Logo CEA IRAMIS
CNRS
Logo Université Paris-Saclay
Recherche

Projet ANR Xstase

XUV STudies of light beams carrying Angular momenta:
Synthesis and Exchanges

 Photons may have quanta of both orbital and spin angular momenta (OAM and SAM) [Lavery 2012]. From the early discussions [Poynting 1909], SAM was clearly associated to light polarization and led to many diagnosis methods for chiral matter or matter under magnetic fields. For OAM, the theoretical literature only gave a formal description, quite far from experimental reach. Only recently was the link explicitly made between OAM and the light field properties [allen 1992]. It corresponds, in textbooks examples, to a spiral wavefront, the polarization remaining linear. The two types of angular momenta carry similarities and differences when they come to interact with matter, justifying their joint study.
So far, it was mostly studied in the visible range for its potential application for optical tweezers, quantum entanglement, to improve data multiplexing or to monitor surfaces. It also triggered a great deal of fundamental questions about its link and interplay with regular SAM. Moreover, when interacting with matter, it may induce unusual nonlinear processes. These specificities are related to the momentum conservation laws. However, fundamental tests could only be partially carried out, either because the IR-visible light induces transitions in usually very crowded regions of the spectrum, or because the reported nonlinear processes remain confined to the 3rd order.

In Xstase we plan to develop XUV sources, with femtosecond to attosecond durations to study ultrashort interaction of matter with either type of angular momentum.
Xstase is a young researcher project funded by the French Agence Nationale de la Recherche. It runs from 2014 to 2019. It is coordinated by Thierry Ruchon from LIDYL laboratory and puts together researchers from this lab exclusively.

Contact Information

Email:
Phone: +33 (0)1 69 08 70 10
Fax: +33 (0)1 69 08 87 07

Overview

Light constitutes a unique probe to trigger and address processes at play in matter. In particular, in the nineties, with the advent of femtosecond light sources two avenues opened: i) pump-probe techniques could be applied at the natural timescale of molecular processes, giving birth to the nowadays flourishing femtochemistry field and ii) due to the high peak value of the light field obtained, the realm of NonLinear Optics (NLO) dramatically widened, setting up new investigation techniques such as Z-scan, confocal microscopy… Xstase will use these two qualities to investigate the interaction of matter with XUV light beams carrying either type of Angular Momentum, Spin (SAM) or Orbital (OAM) through linear and nonlinear processes. We will in particular try to synthesize XUV attosecond pulses through High Harmonic Generation (HHG) with angular momenta and precisely characterize them (1 as= 10-18 s). A typical HHG setup is sketched below.

In a typical experiment, a femtosecond laser (10-50 fs) is focused on a gas jet where it produces high energy photons through extreme non linear processes. These photons, which are in the XUV range are analysed on a photon spectrometer (inset) or used to study photoionization processes with charged particles detectors (intermediate gas inlet, e.g. Time of Flight (TOF) Magnetic Bottle Electron Spectrometer (MBES)). In Xstase, we will try to impart angular momenta on these photons.

Orbital and Spin Angular Momenta of XUV light

In the great majority of the applications of ultrafast visible lasers, only the linear momentum and the energy of the photons are put at play, not their angular momenta. The ultimate goal of Xstase is to open a series of new application areas of these ultrafast light sources by i) upconverting them to the XUV spectral range and ii) using their currently largely ignored degrees of freedom, namely their angular momenta. We will use the spin and orbital angular momenta (SAM and OAM, Figure) of femtosecond light sources to develop two highly complementary approaches. The first one, dedicated to gas phase sample, will primarily inform on molecular potential surfaces through SAM. The second, which will have imaging capabilities with a few tens of nanometers resolution will use light sources with OAM to develop new form of spectroscopies for solid state physics.

Left and respectively right: polarization vector (blue) and wavefront (orange surfaces) of a light beam carrying a SAM or an OAM. The Spin Angular Momentum is associated to the circular polarization of light. The Orbital Angular Momentum to an helical wavefront. Both may be associated or not.

Our program will altogether deal with the development and characterization of femtosecond and attosecond sources carrying an angular momentum, and, as far as it becomes possible, to develop associated applications.

A few words about chiroptical effects in the XUV

It has long been recognized that XUV light provides invaluable insights into molecular and atomic physics, in particular in the gas phase for wavelengths corresponding to energies in the tens to the few tens of eV range, which allow direct ionization of the samples. Indeed, the demand for synchrotron light shifts is ever increasing (e.g. 10% more users per year @ SLAC), though the number of facilities also increases (  50 worldwide). Using XUV light, the readout can be done either on the transmitted photons, or the created charged particles. The ultimate tool in this field became the ion-electron coincidence imaging devices allowing retrieving the full 3D map probability of ejection of the electrons in the molecular/atomic frame. Photoionization being a single photon process, it yields results corresponding to well-established theories and, of the utmost importance, fairly high signals.

In the gas phase, it was verified that the photoionization of chiral molecules by circularly polarized XUV light may yield an asymmetry up to 10% in the number of photoelectrons emitted in the forward and backward directions with respect to the direction of propagation of light (see e.g. Desirs beamline @ SOLEIL, or PECD @ Nothingham). Indeed, alike regular absorption, this process christened PECD relies on a one photon electric dipole transition, way more intense that regular natural circular dichroism. Developping ultrafast XUV sources with a SAM is thus of great promises.

