS: INTERSTROKE is an international, multicenter case-control study. Cases are patients with a first stroke within 72 h of hospital presentation in whom CT or MRI is performed. Proxy respondents are used for cases unable to communicate. Etiological and topographical stroke subtype is documented for all cases. Controls are hospital- and community-based, matched for gender, ethnicity and age (+/-5 years). A questionnaire (cases and controls) is used to acquire information on known and proposed risk factors for stroke. Cardiovascular (e.g. blood pressure) and anthropometric (e.g. waist-to-hip ratio) measurements are obtained at the time of interview. Nonfasting blood samples and random urine samples are obtained from cases and controls. Study Significance: An effective global strategy to reduce the risk of stroke mandates systematic measurement of the contribution of the major vascular risk factors within defined ethnic groups and geographical locations.

Non-radially polarized THz pulse emitted from femtosecond laser filament in air.

Femtosecond laser filament could produce THz wave in forward direction. In our experiment, THz pulse emitted from a femtosecond laser filament has been investigated. It was found that the polarization of the studied THz pulse mainly appears as elliptical. This observation supplements the previous conclusion obtained by C. D'Amico et al. that THz wave emitted by a filament is radially polarized. The mechanism of generating elliptically polarized THz wave has been interpreted by either four-wave optical rectification or second order optical rectification inside the filament zone where centro-symmetry of the air is broken by the femtosecond laser pulse.

Experimental and theoretical investigation of high-power laser ionization and dissociation of methane.

The main purpose of this paper is to report the high-power laser ionization-dissociation of CH(4) at various femtosecond (fs) laser intensities (from 1 x 10(14) W/cm(2) to 2 x 10(15) W/cm(2)) with a laser pulse duration of 48 fs. The generalized molecular Keldysh theory has been applied to calculate the ionization yields for CH(4)+ and CH(4)++. Outside the influence of the fs intense laser, we propose to calculate the mass spectra due to the decomposition of CH(4)+ and CH(4)++, using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The agreement between the experimental mass spectra and calculated mass spectra seems to be reasonable.

Bragg gratings photoinduced in ZBLAN fibers by femtosecond pulses at 800 nm.

Fiber Bragg gratings were written in thulium-doped and undoped single-mode ZBLAN fibers by focusing femtosecond laser pulses on the fiber core through a phase mask. Maximum index modulation of the order of 1 x 10(-3) was induced in both types of fibers. Measurements of the transverse refractive index changes across the core and cladding regions indicate that the grating formation originates from a negative index change.

Remote time-resolved filament-induced breakdown spectroscopy of biological materials.

We report, for what we believe to be the first time, on the feasibility of remote time-resolved filament-induced breakdown spectroscopy (FIBS) of biological materials. The fluorescence from egg white and yeast powder, induced by femtosecond laser pulse filamentation in air, was detected in the backward direction with targets located 3.5 m away from the detection system. The remarkably distinct spectra of egg white and yeast allow us to propose that this technique, time-resolved FIBS, could be potentially useful for remote detection and identification of harmful biological agents.

Transverse evolution of a plasma channel in air induced by a femtosecond laser.

We investigate the evolution of filamentation in air by using a longitudinal diffraction method and a plasma fluorescence imaging technique. The diameter of a single filament in which the intensity is clamped increases as the energy of the pump light pulse increases, until multiple filaments appear.

Simple 3-D characterization of ultrashort laser pulses.

The space and time distribution of electromagnetic energy is essential information for any laser pulse applications that require precision. Although many instruments quantify the temporal profile of ultrashort laser pulses, they are generally limited to space-averaged measurement. In this work, we present an extremely simple technique to characterize the spatial distribution of fluence, pulse duration and chirp of ultrashort light pulses. This technique is based upon imaging the two-photon fluorescence distribution generated by the laser pulse as it propagates through a dispersive medium. It is expected that this technique will provide to less specialized users a precise in-situ analysis of their ultrashort laser beam.

Destruction of polymer growth substrates for cell cultures in two-photon microscopy.

