Lasers de puissance femtoseconde à la longueur d’onde de 1,7 µm pour la microscopie à trois photons

Sujets de thèse 2014

Intitulé de la thèse
Lasers de puissance femtoseconde à la longueur d’onde de 1,7 µm pour la microscopie à trois photons
Publication du sujet sur le site de l’ABG : NON
Nature du financement : Financement institutionnel, Contrat Doctoral, Financement régional, Contrats université sur projets,)
Spécialité de doctorat : Electronique des hautes fréquences, Photonique et Systèmes

Lieu de travail
Université de Limoges
Faculté des Sciences et Techniques
XLIM
Laboratoire d’accueil : XLIM/PHOTONIQUE

Présentation de l’équipe de recherche
Le département Photonique est un groupe d’une quarantaine de chercheurs (permanents, doctorants et post-doctorants) qui développent de nouvelles fibres optiques, de nouvelles sources laser, de nouveaux instruments de mesures et d’imagerie, de nouveaux composants et systèmes optiques et optoélectroniques.

Le candidat sera intégré dans une équipe spécialisée dans la conception, la fabrication et la caractérisation de fibres optiques spéciales pour applications aux lasers de puissance.

Résumé de la thèse en français
La microscopie multiphotonique, utilisant des lasers femtosecondes à 800 nm, est une technologie-clé pour visualiser in vivo, au niveau cellulaire, la morphologie et la physiologie des tissus. La longueur d’onde laser optimale (1700 nm) pour une imagerie en profondeur (millimètres) est inemployée par manque de lasers adéquats.
Nous souhaitons réaliser notre propre laser femtoseconde émettant à 1700 nm, l’implémenter dans un microscope disponible à Xlim et imager des cellules vivantes à grande profondeur. Pour cela, nous mettrons à profit les possibilités offertes par la technologie des fibres optiques.
Nous réaliserons un laser à fibre à 1550 nm et décalerons la longueur d’onde d’émission vers 1700 nm en utilisant les propriétés physiques d’une fibre optique spéciale, conçue et fabriquée spécifiquement dans le cadre de cette thèse. Finalement nous réaliserons des images sur des cellules vivantes, le noyau de cellules cancéreuses par exemple.

Résumé de la thèse en anglais
Multiphoton microscopy, powered by cumbersome Ti:Sa femtosecond laser at 800 nm, is a key-enabling technology to visualize in vivo tissue morphology and physiology at a cellular level. The imaging depth is limited by scattering and water absorption. The optimum wavelength is located in the infrared, at 1700 nm. However, there is no direct laser emission at this wavelength.
We aim at demonstrating three-photon microscopy in order to open new significant possibilities in biology and medicine. We will implement three-photon microscopy by capitalizing on our recent achievements in fiber optics technology.
The goal is to develop a novel 1700-nm femtosecond laser and to implement it in a commercial nonlinear microscope, available at Xlim. We will design and fabricate an all-fiber format femtosecond laser at 1550 nm and shift its wavelength to the 1700-nm range by using the physical properties of a specially designed delivery fiber. Finally we will image living cells.

Description complète du sujet de thèse
The availability of intense ultra-short laser pulses has made possible the study and exploitation of high-order nonlinearity processes. Among them, two-photon excitation and fluorescence have revolutionized optical microscopy. It becomes possible to observe intact tissues hundreds of micrometers below the surface. Two-photon microscopy is now recognized as a key-enabling technology to visualize in vivo tissue morphology and physiology at a cellular level within scattering tissue. Both the scattering properties of the turbid medium and the water-related absorption decrease the signal-to-background noise ratio and limit the imaging depth. As a result of the conjunction of these two effects, the optimal femtosecond laser wavelength for deep tissue imaging is 1700 nm. The lack of fluorescent indicators at the 1700 nm spectral excitation window for two-photon excitation calls for the exploitation of three-photon fluorescence microscopy (3PM). Three-photon excitation at 1700 nm is equivalent to one-photon excitation at 560 nm, thus enabling the use of a wide variety of fluorophores. Moreover, under three-photon excitation, the fluorescence decays around the focal volume as 1/z^4, thus dramatically enhancing the fluorescence spatial confinement compared to one- or even two-photon excitation. Powered by 1700-nm femtosecond laser, 3PM has led very recently to dramatic improvements in image quality at deep observation depth (Xu and Wise, Nature Photonics 7, 875, Nov. 2013). However according to the Rayleigh criterion the spot size increases with the squared wavelength λ². Radially-polarized or polarization-vortex beams exhibit unusual properties such as tight focusing beyond the diffraction limit. In conjunction with higher-order nonlinear processes, such as three-photon excitation, nonlinear nanoscopy with vortex modes would break the limit of usual microscopy both in terms of penetration depth and resolution, with direct implications in analysis of living tissues at the nanoscale.
First of all, there is no direct laser emission at 1700 nm. The radiation from a femtosecond erbium-doped fiber laser (at 1550 nm) can be conveniently frequency-converted in a vortex fiber provided the radially-polarized beam also exhibits unusually strong and anomalous GVD (through the soliton self-frequency effect). The goal of this project is then to devise such a specialty fiber and elaborate an optimal light source for three-photon microscopy to reach unprecedented high lateral resolution.

