Femtosecond Laser Filament Research Articles

Supercontinuum enhancement and transfer induced by a plasma grating in air

We experimentally investigate the plasma grating created by two intersecting ultrashort laser pulses, and its effects on the supercontinuum generated from femtosecond laser filamentation. The enhanced supercontinuum generation is observed, with a plasma grating formed. And an energy transfer of the supercontinuum is demonstrated, from one pulse to the other, due to the diffraction property of plasma grating. The theoretical calculation accords well with the experimental result.

Energy transfer between laser filaments in liquid methanol

We demonstrate energy exchange between two filament-forming femtosecond laser beams in liquid methanol. Our results are consistent with those of previous works documenting coupling between filaments in air; in addition, we identify an unreported phenomenon in which the direction of energy exchange oscillates at increments in the relative pulse delay equal to an optical period (2.6 fs). Energy transfer from one filament to another may be used in remote sensing and spectroscopic applications utilizing femtosecond laser filaments in water and air.

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. Because of distinct excitation mechanisms, the signals of the two fluorescence lines increase with the laser intensity at different orders of nonlinearity. An empirical formula has been deduced according to which laser peak intensity could be simply determined by the fluorescence ratio.

Enhancement of peak intensity in a filament core with spatiotemporally focused femtosecond laser pulses

We demonstrate that the peak intensity in the filament core, which is inherently limited by the intensity clamping effect during femtosecond laser filamentation, can be significantly enhanced using spatiotemporally focused femtosecond laser pulses. In addition, the filament length obtained by spatiotemporally focused femtosecond laser pulses is \ensuremath{\sim}25 times shorter than that obtained by a conventional focusing scheme, resulting in improved high spatial resolution.

Optical breakdown of air triggered by femtosecond laser filaments

We report experiments on the generation of dense plasma channels in ambient air using a dual laser pulse excitation scheme. The dilute plasma produced through the filamentation of an ultraintense femtosecond laser pulse is densified via avalanche ionization driven by a co-propagating multi-Joule nanosecond pulse.

Triggering of high voltage discharge by femtosecond laser filaments on different wavelengths

The triggering and guiding of high voltage discharge by plasma filaments generated by 400 nm and 800 nm femtosecond laser were studied experimentally. The detailed diagnostics for characteristics of filaments, such as amount of free electrons, diameter and electron density, were performed using sonographic method, fluorescence imaging and resistivity measurement. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser pulse filaments was observed. It is shown that the 400 nm laser pulses demonstrated stronger triggering ability than 800 nm laser pulse under same pulse energy. This behavior of 400 nm femtosecond pulse is connected to the rich population of highly excited particles in filaments.

An intense tunable femtosecond gas-plasma THz source: Application in spectroscopic studies of polycyclic aromatic hydrocarbons

Abstract We present a review on the emission of intense THz pulses from 2-color femtosecond laser filaments in gases. Through the tailoring of the filamentation process tuning of the emitted THz pulses is possible and is described in details. Using these THz pulses as a spectroscopic tool, the absorption spectra of small polycyclic aromatic hydrocarbons and their halogenated compounds are studied. Distinct resonance features for all the examined compounds have been observed, demonstrating the potential of the approach for rapidly discriminating the molecular structure of large organic molecules, in their crystalline form.

Efficient generation of third harmonic radiation in air filaments: A revisit

In this paper we revisit the third harmonic generation from a femtosecond laser filament in air and its significant enhancement (~ 100 times) with a intercepting pump pulse, which has been reported very recently. The enhanced third harmonic is studied as a function of the pulse polarization, time delay between the pump and signal pulses, laser pulse energy, etc. We provide an explanation for the enhancement of third harmonic based on a quenching of interference effects present near filamentation threshold. Simulations based on a two-color propagation code reproduce well the experimental observations and confirm our interpretation.

Remote generation of high-energy terahertz pulses from two-color femtosecond laser filamentation in air

We experimentally investigated the dynamic behavior of remote terahertz (THz) generation from two-color femtosecond laser-induced filamentation in air. A record-high THz pulse energy of 570 nJ at frequency below 5.5 THz was measured by optimizing the pump parameters at a controllable remote distance of 16 m, while super-broadband THz (300 THz) pulse energy was up to 2.8 \ensuremath{\mu}J. A further energy-scaling possibility was proposed. By analyzing simultaneously the fluorescence from both neutral ${\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}$${}^{+}$ in the filament, we found that the enhancement of THz radiation was due principally to guiding of the weak second-harmonic pulse inside the filament of the first strong fundamental pulse.