A few words about XUV beams with an OAM

XUV sources carrying an OAM were almost non existent when Xstase was written. The scientific study of HHG with OAM beams was essentially a blank canvas, just started with two publications during the last two years [ Gariepy2014, Zurch2012]. From a theoretical perspective there is no code of HHG taking into account all multipoles and the field’s propagation while from an experimental standpoint the progressive transition from the perturbative regime to HHG has not been explored. The HHG characterization that has been reported in the above cited work could be much improved using either interferometric or adapted diffractive schemes as planned in Xstase. The study of photoionization selection rules with XUV OAM beams has so far received only theoretical considerations. During the project, it will be thoroughly tested. Having no well characterized XUV source carrying an OAM, the application field is absolutely clear.

Moments angulaires portés par la lumière. En haut à gauche, onde plane polarisée circulairement. un tel faisceau porte un moment angulaire de spin (ou "spin") de ℏ et aucun moment angulaire orbital (MAO).À droite, onde polarisée linéairement mais dont le plan d'onde est hélicoïdal, avec une rotation de 2π par unité de longueur d'onde. Ce faisceau porte un MAO de ℏ et aucun spin. Les deux propriétés, spin et MAO, peuvent être a priori combinées arbitrairement.Alors que le spin est sensible à la chiralité, c’est-à-dire la symétrie gauche-droite des objets individuels (symétrie interne), le MAO permet de faire tourner des objets macroscopiques par rapport à l’axe du faisceau (degrés de liberté externes).En bas : plans d'onde de faisceaux laser porteurs de moments angulaires orbitaux de ℓ = 1,2 ou 3, vus selon leur direction de propagation et en représentation 3d. L'indice ℓ donne le nombre d'hélices imbriquées. Le faisceau laser est annulaire, avec un rayon fonction du moment angulaire porté.

Objectives

Based on the current state of the art for XUV ultrashort sources carrying angular momenta, we identified the following objectives to be realistic within a 5 year period.

Orbital Angular Momentum

  • Identify in the extreme non linear regime of HHG the conservation rules of OAM
  • Test and report on the most suitable ways to measure the OAM of a femtosecond light beam carrying OAM
  • Design XUV femtosecond light beam carrying arbitrary OAM
  • Identify the role of phase matching during the upconversion of a visible beam through HHG

Spin Angular Momentum

  • Study and report on different approaches to provide XUV femtosecond HHG based sources carrying SAM
  • Provide the most complete measurement of polarization of HHG, including helicity
  • Establish the time-resolved PECD technique (TR-PECD) as a diagnosis tool of chemical physics.
  • Probe the chirality of excited states of chiral molecules.
  • Contribute to the understanding of the mechanisms governing the conservation or loss of molecular chirality after an electronic excitation using pump-probe schemes at the femto and attosecond timescales.

Typical arrangement of our attosecond beamlines. This setup allows both spectral phase and amplitude measuremements at once. It also allows comparing two sources, one of them being pumped.

Attosecond beamlines

Our attosecond beamlines are constructed to offer the most complete diagnosis of the XUV emission. Our current setups, which are highly flexible usually look like the one depicted in the figure below. Basically, we intend to be able to measure a spectral amplitude and a spectral phase of the source using two-photon spectroscopy with the RABBIT technique (see e.g. Ruchon & Camper)) and a photon spectrometer positioned downstream. During Antoine Camper’s PhD thesis, we implemented an extra option by which we can perfom comparisons as for the phase of two adjacent HHG sources, one being possibly pumped (see Antoine Camper’s Thesis). The trick to get an excellent interferometric stability between the sources is to use a binary phase mask to create them. We are now building up a more steady such beamline to be implemented on FAB10 with little flexibility, while keeping two beamlines on FAB1 with a high versatility.

Laser systems

We are carrying most of our research locally, on the SLIC laser systems . There are currently two Ti:Sapphire lasers on which we run attosecond beamlines: FAB1 just getting started and LUCA. LUCA is a 20 Hz, 50 fs, 30 mJ laser, which mode is spatially filtered by a fiber (see Ge et al.)). FAB1 is working at a repetition rate of 1 KHz, and delivers pulses of about 25 fs, 13 mJ. It may be coupled to a light conversion high energy topas, yielding a full tunability between 1200 nm and 1600 nm wavelength, finally yielding a continuously accordable XUV spectrum.

We are currently moving to the new Attolab facility where FAB1 is running since July 2016 and FAB10 is getting ready. This last system works at a 10 kHz repetition rate. It will deliver 20-25 fs long pulses with 2 mJ with a stabilized carrier envelope phase. The commissioning of the associated XUV beamlines is expected late 2016/beginning 2017.

The Xstase team brings together researchers, technicians and early stage researchers with many highly complementary skills from the LIDYL laboratory. A total of almost 200 man-months of working time is planned.

Attophysics group

Reaction Dynamics group

SLIC team

Expertises (with initials of experts)