Generation of ultrabroadband terahertz radiation under optical breakdown of air by two femtosecond pulses of different frequencies

Experimental results on the generation of terahertz (THz) electromagnetic radiation under ionization of air by femtosecond pulses at fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. The mean power of the generated THz radiation as a function of the time delay between two pulses of different frequencies is found to be quasi-periodical. Theoretically, we show that efficient generation of THz radiation is governed by the inertial part of the nonlinear response of the medium, which is determined by the dynamics of population of highly excited states with subsequent emission of electrons.     

High-order harmonics generation by few-cycle and multi-cycle femtosecond laser pulses

This paper investigates experimentally high-order harmonic generation(HHG) of neon gas with 5-fs and 25-fs driving laser pulses.It has been demonstrated that the cutoff energy of the harmonic extreme ultraviolet photons is extended to 131 eV and the HHG spectrum near the cutoff region becomes continuum as the driving laser pulse duration is 5 fs;whereas much lower cutoff photon energy and discrete harmonic spectrum near the cutoff region are presented as the laser pulse duration is 25 fs.The results can be explained by the fact that neutral atoms can be exposed to more intense laser field before they are depleted by ionization because of the extremely short rising time of the few-cycle pulse.The 5-fs driving laser pulse paves the way of generation of coherent x-ray in the water window and single attosecond pulse.     

All-reflective self-referenced spectral interferometry for single-shot measurement of few-cycle femtosecond pulses in a broadband spectral range

An all-reflective self-referenced spectral interferometry based on the transient grating(TG)effect is proposed for single-shot measuring of the amplitude and phase of ultrashort pulses in a broadband spectral range.Except for a thin third-order nonlinear medium,which was used to generate the TG signal,no transmitted optics were used in the proposed device,and few-cycle pulses in a broad spectral range from deep UV to mid-IR can be characterized.With a homemade compact and alignment-free device,a 5.0 fs pulse at 800 nm corresponding to about two cycles and a 14.3 fs pulse at 1800 nm corresponding to less than three cycles were successfully characterized.     

Spectral features of the interaction of femtosecond light pulses of different frequencies near the boundary of a Kerr medium

The interaction of probe ultraviolet (UV) and intense infrared (IR) pump pulses with a duration of 150 fs in a thin 2-mm-long sample of fused silica has been studied theoretically and experimentally. The spectra of UV radiation at the output of the sample have been measured depending on the time delay between the pulses at the sample input. The maximum shifts of the spectrum, attaining up to 0.22 nm for an IR power density of 3 × 10¹¹ W/cm², have been observed under conditions of coincidence of the pulses at the sample input and output, which corresponds to a predominant interaction of the probe radiation with the leading and trailing edges of the pump pulse near the boundary of the medium. The observed dependences are interpreted as a manifestation of the cross-phase modulation due to the Kerr nonlinearity of the medium and the dispersion of the group velocities of the UV and IR pulses. The numerical simulations performed taking into account these effects agree well with the experimental data.     

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     

少周期激光脉冲与气体作用产生的离化电流和THz波辐射

本文通过理论和数值模拟,研究少周期激光脉冲电离气体原子产生的离化电流 以及相应的THz波辐射.研究表明,少周期激光脉冲离化气体后能产生较大的离化电流, 因而可以产生较强的THz辐射.不同的少周期激光脉冲相位导致电离出的 电子初始速度和电离起始时刻不同,从而产生的离化电流有所不同, 辐射的THz波随激光脉冲的相位成周期性变化.该理论得到一维PIC数值模拟的验证. 对于给定的激光脉冲相位,离化电流和THz辐射振幅并没有随入射激光振幅的增加而单调增加, 而是存在一些极值点.与均匀分布气体相比,当气体分布具有一定梯度时, 辐射表现相似的规律,但频谱会发生一定的变化.     

Nonlinear spectral cleaning of few-cycle pulses via cross-polarized wave (XPW) generation

The characterization of a temporal filter based on cross-polarized wave generation working in the few-optical-cycle regime is presented. We show that this device dramatically improves the spectral quality of the ultrashort seed pulses, opening the way to the production of sub-10?fs pulses with high incoherent and coherent contrast. The dispersion compensation conditions for an optimized behavior of the process are experimentally and theoretically discussed.     

