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.

Nonlinear optical methods are becoming ubiquitous in many areas of modern photonics. They are, however, often limited to a certain range of input parameters, such as pulse energy and average power, since restrictions arise from, for example, parasitic nonlinear effects, damage problems and geometrical considerations. Here, we show that many nonlinear optics phenomena in gaseous media are scale-invariant if spatial coordinates, gas density and laser pulse energy are scaled appropriately. We develop a general scaling model for (3+1)-dimensional wave equations, demonstrating the invariant scaling of nonlinear pulse propagation in gases. Our model is numerically applied to high-order harmonic generation and filamentation as well as experimentally verified using the example of pulse post-compression via filamentation. Our results provide a simple recipe for up-or downscaling of nonlinear processes in gases with numerous applications in many areas of science.Nonlinear interactions of intense short laser pulses with gaseous media form the basis behind a wealth of interesting phenomena such as multiphoton ionization [1] and plasma formation [2], spectral broadening (which can be used for pulse compression [3][4][5]), harmonic generation and wave-mixing [6], as well as the creation of attosecond pulses [7] and the formation of electron or ion beams [8]. Advances in femtosecond laser technology constantly yield shorter pulses, higher pulse energies, and higher repetition rates [9][10][11]. However, to fully explore this newly available parameter regime, which gives access to e.g. faster time scales and higher intensities, is often challenging because of damage problems, additional (unwanted) nonlinear effects, or geometrical restrictions. We illustrate this challenge for two important applications of nonlinear optics, filamentation in gases used e.g. for laser pulse compression, and high-order harmonic generation (HHG) providing the basis for attosecond science.The propagation of an intense short laser pulse in a transparent medium induces nonlinear effects caused e.g. by the intensity dependence of the refractive index. When self-focusing due to the Kerr effect balances defocussing caused by diffraction and plasma generation, a filament can be created. In addition, self-phase modulation and self compression may take place in the filament, resulting, possibly after further compression, in ultrashort pulses close to the fundamental limit of a single cycle [12]. Forming a filament requires a certain power, known as the critical power for self-focusing [13,14]. At slightly higher power, limitations arise and multiple filaments are created [15]. Different attempts were suggested to increase the output energy [12,[16][17][18][19][20]. However, pulse compression using filaments (or similarly hollow fibers) is still limited to pulse energies of typically a few mJ [21,22], which is approximately two to three orders of magnitude below the maximum pulse energies available from today's femtosecond laser sources. To scale up pulse po...

International audienceThe efficient generation of isolated attosecond pulses (IAPs), giving access to ultrafast electron dynamics in various systems, is a key challenge in attosecond science. IAPs can be produced by confining the extreme ultraviolet emission generated by an intense laser pulse to a single field half-cycle or, as shown recently, by employing angular streaking methods. Here, we experimentally demonstrate the angular streaking of attosecond pulse trains in a noncollinear geometry, leading to the emission of angularly separated IAPs. The noncollinear geometry simplifies the separation of the fundamental laser field and the generated pulses, making this scheme promising for intracavity attosecond pulse generation, thus opening new possibilities for high-repetition-rate attosecond sources

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We present a compact implementation of the ultrashort pulse measurement technique based on dispersion scans (d-scan), allowing single-shot measurement of few-cycle pulses. The main novelty in our design, making our setup extremely compact and simple, is the use, after a prism, of a spherical mirror in an off-axis geometry. The intentionally introduced strong astigmatism makes it possible to image the output of the crystal in one direction while focusing it in the other direction, resulting in the output face of the prism being imaged into a line in the second-harmonic crystal. The technique is validated by comparing measured dispersion scans, retrieved spectral phases and temporal profiles of this single-shot system with standard d-scan results.