Antoine Camper

Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20–26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

Synthesis and characterization of attosecond light vortices in the extreme ultraviolet

Infrared and visible light beams carrying orbital angular momentum (OAM) are currently thoroughly studied for their extremely broad applicative prospects, among which are quantum information, micromachining and diagnostic tools. Here we extend these prospects, presenting a comprehensive study for the synthesis and full characterization of optical vortices carrying OAM in the extreme ultraviolet (XUV) domain. We confirm the upconversion rules of a femtosecond infrared helically phased beam into its high-order harmonics, showing that each harmonic order carries the total number of OAM units absorbed in the process up to very high orders (57). This allows us to synthesize and characterize helically shaped XUV trains of attosecond pulses. To demonstrate a typical use of these new XUV light beams, we show our ability to generate and control, through photoionization, attosecond electron beams carrying OAM. These breakthroughs pave the route for the study of a series of fundamental phenomena and the development of new ultrafast diagnosis tools using either photonic or electronic vortices.

Spectrally resolved multi-channel contributions to the harmonic emission in N 2

When generated in molecules, high-order harmonics can be emitted through different ionization channels. The coherent and ultrafast electron dynamics occurring in the ion during the generation process is directly imprinted in the harmonic signal, i.e. in its amplitude and spectral phase. In aligned N2 molecules, we find evidence for a fast variation of this phase as a function of the harmonic order when varying the driving laser intensity. Basing our analysis on a three-step model, we find that this phase variation is a signature of transitions from a single- to a multi-channel regime. In particular, we show that significant nuclear dynamics may occur in the ionization channels on the attosecond timescale, affecting both the amplitude and the phase of the harmonic signal.

Tunable orbital angular momentum in high-harmonic generation

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light–matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photons angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

Combined high-harmonic interferometries for vectorial spectroscopy

We present a new method to characterize transverse vectorial light produced by high-harmonic generation (HHG). The incoherent sum of the two components of the electric field is measured using a bi-dimensional transient grating while one of the components is simultaneously characterized using two-source interferometry. The combination of these two interferometric setups enables the amplitude and phase measurement of the two vectorial components of the extreme ultraviolet radiation. We demonstrate the potential of this technique in the case of HHG in aligned nitrogen, revealing the vectorial properties of harmonics 9-17 of a Ti:sapphire laser.

Amplitude and Phase of Resonant High Harmonic Emission in SF6

We study high harmonic generation in SF6 molecules in the vicinity of a shape resonance. Close to the resonance we observe significant distortions of the harmonic plateau and the corresponding emission times.

High Harmonic Phase Spectroscopy Using Long Wavelengths

We report on two high harmonic spectroscopy studies, one in Argon, the other in Nitrogen. We focus on intensity and group delay measurements of the attosecond pulse trains generated with long wavelength laser fields using the RABBITT technique. Our results are compared to different models showing a good degree of agreement with the experiment. These two studies emphasize the ability offered by high harmonic spectroscopy to measure atomic and molecular features with good accuracy.

High harmonic generation in aligned N2O for orbital tomographic imaging

We characterized the high harmonic emission of aligned N2O and CO2 molecules in intensity and spectral phase using the RABBIT technique. The behavior of the observed phase jump is discussed regarding structural and multi-orbital contributions.