The choice of the growth substrate for cell cultures used in fluorescence microscopy is guided by several factors including the type of cells studied and the type of microscopy used. Usually, cells can be cultured on either polymer or glass substrates. One type of polymer, termed Aclar, presents several attractive features: the adhesive properties are better than those of glass, the optical properties are comparable to those of glass, it is biochemically inert, unbreakable, flexible and has a high surface tension, convenient for seeding cells on the cover slip. However, here we show that when imaging with two-photon microscopy, which is based on a femtosecond pulsed laser source, local damage of the Aclar substrate occurs, starting at an average intensity of 10(5) W cm(-2) at the focal point and for exposure times insufficient to cause cell damage. This leads to the appearance of gas bubbles on cultures plated on Aclar cover slips, which perturb the imaging. By contrast, this phenomenon does not occur on borosilicate cover slips, probably because of their different physical (thermal conductivity, absorbance, melting point) and material homogeneity properties. Thus, for cell culture applications using pulsed lasers with high intensities, the use of glass is preferable to Aclar. The results also reveal that substrates can be more susceptible to thermal damage than the cells themselves.

Experiment and simulations on the energy reservoir effect in femtosecond light filaments.

We report the results of an experiment and numerical simulations that demonstrate the large spatial extent and the effect of the so-called energy reservoir during the filamentation of femtosecond laser pulses in air. By inserting pinholes of different sizes in the filament path we observe different stages of development ranging from the termination of the filament, through its partial survival, to undisturbed propagation. A background containing up to 50% of the pulse energy is found to be necessary to maintain the filament formation, including a first refocusing.

Ultra-broad and coherent white light generation in silica glass by focused femtosecond pulses at 1.5 um.

We report on the observation of efficient and ultra-broadband white light supercontinuum generated by focusing femtosecond pulses from an optical parametric amplifier at 1.5 microm in silica glass. The characteristic white light spectrum is extending from 400 nm up to at least 1750 nm. At sufficiently high input powers stable white light patterns associated with the interference of spatially coherent filamentary sources were observed and analyzed. Unlike focusing with 800 nm pulses from a Ti-sapphire laser, the stable fringes formed for each spectral component were pronounced owing to significantly reduced destructive impact of optical breakdown on filamentation of femtosecond pulses at 1.5 microm. By taking advantage of this property, the formation of optical waveguides in silica glass with considerably broader range of writing parameters as compared to those fabricated with 800 nm pulses, was demonstrated.

Nonlinear propagation of fs laser pulses in liquids and evolution of supercontinuum generation.

Nonlinear propagation of fs laser pulses in liquids and the dynamic processes of filamentation such as self-focusing, intensity clamping, and evolution of white light production have been analyzed by using one- and two-photon fluorescence. The energy losses of laser pulses caused by multiphoton absorption and conical emission have been measured respectively by z-scan technique. Numerical simulations of fs laser propagation in water have been made to explain the evolution of white light production as well as the small-scale filaments in liquids we have observed by a nonlinear fluorescence technique.

Sensing of halocarbons using femtosecond laser-induced fluorescence.

A femtosecond laser-induced clean fluorescence technique was explored as a means to monitor halogenated alkanes in the atmosphere. Characteristic difluorocarbene radical (CF2) fluorescence in the UV-vis can be generated inside a femtosecond laser-induced filament for different halocarbons. We show that, due to different dissociation and excitation kinetics leading to fluorescence emission, it is possible to temporally resolve the characteristic fluorescence of CF2-containing halocarbons from that of background species, therefore enhancing the signal-to-noise ratio. Laboratory-scale experiments demonstrate the potential use of femtosecond laser-induced clean fluorescence for the remote sensing of halocarbons in the atmosphere. The combination of this detection strategy with LIDAR could allow the long-range monitoring of several atmospheric species with a single laser source, eventually leading to a better understanding of chemical and dynamic processes affecting global warming, ozone loss, tropospheric pollution, and weather prediction.

Optical breakdown versus filamentation in fused silica by use of femtosecond infrared laser pulses.

The competition between optical breakdown (OB) and laser-pulse filamentation (FL) in bulk fused silica is investigated by using a 1-kHz femtosecond infrared laser. We measure input powers corresponding to the threshold of OB and FL in terms of external focusing conditions. The results demonstrate that OB precedes FL for tight focusing, whereas for sufficiently long focal lengths FL takes places at a lower power than OB does.

Third-harmonic generation and self-channeling in air using high-power femtosecond laser pulses.

It is shown, both theoretically and experimentally, that during laser pulse filamentation in air an intense ultrashort third-harmonic pulse is generated forming a two-colored filament. The third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length. We argue that this is due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch. A rich spatiotemporal propagation dynamics of the third-harmonic pulse is predicted. Potential applications of this phenomenon to other parametric processes are discussed.