Time-resolved measurements of high order harmonics confined by polarization gating

We investigate the temporal confinement of high order harmonic pulses generated by a femtosecond (fs) infrared (IR) pulse with a time varying polarization. We use a set of two birefringent quartz plates to modulate the IR polarization. It produces a short temporal gate of linear polarization where harmonics are efficiently generated during a small fraction of the IR pulse. By rotating one of the plates, the gate width can be continuously varied between 70 fs down to 7 fs. The XUV pulse duration is measured by cross-correlation with a probe IR pulse of 12 fs. When the gate width is decreased, a clear temporal confinement of the XUV emission is observed through the cross correlation signal. This experiment is the first direct experimental evidence in the temporal domain that the polarization gating technique can be used to significantly shorten the harmonic pulse duration. PACS 32.80.Wr; 42.65.Ky; 42.65.Re     

Frequency-resolved optical gating of isolated attosecond pulses in the extreme ultraviolet

The pulse shape and phase of isolated attosecond extreme ultraviolet (XUV) pulses with a duration of 860 asec have been determined simultaneously by using frequency-resolved optical gating based on two-photon above-threshold ionization with 28-eV photons in He. From the detailed characterization, we succeeded in shaping isolated XUV pulses on an attosecond time scale by precise dispersion control with Ar gas density or by changing the driving pulse width. These results offer a novel way to excite and observe an electron motion in atoms and molecules.     

On the population dynamics induced by an attosecond train interacting coherently with an atomic system within the electric dipole approximation

The population dynamics induced in an atomic system by XUV-attosecond radiation have been investigated within the electric dipole approximation. The main characteristics of the dynamics are explained in simple terms, extending our previous work on coherent laser-atom interactions in the XUV spectral region. Attosecond beating appearing in a second-order autocorrelation of a coherent superposition of high order harmonics, is investigated from the perspective of a superposition of Rabi oscillations, as well as of coherent population return (CPR). The work reveals the strength of attosecond radiation as a tool for time domain studies of coherent manipulations of quantum systems.     

等离子体光栅靶的表面粗糙度对高次谐波产生的影响

极紫外光和软X射线由于其波长和脉冲持续时间极短,可用于超快物理过程和物质微观结构的探测.最近几年,研究人员发现激光和等离子体相互作用可以产生持续时间极短(阿秒)且相干性较好的高次谐波辐射,其波长可接近甚至达到水窗波段.然而,实验研究指出,理论上应出现的一些谐波在实验中并没有出现.本文针对超短超强激光与非理想条件下的等离子体光栅靶相互作用产生高次谐波的物理过程进行了理论分析和粒子模拟.研究结果表明,等离子体光栅的周期性结构对于高次谐波的频谱和辐射角分布存在显著调制效果.光栅靶表面粗糙度直接影响光栅的光学调制效果,改变高次谐波的频谱分布和辐射角分布.理想光栅条件下,满足光栅匹配条件的特定阶数谐波明显获得增强,且辐射张角集中在平行靶面的方向.靶表面粗糙度的出现,导致光栅匹配条件失效,高次谐波能量向各阶分散且辐射张角逐渐偏离靶表面方向.研究结果较好地解释了实验中观测到的谐波频谱分布,为进一步的研究提供了一定参考.     

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 the wavelength-tunable XUV pulse using the two-color and three-color infrared pulses

We present an efficient method to generate an ultrashort wavelength-tunable XUV pulse by using the harmonic selec- tive enhancement scheme. The results show that by properly controlling the delay times of a two-color field or a three-color field, selective enhancement of the harmonics with photon energies between 80 eV and 315 eV can be obtained. Fur- ther, a wavelength-tunable and bandwidth-controllable XUV radiation can be obtained by Fourier transformation of these enhanced harmonics.     

