PhD subjects

Many complex molecular systems absorbing light in the near UV spectral range, including those of paramount biological importance, like DNA bases or proteins, are endowed with mechanisms of excited-state deactivation following UV absorption. These mechanisms are of major importance for the photochemical stability of these species since they provide them a rapid and efficient way to dissipate the electronic energy in excess into vibrations, thus avoiding photochemical processes to take place and then structural damages which affect the biological function of the system. In this context, the study of gas phase bio-relevant systems such peptides as proteins building blocks should lead to a better understanding of the photophysical phenomena involved in the relaxation mechanisms of life components. This Ph. D project aims at both investigating the electronic dynamics of bio-relevant model systems, i.e. building block of life components, and documenting the basic phenomena controlling the lifetime of the excited states, through a dual approach using most recent methodological tools, consisting of:



i) An experimental characterization of i) the lifetimes, in nano-, pico- and femtosecond pump-probe experiments, and ii) the nature of the electronic states formed. Sophisticated diagnostic techniques, such as a photo-electron velocity map imaging diagnosis, will be used. These experiments will allow us to identify the relaxation pathways followed by the system, and in particular to assess the role of the several excited states together with the effect of its environment.

ii) A theoretical modeling of the processes involved, in particular to assess the role of specific regions of the potential energy surface (PES), namely the conical intersections, and to determine the motions that trigger deactivation. The systems’ size, their flexibility, the non-covalent interactions, which govern the structures, and the nature of the excited states require the implementation of a computational strategy using sophisticated quantum chemical methods dedicated to excited states (non-adiabatic dynamic, coupled cluster method and multireference configuration interaction method) in order to characterize the first excited states, simulate their PES and eventually determine the relaxation pathways.



Moreover, this work will take place in the following of the ANR project, ESBODYR, for «Excited States of BiO-relevant systems: towards ultrafast conformational Dynamics with Resolution» (Coord V. Brenner, 2014-2018). Finally, the theoretical part will benefit from an access to the national High Performance Computing resources (GENCI: TGCC/ IDRIS / CINES) as well as from access to both the femtosecond SLIC server of IRAMIS (Saclay) and the Laser Center of the University Paris-Sud (CLUPS).

Many complex molecular systems absorbing light in the near UV spectral range, including those of paramount biological importance, like DNA bases or proteins, are endowed with mechanisms of excited-state deactivation following UV absorption. These mechanisms are of major importance for the photochemical stability of these species since they provide them a rapid and efficient way to dissipate the electronic energy in excess into vibration, thus avoiding photochemical processes to take place and then structural damages which affect the biological function of the system. In this context, the study of gas phase bio-relevant systems such peptides as proteins building blocks should lead to better understanding the photophysical phenomena involved in the relaxation mechanisms of life components. The size of the systems, their flexibility, the existence of non-covalent interactions which governs structures and the nature of the excited states require the use of sophisticated theoretical models in order to characterize the preferentially formed conformations in gas phase as well as to investigate the electronic deactivation mechanisms of the first excited states.



The focus of the PhD project concerns the implementation of a computational strategy to both characterize the first excited states and simulate their potential energy surfaces in order to determine the relaxation pathways. This theoretical research project contains then the development, evaluation and validation of modern quantum chemical methods dedicated to excited states. It will be backed up by key gas phase experiments performed in our group on bio-relevant systems using recent spectroscopic techniques which provide precise data on their spectroscopic properties and their electronic dynamics of relaxation.



Moreover, it will take place in the context of the following of the ANR project, ESBODYR or «Excited States of BiO-relevant systems: towards ultrafast conformational Dynamics with Resolution» (Coord V. Brenner, 2014-2018) and will benefit from an access to the national High Performance Computing resources (GENCI: TGCC/ IDRIS / CINES).

This Phd subject is a fundamental approach to a better understanding of the triplet states of organic molecules with complete layer. These molecules thus excited are more reactive than in their ground state. Unfortunately, these triplet states are not well known. Also, this Phd subject offers the possibility to overcome this poor knowledge by carrying out the spectroscopy and the dynamics of the triplet states of organic molecules via a resonant excitation (UV, Vis) since the ground state. The molecule will be deposited in a helium cluster. To succeed this direct excitation of the triplet state, it will be necessary to use the particular properties of the quantum medium that are the helium clusters: cooling of the molecule in its electronic and vibrational fundamental state, weak interaction with the host molecule. Because of the long life of the triplet states, it will also be possible to perform the IR spectroscopy of the hosted molecule in its triplet state. The electron relaxation dynamics of the triplet state will be studied later.



