Characterization of the third-harmonic radiation generated by intense laser self-formed filaments propagating in air

We perform laboratory experiments to study ultraviolet radiation generated by intense self-formed laser filaments produced by propagating high-power femtosecond laser pulses in air. The laser used in the experiment is a 0.5 TW Ti:sapphire system with the center wavelength at 800 nm. The observed ultraviolet emission occurs in the form of the third harmonic and frequency-upshifted radiation from the fundamental. We present direct characterization of the generated harmonic and frequency-upshifted radiation, including transverse imaging and spatially resolved spectral measurements.     

Measurement of filament length generated by an intense femtosecond laser pulse using electromagnetic radiation detection

We present a new method to measure the length of a filament induced by the propagation of intense femtosecond laser pulses in air. We used an antenna to detect electromagnetic pulses radiated from multipole moments inside the filament oscillating at the plasma frequency. The results are compared with the values detected from the backscattered fluorescence induced by multiphoton ionization of nitrogen molecules excited inside the filament. The values are found to be in good agreement. Received: 6 November 2002 / Revised version: 27 January 2003 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: shosseini@phy.ulaval.ca     

Forward acceleration and generation of femtosecond megaelectronvolt electron beams by an ultrafast intense laser pulse

We present a new mechanism of energy gain of electrons accelerated by a laser pulse.It is shown thatwhen the intensity of an ultrafast intense laser pulse decreases rapidly along the direction of propagation,electrons leaving the pulse experience an action of ponderomotivc deceleration at the descending part ofa lower-intensity laser field than acceleration at the ascending part of a high-intensity field, thus gain netenergy from the pulse and move directly forward. By means of such a mechanism, a megaelectronvoltelectron beam with a bunch length shorter than 100 fs could be realized with an ultrafast(≤30 fs),intense (>10~(19)W/cm~2)laser pulse.     

High-power third-harmonic flat pulse laser generation

The generation of a high-power laser pulse at 266 nm that is longitudinally shaped according to a prefixed intensity profile is reported. The main features of the pulse shape modifications due to second- and third-harmonic conversions are measured, and the results are in good agreement with the theory. The UV temporal shape depends on the chirp of the fundamental pulse and on the crystal phase-matching angle. Exploiting the large stretching imposed on the third-harmonic signal, we show that the pulse intensity profile can be obtained by spectral single-shot measurements.     

Multiframe observation of an intense femtosecond optical pulse propagating in air

We have demonstrated femtosecond time-resolved and picosecond time-interval successive observations of a single femtosecond optical pulse propagating in air with ultrafast self-modulation such as filamentation. A quadruple femtosecond probe pulse crossing an intense propagating pulse at picosecond intervals was able to capture directly four successive images of the propagating pulse as in a movie but with femtosecond time resolution. From this observation, we can directly analyze and recognize the propagation process, which is significantly affected by pulse-energy fluctuation, and (or) atmospheric turbulence from shot to shot.     

Harmonic generation by an intense laser pulse in neutral andionized gases

Reported are the results of a harmonic generation experiment in a simple gas (hydrogen) using 1-ps, 1-μm laser pulses with a range of intensities extending from below to far above the laser ionization saturation threshold. The scaling with intensity above saturation of the third harmonic generated by a single laser-pulse in a filled gas cell is observed not to fit with a simple model that takes into consideration volume ionization effects alone. In another experiment, a pump-probe type, an upper limit on the conversion efficiency of third-harmonic generation in a preformed plasma is determined. It is found to be in agreement with the efficiency predicted by a relativistic harmonic generation theory     

Second-and third-harmonic generation by carbon nanotubes irradiated with femtosecond laser pulses

Femtosecond pulses of a Cr:forsterite laser are used to study second-and third-harmonic generation in a layer of single-wall carbon nanotubes produced by low-velocity spraying. The harmonic amplitude in our experiments scales as (I _p)ⁿ as a function of the pump intensity I _p, with n=2 and 3 for the second and third harmonics, respectively. This scaling law holds up to pump intensities on the order of 10¹²W/cm². The ratio of the maximum signal to the averaged background in the spectra of the second and third harmonics is estimated as 50 and 30, respectively. The second and third harmonics produced by a linearly polarized pump field are also linearly polarized, with their polarization vectors oriented along the polarization direction of the pump field. The capabilities of nonlinear-optical methods for structural and morphological analysis of carbon nanotubes are discussed, as well as ways to create solid-state carbon-nanotube generators of optical harmonic.     

超短脉冲强激光在空气中的传输

超短脉冲强激光在空气中传输时由于非线性克尔自聚焦效应会使激光光束聚焦，造成空气的离化而形成等离子体，等离子体对激光光束又会产生散焦作用，这两种过程的动态平衡可以形成很长的等离子体通道，从而产生一系列复杂而有趣的现象．文章对通道形成的机理、锥角辐射、超连续谱和三次谐波的产生进行了阐述，并介绍了目前等离子体通道形成的几种理论模型．文章还对通道内的各种复合和辐射机制进行了分析．     

Ultrafast electron dynamics on the silicon surface excited by an intense femtosecond laser pulse

The electron dynamics on the silicon surface during the pump ultrashort infrared laser pulse is studied by time-resolved optical microscopy and electron-emission measurements. It is found that the optical response of the material under the conditions where a dense electron-hole plasma is formed is determined by the renormalization of the band spectrum of the material rather than by intraband transitions of photoexcited carriers. Nonlinear Auger recombination in the plasma enhanced by the plasma-induced renormalization of the band gap and accompanied by the generation of hot charge carriers stimulates intense prompt emission of such carriers from the surface of the photoexcited material, whose work function decreases owing to the large plasma-induced renormalization of the energies of higher conduction bands.     

