Electron dynamics induced by resonant absorption of light is of fundamental importance in nature and has been the subject of countless studies in many scientific areas. Above the ionization threshold of atomic or molecular systems, the presence of discrete states leads to autoionization, which is an interference between two quantum paths: direct ionization and excitation of the discrete state coupled to the continuum. Traditionally studied with synchrotron radiation, the probability for autoionization exhibits a universal Fano intensity profile as a function of excitation energy. However, without additional phase information, the full temporal dynamics cannot be recovered. Here we use tunable attosecond pulses combined with weak infrared radiation in an interferometric setup to measure not only the intensity but also the phase variation of the photoionization amplitude across an autoionization resonance in argon. The phase variation can be used as a fingerprint of the interactions between the discrete state and the ionization continua, indicating a new route towards monitoring electron correlations in time.

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We study two-color high-order harmonic generation using an intense driving field and its weak second harmonic, crossed under a small angle in the focus. Employing sum-and difference-frequency generation processes, such a noncollinear scheme can be used to measure and control macroscopic phase matching effects by utilizing a geometrical phase mismatch component, which depends on the noncollinear angle. We further show how spatial phase effects in the generation volume are mapped out into the far field allowing a direct analogy with temporal carrier envelope effects in attosecond pulse generation. DOI: 10.1103/PhysRevLett.112.143902 PACS numbers: 42.65.Ky, 42.65.Re High-order harmonic generation (HHG) in gases using multicolor optical fields is becoming a common tool in attosecond science. Multicolor HHG has been implemented by mixing the fundamental field with waves at low harmonic [1][2][3] and at incommensurate [4,5] frequencies. Other schemes employ attosecond pulse trains mixed with the fundamental field [6] as well as synthesized light transients [7] for driving the HHG process. The control and flexibility brought about by using two or more driving fields is used, for example, to improve the efficiency of the generation process [3,8], to monitor the single atom response [9,10], and to implement gating techniques [11]. High-order wave mixing can further be beneficial for achieving phase matching [1,[12][13][14][15].Using multiple optical driving fields also allows for noncollinear geometries. This concept, used extensively in low-order frequency conversion processes such as optical parametric amplification, has barely been applied to attosecond science. In an early work, Birulin and co-workers [16] addressed theoretically the general concept of HHG in a noncollinear geometry. By mixing the fundamental and a weak second harmonic at an angle, Bertrand and co-workers [17] demonstrated the spatial separation of multiple extreme ultraviolet (XUV) beams corresponding to different sets of absorbed photon numbers from the two fields. A similar scheme was recently used [18] for reconstructing the spatiotemporal characteristics of the generated attosecond pulses. Other authors [19][20][21] concentrated on noncollinear sum-frequency generation (SFG) processes driven by two identical laser fields. Such a scheme allows for separation of the harmonics from the fundamental but suffers from phase matching problems.In this Letter, we theoretically and experimentally examine the influence of noncollinear geometries in HHG, analyzing in detail the macroscopic aspects. We show for the first time, to our knowledge, how a weak noncollinear field can be used as a probe to monitor and control phase matching. We show that difference-frequency generation (DFG), where a photon (or several photons) is emitted in the conversion process, is in general more favorable than SFG [22], which becomes very inefficient with increasing noncollinear angle. This leads to the counterintuitive result that the XUV radiation is dominantly emitted ou...

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