Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1 as = 10~(-18) s) timescales. A frequency 'comb' of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train. Laser pulses spanning many optical cycles have been used for the production of such light bunching, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts. If these bursts are intense enough to induce a nonlinear process in a target system, they can be used for subfemtosecond pump-probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization and ultrafast dynamics rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.
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机译：对多个基本动力学过程的时间探测需要飞秒或什至十亿分之一秒（1 as = 10〜（-18）s）时标上的强烈脉冲。在亚皮秒激光脉冲与稀有气体的相互作用中，很容易产生极紫外奇谐波的频率“梳”：如果这种梳子中的光谱成分彼此之间具有适当的相位关系，则它们的傅立叶合成会产生一个阿秒脉冲培养。已经使用跨越许多光学周期的激光脉冲来产生这种光聚集，但是在少数周期脉冲的限制下，同一过程会产生孤立的阿秒脉冲。如果这些爆发的强度足以在目标系统中引发非线性过程，则可以将它们用于飞秒级的超快过程的泵浦探针研究。迄今为止，用于定量研究阿秒光定位和超快速动力学的所有方法都依赖于极端紫外线脉冲与产生它们的基本激光场之间的互相关过程的建模。在这里，我们报告通过测量一秒的阿秒脉冲序列的二阶自相关迹线，直接确定亚飞秒范围内的脉冲的时间特性。该方法展现出独特的功能，可用于在原子科学中的众多应用中表征和利用阿秒脉冲。
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