Phd position in Tailoring XUV Beams and Attosecond Vortices for Time-resolved Imaging, Talence, France (Deadline: 01.05.2026)

Laboratory : Centre Lasers Intenses et Applications (CELIA) Domaine du Haut Carré, 43 rue Pierre Noailles,
33400 Talence, France

Supervision : Constance Valentin; constance.valentin@u-bordeaux.fr
Eric Mével; eric.mevel@u-bordeaux.fr

The general context of the project is the attoscience. It aims at generating attosecond (1 as = 10-
18 s) intense and coherent XUV (200-10 nm) radiation. Applications are mostly twofold:

1) Tracking attosecond dynamics in matter through XUV pump/XUV probe experiment.
This is a major grail in Attoscience.

2) Observing structural dynamic changes in nano structures through time-resolved XUV
imaging.

Both of those frontier applications require high XUV intensities (near and beyond 1013 W. cm-2)
and a well controlled multidimensional XUV field (spatial mode and phase, pulse time-structure
and polarization shaping). High order harmonic generation in gases produced by focusing
intense femtosecond laser pulses provides a suitable source for those studies. It nevertheless
exhibits a low efficiency in laser/XUV energy transfer. Furthermore, XUV inherent chromatic
effects resulting in spatio-temporal couplings are limiting achievable intensities. Indeed, spatial
properties of the XUV beam are harmonic order dependent and thus prevents a perfect overlap
when focusing over broad XUV bandwidth. Based on experimental reconstruction of XUV foci,
simulations show this has a major impact on achievable intensities and results in distorting the
attosecond structure throughout the XUV focal volume. This is now quite well documented [1 –
3] and adequate spatial shaping of the driving laser is found to efficiently control and mitigate
the chromatic effects. Surprisingly enough, temporal shaping is also found to help reducing the
chromatism [4]. However, no direct time measurement has confirmed this prediction so far.

The first goal of the PhD project will be to set a spatially resolved FROG-CRAB derived
technics to characterize the impact of the chromatism control on the attosecond pulse
structure. Comparison will be investigated between IR laser Gaussian beam and Flat-top or
hollow Gaussian beam driven harmonics. This later case will be eventually investigated with the
Lund Laser Center (Anne L’Huillier’s group) as harmonics generated with a “donut”-shaped
driving laser exhibit an enhanced intensity in the intermediate field when refocused [5]. The
influence of the temporal shaping of the driving laser will also be investigated within a current
collaboration with the King’s College London and the Institut Lumière Matière at the University
of Lyon I. Finally spatial and temporal shaping of the driving field will be combined. This will
contribute to an IA driven approach through physics-informed machine learning to optimize
attosecond pulse intensities suitable for XUV pump/XUV probe experiments.

A second objective will be to directly shape an XUV beam with orbital angular momentum
(OAM) of topological charge l = 1. In high-harmonic generation, OAM beams have typically
been produced by driving the process with an IR beam carrying OAM. However, momentum
conservation imposes a topological charge of q x l on the qth harmonic. At high orders, the
resulting large OAM is extremely sensitive to aberrations, making the beam difficult to focus. It
also produces a “light-spring” attosecond structure with very large extrinsic OAM, which is
impractical for most applications. The proposed approach is inspired by recent works using two
or four mirror geometries to imprint OAM onto a reflected beam [6, 7]. Simulations show that a
simple two-mirror “scissor” arrangement can generate a fairly pure OAM mode l = 1 after
focusing. Because the method relies solely on reflection, it is fully compatible with attosecond
pulses. This new approach will be investigated with our colleagues from LIDYL from CEA-Saclay
within a joint experiment at CELIA supported by the PEPR LUMA. The Eclipse high energy
harmonic beamline equipped with a EUV wavefront sensor is fully adapted to deliver the
properly controlled XUV beam and to characterize the spatial mode including the measurement
of its inherent azimuthal phase. Imprinting a l=1 topological charge has been realized in the X-
ray domain using synchrotron or XFEL facilities. Such OAM X-ray beams are well suited to study
static properties of chiral systems or magnetic materials through helical dichroism while
matching sub-100 nanometer sized objects. Using XUV high order harmonics pulses carrying
topological charge may also spatially resolve similar small sized objects but with the added
capability of probing ultrafast dynamics [8, 9]. There is therefore a strong incentive for the
generation of HHG beam carrying controllable OAM.

Finally, in a more general frame, collaboration with the Lidyl is organized within the PEPR
LUMA and the infraserv Lasers4EU project and will tackle nanometric surface metrology using
XUV ptychographic technics. The so-called Nanophare (NANOmetric PHAse REflectometry)
project has been accepted by the scientific advisory committee of the Ultrafast platform from
the LUMA PEPR. The objectives are two-fold: demonstrating the capability to tailor an XUV
beam with a l = 1 OAM using a silicon nitride reflective phase plate but also performing
reflective ptychographic reconstruction of the surface phase object using coherent diffraction
imaging [4] with the focused harmonic beam.

References:

[1] E. Frumker et al., Optics Express 20, 13870 (2012)
[2] K. Veyrinas et al., New J. of Phys. 25.2, 023017 (2023)
[3] S. Prawdziak, PhD manuscript (2025).
[4] B. Miller et al., in preparation
[5] R. Martín-Hernández et al., https://arxiv.org/html/2507.04550v1
[6] G. Fauvel and P. Balcou, J. of Lightwave Techn. 41, 0733 (2023)
[7] Z. Zhou et al., Phys. Rev. Lett. 134, 205001 (2025).
[8] M. Fanciulli et al., Phys. Rev. Lett. 134, 156701 (2025).
[9] R. Géneaux, Phys. Rev. Lett. 133, 106902 (2024)

Required profile:
The candidate should have a background in laser optics, atomic physics with a particular
interest and skills in experimental work, programming knowledge for data processing and
numerical simulations is desired.