High harmonic XUV spectral phase interferometry for direct electric-field reconstruction

We demonstrate the first experimental complete temporal characterization of high-harmonic XUV pulses by spectral phase interferometry, with an all-optical setup. This method allows us to perform single-shot measurements of the harmonic temporal profile and phase, revealing a remarkable shot-to-shot stability. We characterize harmonics generated in argon by a 50 fs 800 nm laser pulse. The 11th harmonic is found to be 22 fs long with a negative chirp rate of -4.8 x 10(27) s(-2). This duration can be reduced to 13 fs by modulating the polarization of the generating laser. The technique is easy to implement and could be routinely used in femtosecond XUV pump-probe experiments with harmonics.     

Generation of attosecond pulses with ellipticity-modulated fundamental

We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated. PACS 42.65.Ky; 42.65.Re; 32.80.Wr     

Temporal confinement of the harmonic emission through polarization gating

We show experimentally that a simple optical setup is able to reduce the duration of the harmonic emission generated by long (many optical cycles) laser pulses. By controlling the time-dependent ellipticity during the laser pulse, one can confine the harmonic emission to a narrow temporal window where the polarization is quasi-linear. The results obtained and their comparison with a simple model show a shortening of a factor 2 for harmonics located in the plateau region. Using shorter laser pulses of 15-20 fs duration should make it possible to isolate a single attosecond pulse.Received: 25 November 2002, Published online: 8 July 2003PACS: 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation - 32.80.Wr Other multiphoton processes     

Time-frequency characterization of femtosecond extreme ultraviolet pulses

We present energy-resolved cross-correlation measurements of an extreme ultraviolet (XUV) pulse, generated as the fifth harmonic (15.5 eV) of an intense 80 fs laser pulse centered at 400 nm. Spectrally resolving the cross-correlation signal allows us to characterize the time-dependent frequency of the XUV pulse. We find that the fifth harmonic has a small negative chirp in excess of that predicted by perturbation theory. In addition, by manipulating the chirp of the driving laser we can induce and measure a positive or a negative chirp on the XUV pulse.     

Femtosecond-resolution measurement of soft-X-ray pulse duration using ultra-fast population increase of singly charged ions induced by optical-field ionization

We demonstrate sub-100-fs resolution of a cross-correlation method for measuring the duration of soft-X-ray pulses. The method uses the ultra-fast increase in a singly charged ion population induced by optical-field ionization as a soft-X-ray -absorption switch. We measured the pulse duration of the 51st harmonic of a Ti:sapphire laser pulse using Kr gas as a soft-X-ray absorption medium and found it to be 60 fs assuming that the harmonic envelope is equal to a squared secant hyperbolic. This confirmed that our method achieves a shorter temporal resolution than the 100-fs pulse duration of the ionizing laser pulse. The temporal resolution obtained in this way is expected to be from one-third to one-half the duration of the ionizing laser pulse, according to our calculation of the time-evolving population of the Kr⁺ ions. The experimental demonstration and calculation show that methods based on optical-field-induced ionization are promising for femtosecond temporal characterization of an ultra-short pulse in the soft-X-ray region. PACS 42.50.Hz; 42.65.Ky; 32.80.Rm; 06.60.Jn     

相对论圆偏振激光与固体靶作用产生高次谐波

超短超强激光与固体靶表面等离子体相互作用可以通过高次谐波的方式产生从极紫外到软X射线波段的相干辐射,获得飞秒甚至阿秒量级的超短脉冲,可用于观测原子或分子中的电子运动等超快动力学过程.本文实验研究了相对论圆偏振飞秒激光与固体靶相互作用的高次谐波产生过程,实验结果表明,在较大入射角下,圆偏振激光也可以有效地产生高次谐波辐射.通过预脉冲控制靶表面的预等离子体密度标长,发现高次谐波的产生效率随密度标长的增加而单调下降.进一步通过二维粒子模拟程序,分析了激光的偏振以及预等离子体密度标长对高次谐波产生的影响,很好地解释了实验观测结果.     

Flexible generation of femtosecond cylindrical vector beams(Invited Paper)

Femtosecond(fs)cylindrical vector beams(CVBs)have found use in many applications in recent years.However,the existing rigid generation methods seriously limit its development.Here,we propose a flexible method for generating fs-CVBs with arbitrary polarization order by employing half wave plates and vortex retarders.The polarization state,autocorrelation width,pulse width,and spectrum features of the input and generated CVB pulses are measured and compared.The results verify that the generated CVBs remain in the fs regime with no appreciable temporal distortion,and the energy conversion efficiency can reach as high as 96.5%,even for a third-order beam.As a flexible way to generate fs-CVBs,this method will have great significance for many applications.     

Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field

We experimentally demonstrate the generation of an extreme-ultraviolet (XUV) supercontinuum in argon with a two-color laser field consisting of an intense 7 fs pulse at 800 nm and a relatively weak 37 fs pulse at 400 nm. By controlling the relative time delay between the two laser pulses, we observe enhanced high-order harmonic generation as well as spectral broadening of the supercontinuum. A method to produce isolated attosecond pulses with variable width and intensity is proposed.     

Maximization of supercontinua in photonic crystal fibers by using double pulses and polarization effects

We show with sub-20 fs pulses both experimentally and theoretically two techniques to control the width and polarization of spectral supercontinua generated in photonic crystal fibers. The first exploits double pulses which interact inside the fiber nonelastically due to stimulated Raman scattering and thus make the supercontinuum wider, whereas the peak intensity can be kept at a reasonably moderate level to prevent fiber damage. The second approach includes polarization manipulations which allow a desirable polarization state to be obtained in a definite spectral range and the spectral shape and width to be varied. PACS 42.65.Tg; 42.81.Dp     

Controlling the spacing of attosecond pulse trains from relativistic surface plasmas

When a laser pulse hits a solid surface with relativistic intensities, XUV attosecond pulses are generated in the reflected light. We present an experimental and theoretical study of the temporal properties of attosecond pulse trains in this regime. The recorded harmonic spectra show distinct fine structures which can be explained by a varying temporal pulse spacing that can be controlled by the laser contrast. The pulse spacing is directly related to the cycle-averaged motion of the reflecting surface. Thus the harmonic spectrum contains information on the relativistic plasma dynamics.     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Compression of the pulses of a Ti:sapphire laser system to 5 femtoseconds at 0.2 terawatt level

High intensity contrast ratio, pre-pulse free pulses with pulse duration of 5 fs and pulse energy of 1 mJ were generated at a repetition rate of 1 kHz by compressing the output pulses of a multi-stage Ti:sapphire laser amplifier system in a Ne-gas-filled hollow fibre. The spatial and temporal properties of the output laser beam were fully characterised. PACS 42.60.Jf; 42.65.Re; 42.81.Qb     

Sub-half-cycle polarization gates in ultra-short laser pulses induced by non-linear propagation effects

We demonstrate the possibility of imprinting a controlled ultra-fast time-varying ellipticity on few-cycle laser pulses through the non-linear propagation in a thin anisotropic crystal. The method is based on the self- and cross-phase modulation induced by the third-order (Kerr) non-linearity when the pulse propagates through the crystal. The feasibility of this technique is demonstrated through the numerical integration of the Maxwell equations, showing the possibility of producing a sub-half-cycle temporal gate of linear polarization. In comparison with alternative approaches, our method results in a polarization gate with higher efficiency, and also it can be controlled by changing the polarization axis of the laser pulse at the entrance of the crystal. We finally demonstrate the efficiency of this scheme in the generation of single attosecond pulses. PACS 42.81.Gs; 42.65.Re; 32.80.Wr     

Time-frequency characterization of high-order harmonic pulses

We present energy-resolved cross-correlation measurements of extreme ultraviolet (XUV) pulses generated as high harmonics of femtosecond pulses from a 1 kHz titanium-sapphire laser. The harmonic pulses are probed by a fraction of the fundamental laser pulse at 800 nm, in a noncollinear geometry, allowing us to vary independently the parameters of the harmonic pump and near-infrared probe pulses. We measure the so-called sidebands in the photoelectron spectrum of argon corresponding to the absorption of a harmonic photon plus or minus one probe photon. Spectrally resolving the cross-correlation signal allows us to characterize the time-dependent frequency of the XUV pulse.Received: 20 December 2002, Published online: 24 April 2003PACS: 42.65.Ky Harmonic generation, frequency conversion     

