Master Internship

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

PhD Thesis

Laser-plasma electron acceleration offers a unique way to produce highly energetic and ultra-short electron bunches, on very short distances. It have risen much interest since the first, pioneering, experiments in the early 2000s (Malka 2002; Faure et al. 2004). The interaction between an intense laser pulse and a target material is responsible of the whole extraction, selection and acceleration process, which makes primordial the understanding of the role of the involved parameters, such as the target density, shape and profile, laser duration, phase and intensity.
Among the research fields which laser-driven particle sources are relevant for, radiation biology opens to the exploration of fundamental aspects of radiation toxicity on living matter, that will be accessible only with a radiation source as short as the physical dose deposition time (Bayart et al. 2019; Favaudon et al. 2000; 2014). In order to make laser-driven electron sources interesting and compatible with radiobiology applications, a number characteristics should be addressed, such as the total charge per accelerated bunch, the spectral features, the stability and the duration. The required improvements demand a deep understanding of the acceleration mechanisms, the design of novel acceleration strategies and schemes.
Throughout the thesis activity, high potential topics for fundamental and applied science will be addressed, in the field of laser-created plasmas, particle acceleration, particle detection or dosimetry and engineering of experimental systems towards applications.

keywords :Ultra-intense lasers, Laser-plasma interaction, Laser-driven electron acceleration, Numerical simulations, Ultra-high dose-rate

Contact : Alessandro Flacco / Cédric Thaury

Our group has recently demonstrated a new scheme to achieve backward lasing from air plasma using circularly polarized 800 nm femtosecond pulses [2-4], which is widely available especially for high energy pulses. Up to now, there exist several important fundamental questions concerning this new scheme of backward lasing. For example, the presence of oxygen molecules is found to decrease the lasing efficiency significantly and the physical mechanism for this detrimental role is unclear. At the same time, the pulsed backward emission has not been characterized in the temporal domain and the dynamics of this lasing process is largely unknown. As to its applications, it is still at an early stage.

The student will participate in a series of research activities in order to clarify the fundamental physical mechanisms involved in the lasing actions of neutral nitrogen, to characterize this transit lasing process in the temporal domain. Another aspect of his/her research is to search for the optimal operational conditions for the backward nitrogen laser and improve its properties such as pulse energy and divergence. Several schemes have been envisaged at this moment.

Contact : Houard Aurelien

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

Contact : Houard Aurelien

Postdoc positions

Context :

Laboratoire d’Optique Appliquée (LOA) is developing a new 3D X-ray imaging technique, called plenoptic, based on the combination of main optic and wavefront sensor. Three systems have been built. The first one was a demonstrator running at PETRA III synchrotron in Germany at an energy of 11 keV. It is now disassembled after producing excellent results. Another camera is set and aims at imaging living biological cells. It is working around 400 eV. The last system is targeting small animal imaging with X-rays of energy around 17 keV. Both systems are tabletop.

Both systems still need to be fully tested and improved. The low energy camera is running at LOA, near Paris, while the high energy system is set at Imagine Optic company in Bordeaux, France. LOA’s team is in charge of running and improving both systems through a collaboration agreement.

Today, we are using three software: those specifically developed for the so-called focused ad unfocused plenoptic cameras and a homemade software integrating the two geometries. These softwares are too slow and complex for generating a 3D image.

Topic of the post-doctoral fellowship:

A post-doctoral position is open in FLUX group for 18 months with possibility of extension. During this period, the post-doc will be in charge of analyzing ability of machine vision software to generate efficiently a 3D image from the raw plenoptic data. Disparity as well as machine learning are two options we consider. The candidate will have to choose one or different techniques, implement it/them and then test it/them on real X-ray plenoptic images.

Experience :

Candidate should have developed strong computational skills related to machine vision. Good knowledge on PYTHON is a plus but not required. The candidate should be rigorous and have a proficiency in working in team. English is the work language.

Contact : philippe.zeitoun@ensta.fr

Context :

Laboratoire d’Optique Appliquée (LOA) is developing a new 3D X-ray imaging technique, called plenoptic, based on the combination of main optic and wavefront sensor. Three systems have been built. The first one was a demonstrator running at PETRA III synchrotron in Germany at an energy of 11 keV. It is now disassembled after producing excellent results. Another camera is set and aims at imaging living biological cells. It is working around 400 eV. The last system is targeting small animal imaging with X-rays of energy around 17 keV. Both systems are tabletop.

Both systems still need to be fully tested and improved. The low energy camera is running at LOA, near Paris, while the high energy system is set at Imagine Optic company in Bordeaux, France. LOA’s team is in charge of running and improving both systems through a collaboration agreement.

Topic of the post-doctoral fellowship :

A post-doctoral position is open in FLUX group for 18 months with possibility of extension. During this period, the post-doc will be in charge of finishing to set the two systems, test them and then optimizing them. On both cases, the main objective consists in generating 3D images of adequate samples in a single exposure. Known samples like USAF 1951 will be used first; we will move later to biological samples provided by our collaborators. The measurement of the dose delivered to generate a 3D image will be performed at every step. Most interesting samples will be imaged by X-ray computed tomography and results will be compared to those obtained by X-ray plenoptic.

Experience :

Candidate should have a strong background on experiment in optics or using complex optical systems. Knowledge on X-rays, Optical design software (ZEEMAX, OSLO etc) or PYTHON is a plus but not required. The candidate should be rigorous and have a proficiency in working in team. English is the work language.

Contact : philippe.zeitoun@ensta.fr

Contexte

L’apparition des systèmes laser ultra-courts de haute puissance à la fin des années 90, et les avancées technologiques récentes dans les amplificateurs pompés par diodes, permettent aujourd’hui d’envisager à moyen terme le développement d’applications inédites des lasers de durée femtoseconde qui ont fait l’objet du prix Nobel de physique en 2018.

Le présent projet consiste à étudier l’utilisation de filaments laser femtoseconde pour produire une antenne plasma « virtuelle » émettant dans la gamme RF [2]. Pour ce faire, il sera nécessaire d’enrichir la colonne de plasma initialement créée par l’impulsion laser femtoseconde à l’aide d’un générateur haute-tension [2,3] ou d’une source micro-onde de puissance [4]. Les deux méthodes seront testées expérimentalement dans les locaux LOA et l’antenne plasma sera caractérisée à l’aide de divers diagnostics (caméra rapide, interférométrie, mesure de rayonnement..).

Profile du candidat

Le candidat devra avoir de solides connaissances en physique des plasmas, en diagnostics optiques ou plasma, et des notions d’optique.

Salaire net mensuel : entre 2100 et 2700 euros suivant l’expérience du candidat Durée du contrat : un à deux ans.

Les travaux étant réalisés dans le cadre d’un contrat de la DGA, le candidat devra être issu de l’Union européenne ou de la Suisse.

Contact : Houard Aurelien