Objectifs scientifiques de la thèse
There is no direct emission at 1700 nm from a fiber laser. The emission band closest to 1700 nm is that of erbium ions (around 1550 nm). The radiation from an ultrafast Er-doped fiber laser can be red-shifted through a third-order nonlinear process referred to as the soliton self-frequency shift effect (SSFS). The project builds on our recent work on femtosecond lasers at 1550 nm.

The parameters of the input pulse and those of the delivery fiber must be carefully chosen to ensure stable propagation of the soliton along the delivery fiber. A soliton is an optical pulse that results from the balance between the third order nonlinearity and the anomalous group-velocity dispersion in a medium. Once the soliton is produced, the SSFS effect transfers its spectrum towards longer wavelengths. Not only does the delivery fiber red-shift the pulse but it also compresses its temporal duration to sub-100 fs which is highly desirable to initiate the three-photon excitation process at relatively low average power, ensuring viability of the samples.

Anomalous dispersion is achieved naturally at 1550 nm. Soliton formation is triggered by the input pulse energy. In usual optical fibers, the threshold is below 1 nJ that is far below the energy level required for 3PM.Thus conventional optical fibers cannot be used. The threshold of soliton appearance increases with the dispersion and decreases with the nonlinearity. There are therefore two ways to increase the soliton energy up to the level required for 3PM: either to increase the anomalous dispersion and/or to increase the mode field area. Both ways will be studied.

To ensure high performance 3PM, the pulse energy at the target surface must be on the order of several tens of nanojoules. However, according to the Rayleigh criterion the spot size increases with the squared wavelength λ². Radially-polarized or polarization-vortex beams, which exhibit unusual properties such as tight focusing beyond the diffraction limit, can be of great help to reach high power density at the target.

The scientific objectives of the thesis are:

1.To study peculiar fiber designs able to produce high-energy soliton and to focus light beyond the diffraction limit.

2.To fabricate and characterize specialty optical fibers.

3.To shift experimentally the pulse central wavelength from 1550 nm to around 1700 nm while preserving high energy.

4.To compress the pulse duration down to 100 fs for initiating the third-order nonlinear process at the targeted tissue.

5.To image deep living tissues.

Compétences à l’issue de la thèse
A l’issue de la thèse l’étudiant(e) aura intégré les principes complexes de la propagation guidée d’ondes vectorielles en régime nonlinéaire.

L’étudiant(e) aura conçu et fabriqué des fibres optiques complexes et les aura intégrées dans des lasers femtosecondes de dernière génération.

Finalement, l’étudiant aura appliqué ses réalisations dans le contexte de la microscopie nonlinéaire à très haute résolution spatiale.

Les résultats envisagés doivent placer le laboratoire à la pointe mondiale dans le domaine des lasers et de la microscopie à haute résolution.

Mots clés (séparés par des virgules)
Fibre optique, propagation vectorielle, microscopie nonlinéaire
Conditions restrictive de candidature (nationalité, âge, …) : NON

Directeur de thèse
Sébastien Février
Adresse mail du directeur de thèse : sebastien.fevrier@unilim.fr
Téléphone Directeur de thèse : 05 55 45 72 41

Co-directeur de thèse
Raphaël Jamier
Cofinancement LABEX SigmaLIM demandé : NON
Thèse pour Action transverse : NON

Recherche

Menu principal

Haut de page