Maite Louisy

Scale-invariant nonlinear optics in gases

Nonlinear optical methods have become ubiquitous in many scientific areas, from fundamental studies of time-resolved electron dynamics to microscopy and spectroscopy applications. They are, however, often limited to a certain range of parameters such as pulse energy and average power. Restrictions arise from, for example, the required field intensity as well as from parasitic nonlinear effects and saturation mechanisms. Here, we identify a fundamental principle of nonlinear light–matter interaction in gases and show that paraxial nonlinear wave equations are scale-invariant if spatial dimensions, gas density, and laser pulse energy are scaled appropriately. As an example, we apply this principle to high-order harmonic generation and provide a general method for increasing peak and average power of attosecond sources. In addition, we experimentally demonstrate the implications for the compression of short laser pulses. Our scaling principle extends well beyond those examples and includes many nonlinear processes with applications in different areas of science.

Spectral phase measurement of a Fano resonance using tunable attosecond pulses.

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.

Gating attosecond pulses in a noncollinear geometry

The 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.

Fast iterative retrieval algorithm for ultrashort pulse characterization using dispersion scans

We present an iterative retrieval algorithm based on data constraint for ultrashort pulse characterization using dispersion scan (d-scan). The proposed algorithm is much faster and leads to a drastic reduction of retrieval times, but, compared to the standard algorithm, it performs less robustly in the retrieval of noisy d-scan traces. The algorithm is tested on several simulated cases and in two different experimental cases in the few-cycle regime.

Phase Measurement of a Fano Resonance Using Tunable Attosecond Pulses

We study photoionization of argon atoms close to the 3s(2)3p(6) -> 3s(1)3p(6)4p Fano resonance using an attosecond pulse train and a weak infrared probe field. An interferometric technique combined with tunable attosecond pulses allows us to determine the phase of the photoionization amplitude as a function of photon energy. We interpret the experimental results using an analytical two-photon model based on the Fano formalism and obtain quantitative agreement.

Compact single-shot d-scan setup for the characterization of few-cycle laser pulses

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.

Scaling Nonlinear Optics in Gases

Extrapolating nonlinear phenomena, such as filamentation, to new parameters as e.g. to higher pulse energy is often challenging. We here present a general scaling model for nonlinear light-matter interactions in gases and proof it experimentally.

High-average power high-harmonic and attosecond sources: Status and prospects

Experiments employing extreme ultraviolet sources based on high harmonic generation often suffer from photon flux limitations. We discuss current status and prospects for scaling such sources to higher repetition rate, pulse energy and average power.

Photoionization time delay measurement close to a Fano resonance using tunable attosecond pulses

We investigate the influence of a Fano resonance on the delays for electron emission in two-photon, near-resonant ionization of argon. The delays were measured using an interferometric method that employed an attosecond pulse train.