Soft-x-ray wavelength shift induced by ionization effects in a capillary

Coherent soft x rays are produced by high-harmonic generation in a capillary filled with Ar gas. We demonstrate that the tuning of the harmonic wavelengths with intensity and chirp arises from changes in the Ar ionization level. Control over the tuning can be achieved either by changing the average intensity of the laser pulse or by varying the quadratic spectral phase of the laser pulse. We observe an ionization-dependent blueshift of the fundamental wavelength that is directly imprinted on the harmonic wavelengths. The harmonic tuning is shown to depend on nonlinear spectral shifts of the fundamental laser pulse that are due to the plasma created by ionization, rather than directly on any chirp imposed on the fundamental wavelength.     

Measurement of the intensity-dependent atomic dipole phase of a high harmonic by frequency-resolved optical gating

The temporal profile and phase of the fifth harmonic of a Ti:sapphire laser were fully characterized by two-photon ionization frequency-resolved optical gating technique for the first time. The fifth harmonic was found to have negative chirp and the pulse compression was demonstrated. The negative chirp is well explained by using a zero-range potential model. This technique is scalable to extreme ultraviolet (XUV) and soft x-ray regions by using currently available light sources, making it possible to measure the pulse duration and phase of vacuum ultraviolet, XUV, and soft x-ray pulses.     

Conclusive evidence of an attosecond pulse train observed with the mode-resolved autocorrelation technique

We report on the direct observation of an attosecond pulse train with a mode-resolved autocorrelation technique. The chirp among the three harmonic fields is specified by analyzing two-photon above-threshold ionization spectra of electrons, resulting in a pulse duration that should be shorter than 450 as, which is, to our knowledge, the first determination of the chirp in the attosecond pulse train with an autocorrelation technique. These results will open the way to full characterization of an attosecond pulse train with its envelope.     

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 high-order harmonic continuum with two-hump-structure infrared laser pulse

We experimentally investigate the high-order harmonic generation in argon gas cell driven by a multi-cycle broadband infrared laser pulse from a tunable optical-parametric-amplifier （OPA） source. The generation of high-order harmonic continuum with the cut-off photon energy up to 110 eV is observed by tuning the chirp of the 800-nm laser pulse which pumps OPA source. The generation of harmonic continuum is understood in terms of the two-hump structure of the OPA output spectrum and the optimal relative phase of the two humps. The demonstrated scheme is of importance for the generation of extreme ultraviolet （XUV） continuum at higher photon energy region.     

Retrieving the Amplitude and Phase Parameters of an Ultrashort Wave Packet by Autocorrelation Methods

Based on an analysis of the signal of an interferometric intensity autocorrelator, we demonstrate theoretically and experimentally a simple technique for retrieving the temporal distributions of the amplitude and phase of an ultrashort light pulse. Assuming that a phase-modulated wave packet has a Gaussian envelope, we derive an analytical expression for the autocorrelator signal in the cases of linear- and quadratic-chirp optical pulses. The experimental and theoretical results are in excellent agreement. In particular, we show that a linear chirp (quadratic phase modulation) can be measured with a good experimental accuracy of no worse than 10%. This yields the possibility of measuring the parameter k₂, which describes the dispersion characteristics of a medium in the second order of the dispersion theory, with the same accuracy. Measuring a quadratic chirp by the proposed method is possible only if such a chirp is sufficiently large.     

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     

二次谐波产生中入射啁啾脉冲对相位的影响

讨论了基波为叔率啁啾脉冲的二次谐波产生过程，结果表明选取合适的啁啾参量可以提高二次谐波的光强，但同时会使其波形和相位发生变化，三次相位的畸变主要取决于入射的基波啁啾脉冲，而与相位匹配的失谐量关系不大。     

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.     

Pulse compression of a high-order harmonic

The fifth harmonic pulses of an intense femtosecond Ti:sapphire laser were experimentally shown to be negatively chirped by using an LiF plate as a positive dispersive medium. The chirp of the harmonic pulse originates from the intensity-dependent atomic dipole phase, which is estimated to be proportional to 25 U_p, where U_p is the ponderomotive energy. Consequently, we have succeeded in compressing the chirped pulses to 13 fs by compensating the intrinsic negative chirp. Chirp effects of the fundamental laser on the pulse width of the fifth harmonic were consistent with the negative chirp of the fifth harmonic.     

测量飞秒激光脉冲啁啾的方法

介绍了一种能够直接检测飞秒脉冲啁啾大小的方法。采用共轴结构的迈克尔逊干涉自相关二次谐波检测系统对自制碰撞脉冲锁模循环染料激光器输出的最窄脉冲宽度约为200fs的脉冲进行了测量。测得的飞秒脉冲自相关二次谐波曲线表明：实测曲线的下包络两翼明显隆起，包络外形很不整齐，对称性较差，说明光脉冲含有啁啾。通过比较干涉测量曲线和理论曲线的形状与面积的大小，可以直观地反映出光脉冲所含啁啾的大小。实验结果分析表明，利用满足光相关检测所需精度的光学元件和满足光相关探测基本条件的实验装置，能测得含有相位及啁啾信息的快干涉项信号，是一种能够直观检测光脉冲所含啁啾大小的可行方法。     

Coherent enhancement of broadband frequency up-conversion in BBO crystal by shaping femtosecond laser pulses

An optimal feedback control of broadband frequency up-conversion in BBO crystal is experimentally demonstrated by shaping femtosecond laser pulses based on genetic algorithm, and the frequency up-conversion efficiency can be enhanced by ∼16%. SPIDER results show that the optimal laser pulses have shorter pulse-width with the little negative chirp than the original pulse with the little positive chirp. By modulating the fundamental spectral phase with periodic square distribution on SLM-256, the frequency up-conversion can be effectively controlled by the factor of about 17%. The experimental results indicate that the broadband frequency up-conversion efficiency is related to both of second harmonic generation (SHG) and sum frequency generation (SFG), where the former depends on the fundamental pulse intensity, and the latter depends on not only the fundamental pulse intensity but also the fundamental pulse spectral phase.     