Note that the experimental GOUTTELIUM device, on which the spectroscopic part of this thesis will be conducted, is unique in France.

Flexible molecules are ubiquitous in Nature (proteins, sugars, ...) and are at the origin of many applications (drugs, molecular machines, ...). By definition, these molecules exist in several conformations that each have physical, chemical or biological properties which can vary greatly from one conformation to another. Among these properties, photoexcitation and relaxation of the electronic states are particularly sensitive to conformation: for example, the lifetime of the first excited electronic state can vary with the conformation by several orders of magnitude. However, the experimental characterisation of such conformational effects on the excited states remains rare due to the difficulty to specifically study one conformation present in a conformational mixture. This thematic is still poorly documented despite i) a need for experimental results to help the development of theoretical models, and ii) a lack of knowledge in a field where fundamental (conical intersections, ultrafast phenomena) and application (photostability, energy transfer) issues are important.



In this context, the LIDYL laboratory brings together several experimental apparatus allowing an original multi-scale (ns-fs) and conformation-resolved study of the dynamics of the electronic relaxation in flexible molecular systems. The research program will focus on biologically-relevant systems and molecular complexes, and will target:



- the observation of conformer-dependent dynamic processes and their rationalisation.

- the characterization of species previously inaccessible to conventional detection techniques.

A vast range of topics in physics such as the star internal structure, the X-ray emission of accretion disks, the dynamics of inertial confinement fusion, or new radiation sources requires an accurate knowledge of the radiative properties of hot plasmas. Such plasmas exhibit spectra consisting of a large number of lines often merging in unresolved arrays. Statistical methods are used for the interpretation of such spectra. Using second quantization and tensor algebra techniques, one may obtain quantities such as the average and variance of transition energies inside these unresolved arrays. Though a wide literature exists on this subject, certain types of transitions, e.g., magnetic dipole transitions inside a given configuration or processes involving several electron jumps, have not been considered up to now. In addition to this analytical study, a numerical work involving the Flexible Atomic Code will be proposed for this thesis. This study will concern plasmas either at thermodynamic equilibrium, or out of equilibrium, where level population is obtained by solving a system of kinetic equations. Several applications may be foreseen: interpretation of recent opacity measurements performed on LULI2000 laser at Ecole Polytechnique, determination of radiative power losses in a tungsten plasma to characterize tokamak operation, or the open subject of the characterization of photoionized silicon plasma analyzed in Sandia Z-pinch in connection with astrophysical observations.

Guanine Quadruplexes (G4) are four-stranded structures formed by guanine rich DNA sequences. They have been correlated with the oxidative damage which perturbs biological functions. In addition, G4 structures are studied in respect to their applications in molecular electronics and nanotechnologies.

The objective of the thesis is to study the generation and the reactivity of guanine radicals (including electron holes, important in charge transport) induced by absorption low energy UV radiation by G4. The investigation will involve the use of several experimental and computational techniques:

o The electrons ejected by photo-ionization and the resulting base radicals will be studied by time-resolved absorption spectroscopy and time-resolved circular dichroism, from nanoseconds to milliseconds.

o The dynamics of the excited states, expected to play a role in the photo-ionization process, will be studied by fluorescence spectroscopy, from femtoseconds to nanoseconds.

o The observed optical spectra will be interpreted by means of quantum chemistry methods.

o The reaction products resulting from UV-induced radicals will be identified using analytical methods.

With the advent of PetaWatt (PW) class lasers capable of achieving light intensities of 10^22W.cm-2 at which matter turns into a plasma, Ultra-High Intensity (UHI) physics now aims at solving two major challenges: can we produce high-charge compact particle accelerators with high-beam quality that will be essential to push forward the horizons of high energy science? Can we reach extreme light intensities approaching the Schwinger limit 10^29W.cm-2, beyond which light self-focuses in vacuum and electron-positrons pairs are produced? Solving these major questions with the current generation of high-power lasers will require conceptual breakthroughs that will be developed during this PhD.



In particular, the goal of this PhD proposal is to demonstrate that so-called ‘relativistic plasma mirrors’, produced when a high-power laser hits a solid target, can provide simple and elegant paths to solve these two challenges.  Upon reflection on a plasma mirror surface, lasers can produce high-charge relativistic electron bunches and bright short-wavelength attosecond harmonic beams. Could we use plasma mirrors to tightly focus harmonic beams and reach extreme light intensities, potentially approaching the Schwinger limit? Could we employ plasma mirrors as high-charge electron injectors in a PW laser capable of delivering accelerating gradients of 100TV.m-1, or in a laser wakefield accelerator, to build ultra-compact particle accelerators?



