RABBITT with tailored fields: Measuring dipole transitions in the continuum

阿秒脉冲可以观察到电子在各种不同系统中运动的阿秒动力学。测量这种超快动力学的一种方法是基于双光子跃迁的干涉重建阿秒跳动(也称为Rabbitt)。顾名思义，Rabbitt方法是基于双光子跃迁步骤的：一串阿秒脉冲电离系统，这是fi第一个光吸收步骤，一个弱探测器field驱动一个连续-连续的偶极跃迁。在这个项目中，我们提出了一个基于扩展的Rabbitt技术的实验，有望获得关于连续统中偶极跃迁的一般信息。其主要思想是比较两个Rabbitt测量，其中两个测量的电离步骤相同，但连续-连续转变的数量不同。这可以通过选择较小的探测频率来实现。例如，当阿秒脉冲序列由频率为2ω的驱动激光产生时，标准Rabbitt方法使用与探测光束相同的频率2ω。但如果使用半频ω，则会涉及两个连续-连续跃迁。这种多色兔方法很容易扩展，产生阿秒脉冲序列，例如三次谐波，并用三次或二次或基波fi电场探测系统。例如，当第二fi场和基波fi场混合时，也可以用多色LED场进行探测。这些定制的fi场可以通过改变其中一个fi场的相位或偏振来进一步操纵。用多色Rabbitt技术获得的知识对于正确解释处理接近电离阈值的阿秒动力学的实验是重要的。

Attosecond pulses allow the observation of attosecond dynamics of electron motion in a variety of different systems. One method to measure such ultrafast dynamics is based on the reconstruction of attosecond beating by the interference of two-photon transitions (also called RABBITT). As the name suggests, the RABBITT method is based on two-photon transition steps: a train of attosecond pulses ionizes the system, this is the first photo absorption step, and a weak probe field drives a continuum-continuum dipole transition. In this project we propose an experiment, based on an extended RABBITT technique, promising to gain general information about dipole transitions in the continuum. The main idea is to compare two RABBITT measurements, where the ionization step is the same for both measurements, but e.g. the number of continuum-continuum transitions differ. This can be realized choosing a smaller probe frequency.When e.g. the attosecond pulse train is generated by a driving laser with a frequency of 2ω, the standard RABBITT method uses the same frequency 2ω as probe beam. But if the half frequency ω will be used, two continuum-continuum transitions are involved. This multi-color Rabbit method is easily extendable generating the attosecond pulse train e.g. with the third harmonic and probe the system with the third or second or fundamental field. It will be also possible to probe with a multi-color field, when e.g. the second and fundamental fields are mixed. These tailored fields can be further manipulated by changing the phase or the polarization of one of the fields. The knowledge gained with the multi-color RABBITT technique is important for the correct interpretation of experiments dealing with attosecond dynamics close to the ionization threshold.