Generation of Continuum Extreme-Ultraviolet Radiation by Carrier-Envelope-Phase-Stabilized 5-fs Laser Pulses

Coherent extreme-ultraviolet （XUV） radiation is studied by interaction of carrier-envelope （CE） phase stabilized high energy 5-fs infrared （800 nm） laser pulses with neon gas at a repetition rate of 1 kHz. A broadband continuum XUV spectrum in the cut-off region is demonstrated when the CE phase is shifted to about zero, rather than modulated spectral harmonics when setting of CE phase is nonzero. The results show the generation of isolated attosecond XUV pulses.     

Efficient output coupling of intracavity high-harmonic generation

We demonstrate a novel technique for coupling extreme-ultraviolet (XUV) harmonic radiation out of a femtosecond enhancement cavity. We use a small-period diffraction grating etched directly into the surface of a dielectric mirror. For the fundamental light, this element acts as a high reflector. For harmonic wavelengths, it acts as a diffraction grating, coupling XUV radiation out of the cavity. Using this method, we observed the third through twenty-first odd harmonics with a dramatic increase in usable power over previous results of high-harmonic generation at high repetition rates.     

Production of narrowband tunable extreme-ultraviolet radiation by noncollinear resonance-enhanced four-wave mixing

Fourier-transform-limited extreme-ultraviolet (XUV) radiation (bandwidth approximately < 300 MHz) tunable around 91 nm is produced by use of two-photon resonance-enhanced four-wave mixing on the Kr resonance at 94 093 cm(-1). Noncollinear phase matching ensures the generation of an XUV sum frequency 2 omega1 + omega2 that can be filtered from auxiliary laser beams and harmonics by an adjustable slit. Application of the generated XUV light is demonstrated in spectroscopic investigations of highly excited states in H2 and N2.     

Controlling the divergence of high harmonics from solid targets: a route toward coherent harmonic focusing

Harmonic generation from relativistically oscillating plasma surfaces formed during the interaction of high contrast lasers with solid-density targets has been shown to be an efficient source of extreme ultraviolet (XUV) and X-ray radiation. Recent work has demonstrated that the exceptional coherence properties of the driving laser can be mirrored in the emitted radiation, permitting diffraction limited performance and attosecond phase locking of the harmonic radiation. These unique properties may allow the coherent harmonic focusing (CHF) of high harmonics generated from solid density targets to intensities on the order of the Schwinger limit of 10²⁹ W cm^-2 with laser systems available in the near future [Phys. Rev. Lett. 93, 115002 (2004)] and thus pave the way for unique experiments exploring the nonlinear properties of vacuum on ultra-fast timescales. In this paper we investigate experimentally as well as numerically the prospect of focusing high harmonics under realistic experimental conditions and demonstrate, using particle in cell (PIC) simulations, that precise control of the wavefronts and thus the focusability of the generated harmonics is possible with pre-shaped targets.     

Generation of 10- microJ coherent extreme-ultraviolet light by use of high-order harmonics

We demonstrate the generation of 10-microJ coherent extreme-ultraviolet (XUV) light at wavelengths from 73.6 to 42.6 nm, using high-order harmonics. The peak power of this coherent XUV light is estimated to be 0.13 GW at 62.3 nm, and the peak brightness achieved was 3x10(28) photons/(mm(2)mrad(2)s) . To our knowledge, this XUV energy is the highest value achieved with high-order harmonics.     

A tunable XUV monochromatic light source based on the time preserving grating selection of high-order harmonic generation

We accomplish a laboratory facility for producing a femtosecond XUV coherent monochromatic radiation with a broad tunable spectral range of 20 eV-75 eV. It is based on spectral selected single-order harmonics from intense laser driven high harmonic generation in gas phase. The time preserving for the selected harmonic radiation is achieved by a Czerny-Turner type monochromator designed with a conical diffraction grating mount for minimizing the time broadening caused by grating diffraction and keeping a relatively high diffraction efficiency. Our measurement shows that the photon flux of the 23-order harmonic(H23) centered at 35.7 eV is 1×10~9 photons/s approximately with a resolving power E/?E ≈ 36.This source provides an ultrashort tunable monochromatic XUV beam for ultrafast studies of electronic and structural dynamics in a large variety of matters.     

