Light pulses of attosecond duration (1e-18 s) are the fastest tool that mankind has developed so far. Producing such pulses at higher and higher intensity will give access to ever more extreme conditions. In the PCO group of LOA, we run a unique home-built laser-system [1] that generates pulses with terawatt peak power at 1 kHz repetition rate, lasting <4 fs, i.e. less than two periods than its light-wave oscillation cycles. This drives a so-called relativistic plasma-mirror [2], a dense surface-plasma on a solid target, which reflects the incident laser. This device is not only a beautiful model system for studying relativistic laser-plasma interactions, but it also compresses the reflected laser pulses to attosecond duration. Their spectrum then spans the whole visible range and into the XUV.

Focusing such pulses to high-intensities is an interesting challenge because optical aberrations very quickly degrade the space-time quality of attosecond pulses [2]. In the next few years, want to tackle this challenge and aim at demonstrating the highest light intensity ever generated by attosecond pulses in a detector.

We propose a mainly experimental M2 project participating to this effort. We will measure the spatial properties of the attosecond XUV light, i.e. its beam divergence and wavefront, and see how we can optimize these to facilitate achieving the best spatio-temporal quality and thus highest refocused attosecond intensity. We will also replace the solid target by a newly developed liquid-sheet target, which has the potential to solve mainly practical issues like target lifetime or debris.

We are looking for a motivated student with a strong taste for practical lab-work and a keen interest in intense ultrashort lasers. Existing knowledge in plasma physics and (ultrafast) optics will be appreciated.

The student will gain experience working in a state-of-the-art ultrafast laser lab, understand the basics of ultrafast optics, and have a first insight into ultra-high intensity laser-plasma interaction.

[1] M. Ouillé et al., Relativistic-Intensity near-Single-Cycle Light Waveforms at kHz Repetition Rate, Light Sci. Appl. 9, 1 (2020).
[2] S. Haessler et al., High-Harmonic Generation and Correlated Electron Emission from Relativistic Plasma Mirrors at 1 kHz Repetition Rate, Ultrafast Science 2022, 9893418 (2022).

Contact : Stefan Haessler

Thesis possibility after internship: YES

Funding: YES, through ANR project BANDITO

Twenty-five years after the discovery of the laser filamentation phenomena in air by Pr. Gerard Mourou, who received the Nobel Price in 2018, the field of femtosecond filamentation is still very active [1]. In particular, the most striking and promising effect observed in the plasma channel produced in the filament is the UV lasing effect from the plasma.

Indeed, a cavity-free laser in the sky could lead to revolutionary improvements in optical remote sensing for atmospheric science. N2 molecules, when pumped by an intense femtosecond laser, exhibit an important optical gain in the near UV regime. We reported intense forward emission around 391 or 428 nm with energy up to several µJ during filamentation of femtosecond laser pulses in atmosphere [2]. Many laboratories are studying this physical effect in USA, Canada, Austria, Russia, Japan, China and France [3-5]. Despite numerous works, the physical origin of this lasing is still not understood and subject to controversies. Also, the intensity of the backward lasing emission in air at atmospheric pressure would need to be improved for real scale remote application.

In this project we want to study on one side the fundamental origin of the lasing emission and, on the other side, improve the efficiency of the lasing effect for remote sensing applications. A large part of the work will be experimental measurements performed on the laser facilities of LOA.

[1] A. Braun, G. Korn, X. Liu, D. Du, G. Mourou, “Self-channeling of high-peak-power femtosecond laser pulses in air”, Optics Letters 20, 73,‎ (1995)

[2] G. Point, Y. Liu, Y. Brelet, S. Mitryukovskiy, P. J. Ding, A. Houard, and A. Mysyrowicz, “Lasing of  ambient air with microjoule pulse energy pumped by a multi-terawatt  femtosecond laser,” Opt. Lett. 39,1725 (2014)

[3] Y. Liu, et al., “Unexpected sensitivity of nitrogen ions superradiant emission on pump laser wavelength and duration,” Phys. Rev. Lett. 119, 203205 (2017)

[4] H. Xu, et al., Sub-10-fs population inversion in N2+ in air lasing through multiple state coupling, Nat. Commun. 6, 8347 (2015)

[5] A. Mysyrowicz, R. Danylo, A. Houard, V. Tikhonchuk, X. Zhang, Z. Fan, Q. Liang, S. Zhuang, L. Yuan, Y. Liu, “Lasing without population inversion in N2+,” APL Photonics 4, 110807 (2019)

Contact : Aurelien Houard

Thesis possibility after internship: YES

Funding: NO

Condensed Matter Physics: YES

Soft Matter and Biological Physics: NO

Quantum Physics: YES

Theoretical Physics: YES

A laser beam usually exhibits a plane wavefront (flat phase) et can be circularly polarized (E field spinning around the propagation axis). This polarization state defines the spin angular momentum of the photons. On the contrary, if the polarization is linear but the wavefront spins around the propagation axis (spiral phase), the photons possesses an orbital angular momentum. It can be quantified using a number ℓ called topological charge characterizing the shape of the wavefront (ℓ=0 flat, ℓ=1 simple helix, ℓ=2 double helix like ADN, etc…).

Such beams can be generated with good conversion efficiencies by a ‘vortex’ phase plate applying a azimuthally-varying phase difference, giving the wavefront its helical shape. Visible or infrared vortex beams have many applications, such as rotating molecules or nanostructures, stimulating forbidden dipolar transitions or allowing STED microscopy measurements with resolutions better than the diffraction limit.