Simulation of third-harmonic and supercontinuum generation for femtosecond pulses in air

Numerical simulations of third-harmonic and supercontinuum generation for femtosecond pulses propagating in air are presented which, for the first time to the best of our knowledge, do not rely on the decomposition of the total field into slowly-varying fields centered around the fundamental and third-harmonic. The simulation results are interpreted using an effective three-wave mixing approach, and together they provide new insights into the relation between third-harmonic and supercontinuum radiation. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

Multiple filamentation of intense femtosecond laser pulses in air

The propagation of focused femtosecond laser pulses with supercritical peak powers in air has been investigated by the methods of optical visualization, profilometry, and calorimetry. Laser pulses with supercritical powers create a bundle of submillimeter filaments with a diameter of about 5 μm ahead of the lens focus; the maximum number of filaments in the beam cross section and their length increase linearly and sublinearly, respectively, with the radiation peak power. The optical visualization and calorimetry indicate that the plasma channels of filaments are optical contrast (a plasma density of 10¹⁸–10¹⁹ cm⁻³), ensuring the refraction of laser radiation incident on them.     

High-order optical vortex harmonics generated by relativistic femtosecond laser pulse

Harmonics generated by an intense femtosecond Gaussian laser pulse normally incident on a spiral-shaped thin foil target were studied.By using two-dimensional(2D) particle-in-cell(PIC) simulation,we observed evident odd harmonics signals in the reflection direction and found that the reflected field has a helical structure determined by the target shape.This method provides a new way to generate an intense ultraviolet vortex with high-order topological charge by use of ultrahigh intense laser-driven harmonics.     

Three-dimensional simulation on explosions of hydrogen atomic clusters irradiated by an intense femtosecond laser pulse

Using classic particle dynamics simulations, the interaction process between an intense femtosecond laser pulse and icosahedral hydrogen atomic clusters H_{13}, H_{55} and H_{147} has been studied. It is revealed that with increasing number of atoms in the cluster, the kinetic energy of ions generated in the Coulomb explosion of the ionized hydrogen clusters increases. The expansion process of the clusters after laser irradiation has also been examined, showing that the expansion scale decreases with increasing cluster size.     

Interaction of light filaments generated by femtosecond laser pulses in air

The interaction of two light filaments propagating in air is simulated. Simulations show that the interaction of the two light filaments displays interesting features such as attraction, fusion, repulsion, and spiral propagation, depending on the relative phase shift and the crossing angle between them. A long and stable channel can be formed by fusing two in-phase light filaments. The channel becomes unstable with the increase of the crossing angle and phase shift. The interaction of two light filaments in different planes is studied and the spiral propagation is observed.     

A LIDAR technique to measure the filament length generated by a high-peak power femtosecond laser pulse in air

We demonstrate a new method for measuring the length of a femtosecond laser pulse induced filament in air using a LIDAR (LIght Detection And Ranging) technique. The LIDAR involves a detector with a fast response time. The back-scattered multiphoton induced fluorescence from nitrogen molecules excited inside the filament is measured, from which the length of the filament can be determined. We find good agreement between the measured filament length and the length estimated from burn patterns on paper. In addition, good qualitative agreement between the experimental measurement and numerical simulations is obtained for the signal features of the filament. We propose that this new method can be used to quantitatively determine filamentation at longer distances. Received: 11 June 2002 / Revised version: 21 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: aiwasaki@phy.ulaval.ca     

Multistep third-harmonic generation of femtosecond laser pulses in periodically-poled and chirped-periodically-poled lithium niobate

Simultaneous generation of the second and third harmonics in periodically-poled and chirped-periodically-poled lithium niobate (PPLN and CPPLN) is studied numerically taking into account mismatches and dispersions of the group velocities of interacting laser pulses. It is shown that CPPLN frequency converters can be successfully used for creating multicolor short laser pulses.     

等离子体通道的声学诊断方法

超短脉冲强激光在空气中传输可以形成很长的等离子体通道，目前对这种等离子体通道有许多诊断方法，文章介绍了一种十分简易但很有效的方法——声学测量方法．该方法可以根据通道不同位置的声音信号，测量通道的长度和通道内电子密度等一些参数，具有其他传统诊断方法无可比拟的优越性．     

Self-focusing of a focused femtosecond laser pulse in air

The regularities of self-focusing and filamentation of spatially focused ultra-short laser radiation in air are theoretically considered. The relationship between beam focusing sharpness and effective characteristics of the filamentation zone is investigated. The formation of ‘narrow’ and ‘wide’ light filaments compared to that of collimated radiation filamentation, upon high-power focused laser pulse propagation, is established.     

Coherent control of high-order harmonics generated with intense femtosecond laser pulses

Experimental results and theoretical analysis on the coherent control of high-order harmonics with chirped femtosecond laser pulses are presented. The coherent control of high-order harmonic generation resulted in sharp harmonic spectra by compensating for induced harmonic chirp with the control of applied laser chirp and it was found to be crucial also in producing sharp and bright harmonics.Received: 18 November 2002, Published online: 8 July 2003PACS: 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation - 32.80.-t Photon interactions with atoms - 42.65.Re Ultrafast processes; optical pulse generation and pulse compression