Enhancing the brilliance of high-harmonic generation

Both the gas jet and the capillary setup have remarkable features for high-harmonic generation (HHG). We demonstrated both the ability of the capillary to tailor its spectral XUV output in unique ways, as well as the feasibility of the jet and its ability to produce photon energies up to the keV range. For maximizing the brilliance in both schemes we have successfully applied the concept of self-learning loops to several of our experiments. We present current results on the focusability of capillary high-harmonic radiation, a wave front measurement on gas jet high harmonics and measurements of the influence of gas pressure and laser focus position on the divergence of gas jet high harmonics. This is embedded in an overview of selected previous and current work of our group.     

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.     

Multi-color XUV interferometry using high-order harmonics

We present a novel interferometric setup operating in the XUV spectral range. The interferometer consists of a combination of a double pinhole (similar to Young’s double slit) and a transmission grating. In the case of a light source consisting of discrete spectral lines, it allows recording interferograms for multi-colors simultaneously. We present two experiments in which high-order harmonics generated by a titanium sapphire laser were used as the light source for the interferometer. First, the temporal coherence lengths of the single harmonics were determined, and second, the index of refraction and the absorption of a thin beryllium foil were measured simultaneously in the range of 17–25 nm.     

Marked Advance in Widths and Heights of Harmonic Plateau

We propose a scheme to simultaneously widen and heighten the high-order harmonic plateau on a large scale. More specifically, by adopting a united two-atom system with a suitable inter-nuclear separation instead of a single-atom, the harmonic plateau is widened from I_p + 3.2U_p to I_p + 8.5U_p; further, by adopting the combined pulse, the extended plateau (harmonics near I_p + 5.6U_p) is selectively heightened in excess of 4 orders of magnitudecompared with the case of the low-frequency pulse alone. By means ofthe wavelet transform for the induced dipole of these harmonics, asingle x-ray pulse as short as 210 asec is achieved.     

Manifestation of attosecond XUV fields temporal structures in attosecond streaking spectrogram

The features of an attosecond extreme ultraviolet (XUV) field are encoded in the attosecond XUV spectrogram. We investigate the effect of the temporal structures of attosecond XUV fields on the attosecond streaking spectrogram. Factors such as the number of attosecond XUV pulses and the temporal chirp of attosecond XUV pulses are considered. Results indicate that unlike the attosecond streaking spectrogram for an attosecond XUV field with two pulses of a half-cycle separation of streaking field, the spectrogram for the attosecond XUV field with three pulses demonstrates fine spectral fringes in separated traces.     

基于阿秒XUV-飞秒IR泵浦-探测的冷靶反冲离子动量成像谱仪

利用多道可见光谱探测系统测量了Hα、CⅢ(464.7nm)和OⅡ(441.5nnm)谱线的时间行为,得出了碳、氢和氧元素的入射通量。在简化模型下算出了氧碳间的化学溅射率,结果表明HT-6M托卡马克边界杂质产生机制主要是氢氧间的化学溅射和氧碳间的化学溅射,因此控制氧杂质尤其重要。     

Low- and high-order harmonic generation inside an air filament

We report on the generation of third-order (TH) and high-order harmonics (HH) directly inside a self-guided femtosecond filament in air. By terminating the filament with a steep density gradient behind a pinhole placed at different distances from the geometric focus, the evolution of the generated radiation is tracked along the nonlinear interaction zone. Spectra are recorded in the visible (VIS), ultraviolet (UV) and extreme ultraviolet (XUV) spectral range under identical conditions. The VIS and UV spectral bandwidth undergoes significant broadening. The input pulse parameters are varied systematically to optimize the TH bandwidth. Recorded spectra show Fourier-limits below 5 fs pulse duration centered at 264 nm wavelength. We observe conversion to HH up to the 25th order and use the HH generation process as a nonlinear probe for the on-axis intensity evolution along the filament.