The mission of the PhD candidate will be to answer these two interrogations ‘on-chip’ using massively parallel simulations on the largest supercomputers in the world. To this end, the successful candidate will make use of our recent transformative developments in ‘first principles’ Particle-In-Cell (PIC) simulations of UHI laser-plasma interactions that enabled the 3D modelling of plasma mirror sources with high-fidelity on current petascale and future exascale supercomputers. These developments were recently implemented, validated and tested in our code PICSAR (https://www.picsar.net). Armed with PICSAR, the candidate will model novel schemes employing plasma mirrors to address the two UHI challenges introduced above.

Summary :



Using tunable attosecond pulses produced with an optical parametric amplifier (OPA) pumped by an intense Titanium:Sapphire laser (ATTOLab Excellence Equipment), the student will investigate the ionization dynamics of atomic and molecular gases close to resonances. The objective is to follow in real time the electron ejection and to measure the buildup of the resonance profile.



Detailed summary :



Recently, the generation of sub-femtosecond pulses, so-called attosecond pulses (1 as=10-18 s), has made impressive progress. These ultrashort pulses open new perspectives for the exploration of matter at unprecedented timescale. Their generation result from the strong nonlinear interaction of short intense infrared (IR) laser pulses (~20 femtoseconds) with atomic or molecular gases. High order harmonics of the fundamental frequency are produced, covering a large spectral bandwidth in the extreme ultraviolet (XUV) range. In the temporal domain, this coherent radiation forms a train of 100 attosecond pulses [1].



With such attosecond pulses, it becomes possible to investigate the fastest dynamics in matter, i.e., electronic dynamics that occur naturally on this timescale. Attosecond spectroscopy thus allows studying fundamental processes such as photo-ionization, in order to answer questions such as: how long does it take to remove one electron from an atom or a molecule? More precisely: how long does it take for an electron wavepacket produced by absorption of an attosecond pulse to exit the atomic/molecular potential? The measurement of such tiny ionization delays is currently a “hot topic” in the scientific community. In particular, the study of the ionization dynamics close to resonances would give access to detailed information on the atomic/molecular structure, such as the electronic rearrangements in the remaining ion upon electron ejection. Recently, we have studied an auto-ionizing resonance, so-called “Fano resonance”. We have shown through 2-photon XUV+IR ionization that it is possible to observe in real time the buildup of the resonance profile [2].



The objective of the thesis is to generalize the technique to the study of other types of atomic/molecular resonances, such as shape resonances. To this end, tunable attosecond pulses will be generated using the mid-IR [1.2-2µm] radiation from an optical parametric amplifier (OPA) pumped by an intense Titanium:Sapphire laser. Finally, the measurement of the photoelectron angular distribution, in combination with the temporal information detailed above, will allow the reconstruction of the full 3D movie of the electron ejection.

The experimental work will include the operation of a setup installed in the FAB1 laser of ATTOLab allowing: i) the generation of attosecond XUV radiation, ii) its characterization using quantum interferometry, iii) its use in photo-ionization spectroscopy (electron detection). The theoretical aspects will also be developed. The student will be trained in ultrafast optics, atomic and molecular physics, quantum chemistry, and will acquire a good mastery of charged particle spectrometry. A background in ultrafast optics, nonlinear optics, atomic and molecular physics is required.

Part of this work will be performed in collaboration with partner French laboratories (ANR CIMBAAD) and members of the MEDEA European Network through joint experiments in the different associated laboratories (Milano, Lund).



References :

[1] Y. Mairesse, et al., Science 302, 1540 (2003)

[2] V. Gruson, et al., Science 354, 734 (2016)

Light in the extreme ultraviolet (XUV) is a universal probe of mater, may it be in diluted or condensed phase: photons associated with this spectral range carry energy of 10 to 100 eV, sufficient to directly ionize atoms, molecules or solids. Large scale instruments such as synchrotrons or the lately developed free electron lasers (FEL) work in this spectral range and are used to both study fundamental light matter interaction and develop diagnosis tools. However these instruments do not offer the temporal resolution require to study light matter interactions at their ultimate timescales, which is in the attosecond range (1as = 10^-18s). An alternative is offered by the recent development of XUV sources based on high order harmonic generation (HHG). They are based on the extremely nonlinear interaction of a femtosecond intense laser beam with a gas target. Our laboratory has pioneered the development, control and design of these sources providing XUV attosecond pulses.