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.     

Resonant generation of few-cycle pulses in hydrogenlike atoms

We show the possibility to produce nearly bandwidth-limited few-cycle pulses based on time-dependent resonant interaction of an incident radiation with the bound states of hydrogenlike atoms. A time-dependence of an atomic resonance is provided by far-off-resonant laser field with intensity much below the atomic ionization threshold via time-dependent tunnel ionization from the excited states and temporal adiabatic Stark splitting of the excited energy levels. Without external synchronization of the spectral components it is possible to produce a train of two-cycle pulses at 121.6 nm with duration 800 as in atomic hydrogen.     

Broadband supercontinuum generation in air using tightly focused femtosecond laser pulses

Supercontinuum generation in air using tightly focused femtosecond laser pulses was investigated experimentally. Broadband white-light emission covering the whole visible spectral region was generated. Spectral broadening extended only to the blue side of the fundamental frequency due to the phase modulation induced by the strong ionization of air. Numerical simulation was also performed to confirm the spectral broadening mechanism. A constant UV cutoff wavelength close to 400 nm was observed in the supercontinuum spectrum. This phenomenon indicated that intensity clamping still plays a role in tight focusing geometry.     

Concatenation of plasma filaments created in air by femtosecond infrared laser pulses

We report the first observation of the attachment of two single plasma filaments created collinearly in the atmosphere by IR femtosecond laser pulses. The linked filamentary structure is electrically conductive and emits sub-THz radiation over its entire length. Concatenation is achieved only for a specific time ordering between the two initial laser pulses. The pulse producing the filament closer to the laser source must be retarded with respect to the other pulse. This special time ordering is attributed to the acceleration of light in a self-guided pulse. Received: 4 March 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/6931-9996, E-mail: stzortz@ensta.fr     

Single-shot measurement of the spectral envelope of broad-bandwidth terahertz pulses from femtosecond electron bunches

We present a new approach (demonstrated experimentally and through modeling) to characterize the spectral envelope of a terahertz (THz) pulse in a single shot. The coherent THz pulse is produced by a femtosecond electron bunch and contains information on the bunch duration. The technique, involving a single low-power laser probe pulse, is an extension of the conventional spectral encoding method (limited in time resolution to hundreds of femtoseconds) into a regime only limited in resolution by the laser pulse length (tens of femtoseconds). While only the bunch duration is retrieved (and not the exact charge profile), such a measurement provides a useful and critical parameter for optimization of the electron accelerator.     

Oxygen,nitrogen and air broadening of HCN spectral lines at terahertz frequencies

The room-temperature nitrogen- and oxygen-broadening coefficients of hydrogen cyanide spectral lines have been measured in the 0.5–3 THz (17–100 cm^?1) frequency range (purely rotational transitions with 5?J?36) by a continuous-wave terahertz spectrometer based on a photomixing source. An improved version of the Robert and Bonamy semiclassical formalism has been used to calculate the oxygen-broadening coefficients and resulted in a good agreement with these measurements. The nitrogen and oxygen data are combined to provide the air-broadening coefficients as used by the HITRAN database. A significant difference is observed between the measured and tabulated values for transitions with high values of the rotational quantum number. A new polynomial representation is suggested for inclusion in HITRAN. A similar polynomial expression has been derived for the nitrogen broadening to aid the studies of Titan's atmosphere.     

Influence of spectral phase on cross-polarized wave generation with short femtosecond pulses

In this paper we present the first comprehensive study of the role of spectral phase on cross-polarized wave (XPW) generation using sub-30 femtosecond (fs) laser pulses. XPW generation improves the temporal contrast and shortens the pulse duration of fs chirped pulse amplification (CPA) lasers. For Ti:Sa lasers, compression below 30?fs is non-trivial and therefore never perfect. We therefore systematically analyze the effect of an arbitrary input spectral phase on the output spectrum and efficiency of the XPW process, both theoretically and experimentally. We derive the maximum acceptable value of residual phase for a given initial pulse duration in order to efficiently drive the XPW process for pulse shortening and contrast improvement.     