二次谐波中大相位失谐下入射啁啾脉冲对相位的影响

讨论了在大相位失谐下,基波为频率啁啾脉冲的二次谐波产生过程,结果表明选取合适的啁啾参量可以使二次谐波的光强提高,但同时会使其波形和相位发生变化.三波相位的畸变主要取决于入射的基波啁啾脉冲,而与相位匹配的失谐量关系不大.     

Harmonic and attosecond pulse enhancement in the presence of noise

<正>In this paper,we theoretically investigate the effect of noise on the photoionization,the generation of the high-order harmonic and the attosecond pulse irradiated from a model He~+ ion.It shows that by properly adding noise fields,such as Gaussian white noise,random light or colored noise,both the ionization probabilities(IPs) and the harmonic yields can be enhanced by several orders of magnitude.Further,by tuning the noise intensity,a stochastic resonance-like curve is observed,showing the existence of an optimal noise in the ionization enhancement process.Finally,by superposing a properly selected harmonic,an intense attosecond pulse with a duration of 67 as is directly generated.     

Quantum control by selective population of dressed states using intense chirped femtosecond laser pulses

The physical mechanism of strong field quantum control using chirped femtosecond laser pulses is investigated. Dressed state control is exerted by making explicit use of the temporal phase changes of the pulse. In our experiment, the dressed state population is mapped by photoelectron spectra from simultaneous excitation and ionization of potassium atoms as a function of the chirp parameter. We show that chirped pulses can be used to selectively steer ground state atoms temporarily into a single dressed state realizing transient Selective Population of Dressed States (SPODS). PACS 32.80.Qk; 32.80.Rm; 33.80.Rv     

Chirp-coefficient bisection iteration method for phase-intensity reconstruction of chirped pulses

A new method is proposed to systematically measure the phase-intensity information of chirped pulses that is based on the chirp-coefficient bisection iteration (CBI) concept. Through the CBI procedure with measured spectrum and temporal intensity profiles (or intensity autocorrelations), spectral and temporal amplitude–phase information can be rapidly retrieved. We experimentally verified that our method has high precision for nanosecond- and picosecond-level pulses and low precision for femtosecond-level pulses. Our proposed method does not require a sophisticated setup and has the advantage of accurate determination of temporal and spectral chirp coefficients with various orders. It also has lower cost, simple operation, in particular covers a wider measurement range than the main current methods. Moreover, the retrieved waveforms can reveal both the pulse shape and the actual intensity with spectral and temporal chirped coefficients of various orders, which can be directly used in various pulse propagation analyses such as chirped pulse amplification.     

大调谐范围的等强度梁光纤光栅波长调节器

利用等强度梁原理,实现了光栅中心波长双向无啁啾调谐,调谐量可达12.52nm,实验结果与理论分析一致,装置构形简单,成本低,易于定标和操作,在已在自制的可调谐激光器,传感网络中的寻址器以及分插复用器中获得诸多应用。     

Temporal dynamics of high repetition rate pulsed single longitudinal mode dye laser

Theoretical and experimental studies of temporal dynamics of grazing incidence grating (GIG) cavity, single-mode dye laser pumped by high repetition rate copper vapour laser (CVL) are presented. Spectral chirp of the dye laser as they evolve in the cavity due to transient phase dynamics of the amplifier gain medium is studied. Effect of grating efficiency, focal spot size, pump power and other cavity parameters on the temporal behaviour of narrow band dye laser such as build-up time, pulse shape and pulse width is studied using the four level dye laser rate equation and photon evolution equation. These results are compared with experimental observations of GIG single-mode dye laser cavity. The effect of pulse stretching of CVL pump pulse on the temporal dynamics of the dye laser is studied.     

高效、高峰值功率蓝光飞秒脉冲产生研究

分析计算了利用棱镜组引进频谱空间啁啾来补偿谐波倍频晶体的相位失配.结果表明,光谱 存在空间啁啾时,选择合适的透镜可在一定程度上补偿由于飞秒光脉冲的宽谱带引起的相位失配.采用此方法在实验上用自制的飞秒自锁模钛宝石激光器和BBO倍频晶体进行了二次谐波 倍频的研究,结果产生倍频蓝光的转换效率高达63%,蓝光平均输出功率达320mW,中心波长 为420nm,光谱带宽达5.5nm,可支持33.6fs的光脉冲.利用钛宝石激光器中的棱镜对进行波 长调谐,可使蓝光脉冲产生404—420nm的调谐范围. 关键词： 飞秒蓝光脉冲 空间啁啾补偿 二次谐波产生 转换效率