红外激光场中共振结构原子对极紫外光脉冲的压缩效应

通过数值求解一维原子的含时薛定谔方程, 研究了具有共振结构的原子在双色场(红外激光(IR)+极紫外光(XUV)) 驱动下发射高次谐波的特征. 研究结果表明, 具有共振结构的原子所发射的高次谐波与无共振结构原子(简称为一般原子)发射的高次谐波有明显不同, 共振结构的原子除了在某一能量附近(原子的共振能量+电离能)高次谐波的强度有很大提高外, 它还对XUV光的响应较一般原子表现得更为敏感, 即使XUV光的强度较弱, 也能够明显提高XUV光脉冲中心频率附近的谐波强度, 更重要的是通过调节双色场的时间延迟, 能使输入的XUV光的脉宽得到明显的压缩, 通过时间-频率分析给出了发生这种现象的原因. 由此提出了通过滤波-连续反馈的方式可使XUV光的脉冲从200 as压缩至120 as左右.     

The control of the electron quantum paths for the generation of isolated attosecond pulse using a three-color laser scheme

We theoretically propose a three-color laser scheme to enhance the high-order harmonic intensity and generate an isolated attosecond pulse. By adding a 3 fs, 1600 nm laser pulse to a synthesized two-color laser field (5 fs, 800 nm and 10 fs, 1200 nm), the harmonic intensity is effectively enhanced and an isolated attosecond pulse with duration 41 as is generated. In this scheme, the short trajectory is suppressed, the selection of the long quantum path can be achieved. We also investigate emission time of harmonics in terms of the time-frequency analysis and the semi-classical three-step model to illustrate the physical mechanism of high-order harmonic generation.     

Controlling coherent and incoherent plasma emissions: the role of electron plasma waves

We present results of our two-pulse time resolved measurements of second harmonic and hard X-ray generation in the interaction of an intense (10¹⁶ Wcm^-2, 100 fs, 800 nm) laser with a preplasma generated on a solid surface. The time-resolved study as a function of both plasma scale length and laser polarization brings out interesting features of electron plasma wave dynamics. The harmonic and X-ray emission show contrary behavior and we interpret the results in terms of Resonance Absorption and Wave-Breaking mechanisms. Simple optimization of the scale length of the preplasma and the polarization parameters of the main pulse results in significant enhancements, up to a factor of 100 for X-rays and 10 for the second harmonic respectively. These results can help us understand the governing mechanisms for higher harmonic generation and for fast particle generation, aiding the development of more efficient sources. PACS 52.35.Fp; 42.65.Ky; 52.38.Ph     

Spectral and temporal analysis of ultrashort ultraviolet pulses generated by high-intensity laser radiation in the atmosphere

We report direct spectral and temporal characterization of ultrashort ultraviolet (UV) pulses resulting from third-harmonic generation by high-intensity Ti: sapphire laser radiation in the atmosphere. The experimental technique implemented in this work enables the measurement of the pulse width and the chirp of the UV field, as well as the electron density of the plasma produced by laser pulses. The bandwidth of the third harmonic generated in our experiments by laser pulses with an initial pulse width of 45 fs supports transform-limited UV pulses as short as 30 fs.     

Simultaneous fs pulse spectral broadening and third harmonic generation in highly nonlinear fibre: experiments and simulations

Experiments and numerical simulations are used to study non-phasematched single-mode third harmonic generation occurring simultaneously with fs pulse spectral broadening in highly nonlinear fibre. Pump pulses around 100 fs at 1560 nm injected into sub-5 cm lengths of commercially-available highly nonlinear fibre are observed to undergo spectral broadening spanning over 700 nm at the -30 dB level, and to simultaneously generate third harmonic radiation around 520 nm. Simulations based on a generalized nonlinear envelope equation are shown to well reproduce the spectral structure of the broadened pump pulses and the generated third harmonic signal. PACS 42.65.-k; 42.81.Dp     

Sampling measurement of soft-x-ray-pulse shapes by femtosecond sequential ionization of Kr+ in an intense laser field

We propose a sampling technique for measuring the shape of ultrashort soft-x-ray pulses. The technique uses the transient state of Kr+ ions that is produced by the femtosecond sequential evolution of Kr ions during optical-field-induced ionization as an ultrafast x-ray-absorption sampling gate. We demonstrate the technique by measuring the pulse shape of the 51st harmonic (15.6 nm) generated by a 100-fs titanium:sapphire laser pulse. The measured pulse duration is 220 fs. Our experimental result confirms that the sequential evolution of Kr+ ions from neutral Kr to Kr2+ is the dominant contribution to the ionization process from the aspect of time-domain measurement.