A vortex beam can also be used to generate XUV high-order harmonics in gases. It was demonstrated that the vortex is conserved during this highly nonlinear process. However the topological charge of the photons of a given harmonic is equal to that of the driver beam multiplied by the harmonic order. Therefore we typically work with beams at 32nm with a charge ℓ=25. Although it is too high to rotate molecules, new phanomena such as helical dichroism (different absorption of photons with opposed orbital momentum) have been predicted, and the conversion to shorter wavelengths is promising towards STED microscopy with even higher resolutions.

We propose to amplify a topologically charged harmonic pulse at 32nm to drastically increase its fluence. XUV laser amplification occurs, as in the visible or infrared domain, in a population-inverted medium. To reach the high energy difference corresponding to XUV photons, the medium is a highly charged ion. The ions are created by an intense laser pulse focused into a high density gas jet. Electrons in the plasma are heated by the laser field and transfer energy to the ions by collisions, thus allowing a population inversion to take place. Since the high-density plasma is strongly defocusing, we set up a plasma waveguide beforehand to allow proper propagation of the main laser beam. This harmonic amplification technique is well-mastered at LOA and has its own dedicated beamline.

During the internship, the student will set up the vortex beam conversion and subsequent high harmonic generation, the harmonic amplification and caracterization using a XUV wavefront sensor. An experimental campaign will take place at LOA, and a strong collaboration with the LASERIX team from Université Paris Saclay is also expected. LASERIX also operates an installation dedicated to high harmonic generation and amplification

The candidate should have a solid training in general physics and optics. Additional knowledge of plasma physics, nonlinear physics or X/XUV opticswill be appreciated

Contact : Fabien Tissandier

Laser-plasma acceleration is a technique that accelerates particles to relativistic energies over millimeter lengths. Physically, the mechanism is as follows: a femtosecond laser pulse is focused in a plasma and excites a very intense plasma wave there. This plasma wave carries an electric field of some 100GV / m which can be used to accelerate the electrons in the plasma. This method makes it possible on the one hand to drastically reduce the size of the particle accelerators: for example, electrons of 1 GeV (the energy of the SOLEIL synchrotron beam) are obtained over a distance of only 1 cm. On the other hand, it is possible to obtain particle beams whose properties are proving to be extremely interesting for a good number of applications of the medical type, or in X-ray imaging.

One of the key points of current research is to be able to realize a laser-plasma accelerator operating at high speed, in particular for applications. To do this, the LOA and Thalès have just launched a joint laboratory whose goal is to develop laser and plasma technologies in order to build a laser-plasma accelerator delivering relativistic electron beams at 100 Hz. This accelerator will then be used for generating an X-ray source by Compton scattering.

The proposed internship fits into this context. The student will participate in this project and will work on two aspects:

– Participation in the development of 100 Hz laser technology in the Thalès laser laboratories in Elancourt. This includes: participation in the development and characterization of new laser amplifier technology operating at 100 Hz

– Modeling work on laser-plasma acceleration with the parameters of the laser built by Thales. The study will be carried out using Particle In Cell simulations and will model the acceleration of electrons as well as the X-rays obtained by Compton scattering.

The candidate will have a solid general training in physics. Knowledge in the following fields will be appreciated: plasma physics, nonlinear physics, optics and laser. The work will mix the experimental as well as numerical modeling.

We strongly hope that the intern will continue his internship with a CIFRE thesis which will be supported by Thalès.

Nota Bene :

The LOA team is a pioneer in laser-plasma acceleration and in particular in the use of high-speed laser in this field of research, see https://loa.ensta-paris.fr/fr/recherche/groupe- app-researcher /

The Thales team is a world leader in the design and manufacture of ultra-intense short-pulse lasers.

Internship manager: Jérôme Faure

Quand une impulsion laser intense de durée femtoseconde se propage dans l’air, elle donne lieu à la filamentation, un processus spectaculaire où le faisceau se contracte spatialement pour former un mince canal de lumière dans lequel l’intensité est maintenue à ~1015 W /cm2. La filamentation s’accompagne de la formation d’une longue colonne de plasma de courte durée de vie générée dans le sillage de l’impulsion laser. Cette colonne présente notamment la capacité d’initier et de guider des arcs électriques de plusieurs mètres avec une grande reproductibilité (voir photo ci-dessous [1]).

Ces dernières années plusieurs applications basées sur les filaments de plasma ont été proposées telles que le paratonnerre laser [2] et l’antenne virtuelle radiofréquence [3].

Pour rendre possibles ces applications, il est nécessaire de caractériser et d’optimiser les paramètres du plasma produit par le laser femtoseconde ainsi que l’arc électrique guidé. Pour ce faire, différentes techniques de spectroscopie résolue en temps, d’interférométrie et d’imagerie [4] seront mises en œuvre dans le cadre de ce stage. Elles seront testées sur des expériences de guidage de décharges électriques en laboratoire dans le cadre du développement d’une antenne plasma et d’une application de paratonnerre laser.

Le candidat devra avoir des connaissances de base en optique ou en physique des plasmas, un bon niveau d’anglais et présenter de solides références scolaires.

Ce stage sera rémunéré et pourra donner lieu à une prolongation en thèse.

[1] B. Forestier, et al., “Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament”, AIP Advances 2, 012151-13 (2012)

[2] J. Kasparian et al. Science 301, 61 (2003)

[3] Y. Brelet, et al., Radiofrequency plasma antenna generated by femtosecond laser filaments in air”, Applied Physics Letters 101, 264106 (2012)

[4] Improving supersonic flights with femtosecond laser filamentation, P.-Q. Elias, et al., Science Advances 4, eaau5239 (2018)

Ce stage pourra-t-il se prolonger en thèse ? Possibility of a PhD ? : OUI

Rémunération du stage/ financial support for the internship : OUI

Financement de thèse envisagé / financial support for the PhD : Ecole doctorale IP Paris

Type de stage et/ou de thèse (expérience/théorie/simulations) : Expérience

Contact : Houard Aurelien