3D features of modified photostructurable glass-ceramic with infrared femtosecond laser pulses

The exclusive ability of laser radiation to be focused inside transparent materials makes lasers a unique tool to process inner parts of them unreachable with other techniques. Hence, laser direct-write can be used to create 3D structures inside bulk materials. Infrared femtosecond lasers are especially indicated for this purpose because a multiphoton process is usually required for absorption and high resolution can be attained. This work studies the modifications produced by 450 fs laser pulses at 1027 nm wavelength focused inside a photostructurable glass-ceramic (Foturan^®) at different depths. Irradiated samples were submitted to standard thermal treatment and subsequent soaking in HF solution to form the buried microchannels and thus unveil the modified material. The voxel dimensions of modified material depend on the laser pulse energy and the depth at which the laser is focused. Spherical aberration and self-focusing phenomena are required to explain the observed results.     

Harmonic generation in the discrete spectral region of xenon using broadband femtosecond laser pulses

The conversion of 34-fs Ti:sapphire laser pulses into the wavelength region 105–210 nm has been studied in xenon for laser intensities up to 5×10¹³ W/cm². A strongly structured, pressure-dependent emission spectrum is observed. Radiation is detected in regions expected for the 5th and 7th harmonics but also in regions in between. In the resonance region (<147 nm), self-phase-modulation processes in the conversion medium together with phase-matched, efficient harmonic generation in negative-dispersive spectral regions explain the observations. Broadband emission is detected in the resonance-free, positive-dispersive spectral region 155–210 nm. Emission from the xenon dimer is superimposed on a direct 5th-harmonic signal. PACS 32.70.-n; 32.80.-t; 42.65.Ky; 42.65.Re     

Frequency comb generation using femtosecond pulses and cross-phase modulation in optical fiber at arbitrary center frequencies

A technique is presented for generating optical frequency combs centered at arbitrary wavelengths by use of cross-phase modulation (XPM) between a femtosecond pulse train and a cw laser beam by copropagating these signals through an optical fiber. We report results from use of this method to place a 90-MHz frequency comb on an iodine-stabilized Nd:YAG laser at 1064 nm and on a frequency-doubled Nd:YVO(4) laser at 532 nm. XPM is verified to be the comb-generating process, and the width of the frequency comb is measured and compared with theory. The spacing of the frequency comb is compared with the femtosecond source, and a frequency measurement with this comb is demonstrated.     

Generation of few-cycle radially-polarized infrared pulses in a gas-filled hollow-core fiber

We perform a numerical study for temporally compressing radially-polarized(RP) infrared pulses in a gas-filled hollow-core fiber(HCF). The dynamic transmission and nonlinear compression of RP pulses centered at wavelengths of0.8 m, 1.8 m, 3.1 m, and 5.0 m in HCFs are simulated. By comparing the propagation of pulses with the same optical cycles and intensity, we find that under proper conditions these pulses can be compressed down to 2–3 cycles. In the transverse direction, the spatiotemporal beam profile ameliorates from 0.8-m to 1.8-m and 3.1-m pulses before the appearance of high-order dispersion. These results show an alternative method of scaling generation for delivering RP infrared pulses in gas-filled HCFs, which can obtain energetic few-cycle pulses, and will be beneficial for relevant researches in the infrared scope.     

空气中激光等离子体通道的三次谐波辐射研究

对超强飞秒激光脉冲在空气中形成的长等离子体通道的三次谐波辐射进行了研究．实验研究发现三次谐波的转换效率在通道范围内基本保持不变.还对等离子体通道锥角辐射中的三次 谐波的辐射角进行了研究，结果表明其辐射角接近6mrad，与理论计算结果符合得很好． 关键词： 三次谐波 等离子体通道     

Numerical modeling of generation of few-cycle pulses in a mode-locked laser

We have performed numerical simulation of a laser generating few-cycle pulses in the regime of coherent mode locking. It is shown that such a laser can support generation of pulses with a duration close to the reciprocal of the main laser transition frequency of the gain medium in a wide range of laser parameters.