During this PhD project, we will develop specific setups to allow these attosecond pulse to carry angular momenta, may it be spin or orbital angular momenta. This will open new applications roads through the observations of currently ignored spectroscopic signatures. On the one hand, the fundamental aspects of the coupling of spin and orbital angular momentum of light in the highly nonlinear regime will be investigated, and on the other hand, we will tack attosecond novel spectroscopies, may it be in diluted or condensed phase. In particular, we will chase helical dichroism, which manifest as different absorptions of beams carrying opposite orbital angular moments. These effects are largely ignored to date.



The student will acquire practical knowledge about lasers, in particular femtosecond lasers, and hands on spectrometric techniques of charged particles. He/she will also study strong field physical processes which form the basis for high harmonic generation. He/she will become an expert in attosecond physics. The acquisition of analysis skills, computer controlled experiments skills will be encouraged although not required.

Full subject available at http://iramis.cea.fr/LIDYL/Pisp/thierry.ruchon/.

Ultrafast nano-photonics science is emerging thanks to the extraordinary progresses in nano-fabrication and ultrafast laser science. Boosting extremely intense electric fields in nano-structured photonic devices has the potential of creating nano-localized sources of energetic photons or particles opening vast applications in science and in the industry. Optoelectronic is extending to the highly non-linear regime. A recent impact of this capability of controlling the response of above band gap electrons under strong fields is the emergence of high harmonic generation (HHG) in crystal [1-6]. 2D and 3D semiconductors exhibits properties of high electron mobility that allows to drive intense electrons currents coherently in the conduction band. HHG are emitted when those electrons recombine to the valence band. This is a pure above band gap non-perturbative phenomena which occurs efficiently in a few 10s to 100s nanometer exit layer of a crystal and down to an atomically thin layer [5,6]. The strong electron current from which HHG originate can be manipulated in space and time. The project will focus in the strong localization in space, and time, at the single optical cycle scale [7,8], of the harmonic generation process. This control can not only revolutionize attosecond science but also prepare a new generation of ultrafast visible to X-ray opto-electronic devices. Based on the CEA group expertise, experimental and theoretical resources [9-12], the fellow will seek for efficient ways to boost the interaction regime through plasmonic amplification and field confinement for the generation of nanoscale, attosecond high harmonic sources in semiconductors. A specific focus will be on 2D materials like graphene, MoS2 and h-BN. Attosecond pulse generation will also be investigated by using harmonic phase measurements available at CEA (RABBITT, FROG techniques). We will also develop an original nanostructured sample that will allow to generate an attosecond light house inside the semiconductor to isolate single attosecond burst of light.

The research will take place in NanoLight facility, a brand new lab equipped with two laser sources: a 100kHz few optical cycles mid-infrared intense OPCPA (tunable from 1,5 to 3.4 µm wavelength) and a 2µm intense MHz rep/rate few optical cycles fiber laser and and ATTOLAB facility equipped with CEP stable Ti:Sa lasers and attosecond metrology.

1. Ghimire, S. et al. Observation of high-order harmonic generation in a bulk crystal. Nat. Phys. 7, 138–141 (2011).

2. Luu, T. T. et al. Extreme ultraviolet high-harmonic spectroscopy of solids. Nature 521, 498–502 (2015).

3. Ndabashimiye, G. et al. Solid-state harmonics beyond the atomic limit. Nature 534, 520–523 (2016).

4. You, Y. S., et al. Anisotropic high-harmonic generation in bulk crystals. Nat. Phys. 13, 345–349 (2017).

5. Liu H. et al. High-harmonic generation from an atomically thin semiconductor. Nature Physics 13, 262–265 (2017).

6. Yoshikawa, N., et al. High-harmonic generation in graphene enhanced by elliptically polarized light excitation. Science, 356, 736-738 (2017).

7. Hohenleuter, M. et al. Real-time observation of interfering crystal electrons in high-harmonic generation. Nature 523, 572-575 (2015).

8. Langer, F. et al., Lightwave-driven quasiparticle collisions on a subcycle timescale. Nature 533, 225–229 (12 May 2016).

9. Franz et al. submitted to Science Advances arXiv:1709.09153

10. Shaaran, T et al. Nano-Plasmonic near Field Phase Matching of Attosecond Pulses. Scientific Reports 2017, 7, 6356.

11. Shi, L. et al. Self-Optimization of Plasmonic Nanoantennas in Strong Femtosecond Fields. Optica 2017, 4, 1038–1043.

12. Nicolas R. et al. Plasmon-Amplified Third Harmonic Generation in metal/dielectric resonators, submitted to ACS Nano (2017).