Experimental Tests of the New Paradigm for Laser Filamentation in Gases

Since their discovery in the mid-1990s, ultrafast laser filaments in gases have been described as products of a dynamic balance between Kerr self-focusing and defocusing by free electric charges that are generated via multiphoton ionization on the beam axis. This established paradigm has been recently challenged by a suggestion that the Kerr effect saturates and even changes sign at high intensity of light and that this sign reversal, not free-charge defocusing, is the dominant mechanism responsible for the extended propagation of laser filaments. We report qualitative tests of the new theory based on electrical and optical measurements of plasma density in femtosecond laser filaments. Our results consistently support the established paradigm.

Pulse characterization during femtosecond laser filamentation in air by two-photon fluorescence measurement

This work reports a temporal characteristic of a laser pulse undergoing filamentation in air. The diagnostic method is based on two-photon fluorescence measurement. The results show that the pulse duration, chirp rate and beam radius could be simultaneously quantitatively retrieved. This simple technique would be useful in practice to trace the underlying dynamics of filamentation in air.

Controlling fluorescence from N2 inside femtosecond laser filaments in air by two-color laser pulses

We experimentally demonstrate the feasibility of controlling the fluorescence emission of nitrogen molecules in air induced by femtosecond laser filamentation by using a pump-probe method. An obvious enhancement or reduction in the filament-induced fluorescence signals of nitrogen molecules can be realized when a blue (400 nm) or an infrared (1338 nm) laser pulse is used as the probe. The completely opposite effect is ascribed to the excitation enhancement of ionization and population trapping of some highly excited states including Rydberg states of nitrogen molecules.

Experimental observation of a traveling plasma grating formed by two crossing filaments in gases

The spatial motion and effective duration of a traveling plasma grating formed by two interfering femtosecond laser filaments in gases is characterized by its Doppler effect imparted on a probe pulse. The shift velocity determined experimentally agrees with the theoretical calculations.

基于双光子荧光原理的空气中飞秒激光成丝区域脉冲特性的测量

基于双光子荧光原理的空气中飞秒激光成丝区域脉冲特性的测量

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. On the other hand, along with the femtosecond laser filamentation, white-light supercontinuum emission in the infrared to UV range is generated, which can be used as an ideal light source for absorption Lidar. In this paper, we present an overview of recent progress concerning remote sensing of the atmosphere using femtosecond laser filamentation.

Tailoring the filamentation of intense femtosecond laser pulses with periodic lattices

We show numerically that by using periodic lattices the filamentation of intense femtosecond laser pulses, otherwise a result of competing nonlinear effects, can be well controlled with respect to its properties. The diffraction induced by the lattice provides a regularizing mechanism to the nonlinear self-action effects involved in filamentation. We demonstrate a new propagation regime of intense lattice solitons bridging the field of spatial solitons with that of femtosecond laser filamentation. The effective filamentation control is expected to have an important impact on numerous applications.

Intensity clamping during dual-beam interference

One has prospected to overcome the intensity clamping phenomenon during femtosecond laser filamentation by superimposing two beams. This experimental scheme is thus studied in our work. We have realized the interference of two ultrashort laser beams and one of them creates a filament in air. The formation of a dynamic plasma grating induced by the interference pattern is observed. However, qualitative analysis of the experimental results indicates that intensity clamping still governs the intensity evolution during the dual-beam interaction. The expectation of greatly increasing laser intensity by dual beams superposition might not be realistic.

Self-Shortening Dynamics Measured along a Femtosecond Laser Filament in Air

The filamentation-induced temporal shortening of a 40 femtosecond pulse propagating in air is traced using impulsive vibrational Raman scattering and measurement of the power spectrum as a function of position along the propagation axis. The N2, O2, and H2O vibrational Raman responses reveal self-shortening of pulse features to 14 fs during the first filamentation cycle and to at least 9 fs in the second cycle. Spectral measurements further demonstrate that the coherent bandwidth generated in the region from 470 to 330 nm during the self-shortening process forms the ∼9 fs pulse.

Coherent control of THz pulses polarization from femtosecond laser filaments in gases

We demonstrate the possibility to rotate the polarization of linearly polarized THz pulses via the accurate control of the 2-color filament surrounding gas pressure. We also show ways to produce elliptically and circularly polarized THz pulses.

Energy Exchange between Femtosecond Laser Filaments in Air

We report on the energy exchange between femtosecond laser filaments in air. A traveling plasma grating formed at the intersection of the filaments is proposed to explain the energy transfer. In this moving plasma grating mediated mechanism the laser energy transfers from the lower frequency pulse to its higher frequency partner, while in a traditional Kerr nonlinearity mediated grating, energy transfer occurs in the opposite manner. Based on this mechanism, we demonstrate an energy transfer ratio as high as 50% for pulses with an energy of several millijoules.